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The high-latitude ionosphere and its effects on radiopropagation

The physical properties of the ionized layer in the Earth’s upper atmosphereenable us to use it to support an increasing range of communications applications.This book presents a modern treatment of the physics and phenomena of thehigh-latitude upper atmosphere and the morphology of radio propagation in theauroral and polar regions.

Chapters cover the basics of radio propagation and the use of radio techniquesin ionospheric studies, as well as descriptions of the behavior and physics of theionosphere at high latitude. Many investigations of high-latitude radiopropagation have previously been published only in conference proceedings andorganizational reports. This book includes many examples of the behavior ofquiet and disturbed high-latitude high-frequency propagation.

Ample cross-referencing, chapter summaries, and reference lists make this bookan invaluable aid for graduate students, ionospheric physicists, and radioengineers.

Cambridge Atmospheric and Space Science Series

Editors: J. T. Houghton, M. J. Rycroft, and A. J. DesslerThis series of upper-level texts and research monographs covers the physics andchemistry of the various regions of the Earth’s atmosphere, from the troposphereand stratosphere, up through the ionosphere and magnetosphere, and out to theinterplanetary medium.

. . is Professor Emeritus at the University of Alaska,Fairbanks and is Senior Partner of RP Consultants in Klamath Falls, Oregon.

His considerable research experience in high- and mid-latitude radio-wavepropagation and ionospheric studies using radio techniques was gained at theGeophysical Institute and Electrical Engineering Department of the University ofAlaska, the Institute for Telecommunication Science (Boulder, Colorado), theBell Labs (Murray Hill, New Jersey) and as a consultant. He has published over100 papers and one book: Radio Techniques for Probing the Terrestrial Ionosphere(1991). From 1995 through 2002 he was Editor-in-Chief of the journal RadioScience.

. . is Senior Research Fellow in the Department ofCommunication Systems of the University of Lancaster, and Senior VisitingFellow of the University of Central Lancashire. He was formerly Senior Lecturerin the Department of Environmental Sciences at the University of Lancaster. Hestudied at the (then) Jodrell Bank Experimental Station of the University ofManchester, and has worked at the Radio Research Station (Slough, England),and the Space Environment Laboratory (Boulder, Colorado). With over fortyyears of research experience, mainly on studies of the upper atmosphere andionosphere by radio methods, he has published 98 papers and two books: TheUpper Atmosphere and Solar–Terrestrial Relations (1979) and TheSolar–Terrestrial Environment (Cambridge University Press, 1992).

Cambridge Atmospheric and Space Science Series

Editors

Alexander J. Dessler

John T. Houghton

Michael J. Rycroft

Titles in print in this series

M. H. ReesPhysics and chemistry of the upper atmosphere

R. DaleyAtmosphere data analysis

J. R. GarrattThe atmospheric boundary layer

J. K. HargreavesThe solar–terrestrial environment

S. SazhinWhistler-mode waves in a hot plasma

S. P. GaryTheory of space plasma microinstabilities

M. WaltIntroduction to geomagnetically trapped radia-tion

T. I. GombosiGaskinetic theory

B. A. KaganOcean–atmosphere interaction and climatemodelling

I. N. JamesIntroduction to circulating atmospheres

J. C. King and J. TurnerAntarctic meteorology and climatology

J. F. Lemaire and K. I. GringauzThe Earth’s plasmasphere

D. Hastings and H. GarrettSpacecraft–environment interactions

T. E. CravensPhysics of solar system plasmas

J. GreenAtmospheric dynamics

G. E. Thomas and K. StamnesRadiative transfer in the atmosphere and ocean

T. I. GombosiPhysics of space environment

R. W. Schunk and A. F. NagyIonospheres: Physics, plasma physics, and chem-istry

I. G. EntingInverse problems in atmospheric constituenttransport

R. D. Hunsucker and J. K. HargreavesThe high-latitude ionosphere and its effects onradio propagation

The high-latitudeionosphere and its effectson radio propagation

R. D. HunsuckerGeophysical Institute, University of Alaska, Fairbanks

J. K. HargreavesUniversity of Lancaster

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo

Cambridge University PressThe Edinburgh Building, Cambridge , United Kingdom

First published in print format

ISBN-13 978-0-521-33083-1 hardback

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© Cambridge University Press 2003

2002

Information on this title: www.cambridge.org/9780521330831

This book is in copyright. Subject to statutory exception and to the provision ofrelevant collective licensing agreements, no reproduction of any part may take placewithout the written permission of Cambridge University Press.

ISBN-10 0-511-06742-9 eBook (EBL)

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Cambridge University Press has no responsibility for the persistence or accuracy ofs for external or third-party internet websites referred to in this book, and does notguarantee that any content on such websites is, or will remain, accurate or appropriate.

Published in the United States by Cambridge University Press, New York

www.cambridge.org

Contents

From the Times of London xv

Preface xvii

Chapter 1 Basic principles of the ionosphere 1

1.1 Introduction 1

1.1.1 The ionosphere and radio-wave propagation 1

1.1.2 Why the ionosphere is so different at high latitude 2

1.2 The vertical structure of the atmosphere 4

1.2.1 Nomenclature 4

1.2.2 Hydrostatic equilibrium in the atmosphere 5

1.2.3 The exosphere 7

1.2.4 The temperature profile of the neutral atmosphere 8

1.2.5 Composition 10

1.3 Physical aeronomy 13

1.3.1 Introduction 13

1.3.2 The Chapman production function 15

1.3.3 Principles of chemical recombination 18

1.3.4 Vertical transport 20

1.4 The main ionospheric layers 23


1.4.2 The E and F1 regions 26

1.4.3 The D region 31

1.4.4 The F2 region and the protonosphere 37

1.4.5 Anomalies of the F2 region 39

1.4.6 The effects of the sunspot cycle 44

1.4.7 The F-region ionospheric storm 46

vii

1.5 The electrical conductivity of the ionosphere 48


1.5.2 Conductivity in the absence of a magnetic field 48

1.5.3 The effect of a magnetic field 48

1.5.4 The height variation of conductivity 50

1.5.5 Currents 50

1.6 Acoustic-gravity waves and traveling ionospheric disturbances 52


1.6.2 Theory 53

1.6.3 Traveling ionospheric disturbances 57

1.6.4 The literature 57

1.7 References and bibliography 58

Chapter 2 Geophysical phenomena influencing the high-latitude ionosphere 61

2.1 Introduction 61

2.2 The magnetosphere 61

2.2.1 The geomagnetic field 61

2.2.2 The solar wind 63

2.2.3 The magnetopause 69

2.2.4 The magnetosheath and the shock 71

2.2.5 The polar cusps 72

2.2.6 The magnetotail 72

2.3 Particles in the magnetosphere 73

2.3.1 Principal particle populations 73

2.3.2 The plasmasphere 74

2.3.3 The plasma sheet 78

2.3.4 Trapped particles 78

2.3.5 The ring current 84

2.3.6 Birkeland currents 85

2.4 The dynamics of the magnetosphere 86

2.4.1 Circulation patterns 86

2.4.2 Field merging 90

2.4.3 Magnetospheric electric fields 91

2.4.4 The dynamics of the plasmasphere 92

2.5 Magnetic storms 93


2.5.2 The classical magnetic storm and the Dst index 94

2.5.3 Magnetic bays at high latitude; the auroral electrojet 95

2.5.4 Magnetic indices 96

viii Contents

2.5.5 Great magnetic storms and a case history 100

2.5.6 Wave phenomena of the magnetosphere 103

2.6 Ionization by energetic particles 105

2.6.1 Electrons 105

2.6.2 Bremsstrahlung X-rays 106

2.6.3 Protons 107


Chapter 3 Fundamentals of terrestrial radio propagation 113

3.1 Introduction 113

3.2 Electromagnetic radiation 113

3.2.1 Basics of line-of-sight propagation in vacuo 113

3.2.2 Principles of radar 116

3.2.3 The significance of the refractive index 118

3.2.4 Interactions between radio waves and matter 121

3.3 Propagation through the neutral atmosphere 122

3.3.1 The refractivity of the neutral atmosphere 122

3.3.2 Terrain effects 124

3.3.3 Noise and interference 127

3.4 Ionospheric propagation 140

3.4.1 Magnetoionic theory 140

3.4.2 Reflection of radio waves from an ionospheric layer 144

3.4.3 Relations between oblique and vertical incidence 149

3.4.4 Trans-ionospheric propagation 147

3.4.5 Principles of radio scintillation 152

3.4.6 Propagation involving reflection from a sharp boundary and full-wavesolutions 159

3.4.7 Whistlers 167

3.5 Ionospheric scatter 169

3.5.1 Coherent scatter 169

3.5.2 Forward scatter 171

3.5.3 Incoherent scatter 171

3.6 HF-propagation-prediction programs 174

3.7 Summary 175


Chapter 4 Radio techniques for probing the ionosphere 181


4.2 Ground-based systems 181

Contents ix

4.2.1 Ionosondes 181

4.2.2 Coherent oblique-incidence radio-sounding systems 187

4.2.3 Incoherent-scatter radars 203

4.2.4 D-region absorption measurements 203

4.2.5 Ionospheric modification by HF transmitters 210

4.3 Space-based systems 215

4.3.1 A history of Earth–satellite and radio-rocket probing 215

4.3.2 Basic principles of operation and current-deployment of radio-beaconexperiments 215

4.3.3 Topside sounders 216

4.3.4 In situ techniques for satellites and rockets 217

4.3.5 Capabilities and limitations 217

4.4 Other techniques 217

4.4.1 HF spaced-receiver and Doppler systems 217

4.4.2 The HF Doppler technique 219

4.4.3 Ionospheric imaging 220

4.5 Summary 220


Chapter 5 The high-latitude F region and the trough 227

5.1 Circulation of the high-latitude ionosphere 227


5.1.2 Circulation patterns 228

5.2 The behavior of the F region at high latitude 234

5.2.1 The F region in the polar cap 234

5.2.2 The effect of the polar cusps 237

5.2.3 The polar wind 239

5.2.4 The F layer in and near the auroral oval 240

5.3 Irregularities of the F region at high latitude 242


5.3.2 Enhancements: patches, and blobs 244

5.3.3 Scintillation-producing irregularities 249

5.4 The main trough 260


5.4.2 Observed properties and behavior of the main trough 261

5.4.3 The poleward edge of the trough 269

5.4.4 Motions of individual troughs 271

5.4.5 Mechanisms and models 273

5.5 Troughs and holes at high latitude 276

x Contents

5.6 Summary 280


Chapter 6 The aurora, the substorm, and the E region 285


6.2 Occurrence zones 286

6.2.1 The auroral zone and the auroral oval 286

6.2.2 Models of the oval 288

6.3 The auroral phenomena 291

6.3.1 The luminous aurora 291

6.3.2 The distribution and intensity of the luminous aurora 291

6.3.3 Auroral spectroscopy 302

6.3.4 Ionospheric effects 302

6.3.5 The outer precipitation zone 305

6.4 The substorm 308

6.4.1 History 308

6.4.2 The substorm in the aurora 308

6.4.3 Ionospheric aspects of the substorm 311

6.4.4 Substorm currents 312

6.4.5 The substorm in the magnetosphere 315

6.4.6 The influence of the IMF and the question of substorm triggering 319

6.4.7 Relations between the storm and the substorm 321

6.5 The E region at high latitude 322


6.5.2 The polar E layer 323

6.5.3 The auroral E layer under quiet conditions 323

6.5.4 The disturbed auroral E layer 323

6.5.5 Auroral radar 326

6.5.6 Auroral infrasonic waves 330

6.5.7 The generation of acoustic gravity waves 331

6.6 Summary and implications 332


Chapter 7 The high-latitude D region 337


7.2 Auroral radio absorption 339

7.2.1 Introduction – history and technique 339

7.2.2 Typical auroral-absorption events and their temporal and spatial properties 340

7.2.3 General statistics in space and time 350

Contents xi

7.2.4 Dynamics 354

7.2.5 The relation to geophysical activity, and predictions of auroral absorption 365

7.2.6 The wider geophysical significance of auroral-absorption events 371

7.3 The polar-cap event 382


7.3.2 Observed properties of PCA events 384

7.3.3 The relation to solar flares and radio emissions 389

7.3.4 Effects arising during the proton’s journey to Earth 390

7.3.5 Non-uniformity and the midday recovery 395

7.3.6 Effects in the terrestrial atmosphere 398

7.4 Coherent scatter and the summer mesospheric echo 406



Chapter 8 High-latitude radio propagation: part 1 – fundamentals and early results 417


8.2 ELF and VLF propagation 419

8.3 LF and MF propagation 429

8.4 HF propagation 439

8.4.1 Tests carried out between Alaska and Scandinavia on fixed frequencies 439

8.4.2 Tests involving transmission between Alaska and the continental USA 448

8.4.3 Other trans-polar HF experiments on fixed frequencies 450

8.4.4 College–Kiruna absorption studies at fixed frequencies 457

8.4.5 Effects of auroral-zone-absorption events on HF propagation 473

8.4.6 Sweep-frequency experiments 473

8.4.7 Other results from HF high-latitude studies from c. 1956–1969 479

8.4.8 Doppler and fading effects on HF high-latitude propagation paths 492

8.5 VHF/UHF and microwave propagation 529

8.6 Summary 531


Chapter 9 High-latitude radio propagation: part 2 – modeling, prediction, andmitigation of problem 537


9.2 Ionospheric ray-tracing, modeling, and prediction of propagation 538

9.2.1 Ionospheric ray-tracing 538

9.2.2 Realistic high-latitude models 538

9.2.3 Validation of ionospheric models 545

xii Contents

9.2.4 The performance of ELP–HF predictions at high latitudes 546

9.2.5 Recent validation of selected ionospheric prediction models usingHF propagation data 553

9.3 Predictions of VHF/UHF propagation 568

9.4 Recent efforts at validation of ionospheric models 568

9.5 Mitigation of disturbance of HF propagation 572

9.5.1 Early attempts 572

9.5.2 Mitigation using solar–terrestrial data 572

9.5.3 Adaptive HF techniques 574

9.5.4 Realtime channel evaluation 580

9.5.5 Recent advances in assessment of HF high-latitude propagation channels 586

9.6 Other high-latitude propagation phenomena and evaluations 591

9.6.1 Large bearing errors on HF high-latitude paths 591

9.6.2 Effects of substorm on auroral and subauroral paths 593

9.6.3 Use of GPS/TEC data to investigate HF auroral propagation 594

9.6.4 The performance of HF modems at high latitude using multiplefrequencies 597

9.7 Summary and discussion 597


Appendix: some books for general reading 612

Index 613

Contents xiii

From the Times of LondonTRANS-ATLANTIC MESSAGEMonday, December 16, 1901 —From our correspondent, St. Johns, NF, Dec. 14;Signor Marconi authorizes me to announce that he received onWednesday and Thursday electrical signals at his experimental stationhere from the station at Poldhu, Cornwall, thus solving the problem oftelegraphing across the Atlantic without wire. He has informed theGovernor, Sir Cavendish Boyle, requesting him to apprise the BritishCabinet of the discovery, the importance of which it is impossible toovervalue.

To Phyllis and Sylvia.For forbearance.

Preface

It is over a century since Marconi’s famous radio transmission across the AtlanticOcean, an experiment closely followed by Kennelly and Heaviside’s suggestionsthat an ionized layer in the Earth’s upper atmosphere had made it possible. Fromthe first, the ionosphere has been put to use, supporting an increasing range ofapplications from point-to-point communication and broadcasting, to direction-finding, navigation, and over-the-horizon radar. After 75 years of active research,the ionosphere can hardly be considered one of the mysteries of the Universe, butin fact some scientific problems and technical difficulties do remain. Many of themconcern the high-latitude regions, which are particularly subject to disturbancesarising initially on the sun.

Since radio propagation depends so strongly on the behavior of the ionosphere,we have tried to bring the two topics together into a single monograph about thepolar regions. The early chapters (1–4) provide introductions to the ionosphere ingeneral, to the influence of the magnetosphere, to the principles of radio propa-gation, and to the major techniques of ionospheric observation. Chapters 5–7describe the various phenomena of the ionosphere that are peculiar to the highlatitudes. The final chapters (8–9) present the results of high-latitude propagationexperiments, many of which have been published only in reports that were notwidely disseminated at the time or have indeed remained unpublished. Short sum-maries are included at the end of each chapter to aid readers in getting a quickoverview of the material in the chapter. Some useful Internet references (URLs)are given within the text.

This book will fill a gap for scientists, engineers and students both at the grad-uate and at the undergraduate level whose interest is in understanding and/or pre-dicting the behavior of radio propagation at auroral and polar latitudes.

xvii

Advanced amateur radio operators and shortwave listeners should also find usefulinformation in this monograph. The book contains interlinking referencesbetween chapters, which, it is hoped, will aid the reader when a deeper under-standing of the phenomena is desired.

Now a word or two about references: The book includes material ranging fromthe classical to the recently published. References to the newer material are givenat the end of each chapter, there divided by section. They are there partly as theusual courtesy to the original authors, but also so that the more inquisitive reader,such as yourself, may follow up topics in more detail by going back to the origi-nal sources. These, moreover, will often cite further valuable references.

It would be impractical to cite all the original authors of material that hasbecome standard in the field through being re-digested and re-presented in numer-ous books and review papers. To support material of this kind (mainly in Chapters1, 2 and 6), a selection of books and conference reports is listed at the end of thechapter, and readers will be able to use these to broaden their knowledge of thefield in general and also to check our own presentation of it if they feel so inclined.(Needless to say, the present authors will appreciate being told of any errors dis-covered.) An appendix lists some books that discuss more broadly the high-latitude phenomena connected with disturbances of the magnetosphere.

We thank the many authors and publishers who have granted permission toreproduce diagrams, including some previously unpublished ones. We are grate-ful in particular to M. Angling, D. H. Bliss, N. J. Flowers, N. Gerson,J. M. Goodman, M. S. Gussenhoven, C. H. Jackman, M. J. Jarvis, V. Jodalen,E. Johnson, L. Kersley, R. L. McPherron, T. I. Pulkinnen, M. H. Rees, J. Secan,P. N. Smith, E. Turunen, M. Walt, J. W. Wright, and M. Wild.

xviii Preface

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Chapter 1

Basic principles of the ionosphere

1.1 Introduction

1.1.1 The ionosphere and radio-wave propagation

The ionosphere is the ionized component of the atmosphere, comprising free elec-trons and positive ions, generally in equal numbers, in a medium that is electri-cally neutral. Though the charged particles are only a minority amongst theneutral ones, they nevertheless exert a great influence on the electrical propertiesof the medium, and it is their presence that brings about the possibility of radiocommunication over large distances by making use of one or more ionosphericreflections.

The early history of the ionosphere is very much bound up with the develop-ment of communications. The first suggestions that there are electrified layerswithin the upper atmosphere go back to the nineteenth century, but the moderndevelopments really started with Marconi’s well-known experiments in trans-Atlantic communication (from Cornwall to Newfoundland) in 1901. These led tothe suggestions by Kennelly and by Heaviside (made independently) that, becauseof the Earth’s curvature, the waves could not have traveled directly across theAtlantic but must have been reflected from an ionized layer. The name ionospherecame into use about 1932, having been coined by Watson-Watt several years pre-viously. Subsequent research has revealed a great deal of information about theionosphere: its vertical structure, its temporal and spatial variations, and the phys-ical processes by which it is formed and which influence its behavior.

Looked at most simply, the ionosphere acts as a mirror situated between 100and 400 km above the Earth’s surface, as in Figure 1.1, which allows reflected

1

signals to reach points around the bulge of the Earth. The details of how reflec-tion occurs depend on the radio frequency of the signal, but the most usual mech-anism, which applies in the high-frequency (HF) band (3–30 MHz), is actually agradual bending of the ray towards the horizontal as the refractive index of theionospheric medium decreases with altitude. Under good conditions, signals canbe propagated in this way for several thousand kilometers by means of repeatedreflections between ionosphere and ground. Reflection from a higher level (the Fregion) obviously gives a greater range per “hop” than does one from a lower level(the E region), but which mode is possible depends on the structure of the iono-sphere at the time. Higher radio frequencies tend to be reflected from greaterheights, but if the frequency is too high there may be insufficient bending and thesignal then penetrates the layer and is lost to space. This is the first complicationof radio propagation.

The second complication is that the lower layers of the ionosphere tend toabsorb the signal. This effect is greater for signals of lower frequency and greaterobliquity. Hence, practical radio communication generally requires a compro-mise. The ionosphere is constantly changing, and the art of propagation predic-tion is to determine the best radio frequency for a given path for the current stateof the ionosphere. Plainly, an understanding of ionospheric mechanisms is basicto efficient radio communication.

Further details about radio propagation are given in Chapter 3, and our centraltopic of how propagation at high latitudes is affected by the vagaries of the high-latitude ionosphere is discussed later in the book.

1.1.2 Why the ionosphere is so different at high latitude

The terrestrial ionosphere may be divided broadly into three regions that haverather different properties according to their geomagnetic latitude. The mid-latitude region has been explored the most completely and is the best understood.There, the ionization is produced almost entirely by energetic ultra-violet and X-ray emissions from the Sun, and is removed again by chemical recombination pro-cesses that may involve the neutral atmosphere as well as the ionized species. The

2 Basic principles of the ionosphere

Figure 1.1. Long distance propagation by multiple hops between the ionosphere and theground.

Ionosphere

300 km

Ground

movement of ions, and the balance between production and loss, are affected bywinds in the neutral air. The processes typical of the mid-latitude ionosphere alsooperate at high and low latitudes, but in those regions additional processes are alsoimportant.

The low-latitude zone, spanning 20° or 30° either side of the magnetic equator,is strongly influenced by electromagnetic forces that arise because the geomag-netic field runs horizontally over the magnetic equator. The primary consequenceis that the electrical conductivity is abnormally large over the equator. A strongelectric current (an “electrojet”) flows in the E region, and the F region is subjectto electrodynamic lifting and a “fountain effect” that distorts the general form ofthe ionosphere throughout the low-latitude zone.

At high latitudes we find the opposite situation. Here the geomagnetic fieldruns nearly vertical, and this simple fact of nature leads to the existence of an ion-osphere that is considerably more complex than that in either the middle or thelow-latitude zones. This happens because the magnetic field-lines connect the highlatitudes to the outer part of the magnetosphere which is driven by the solar wind,whereas the ionosphere at middle latitude is connected to the inner magneto-sphere, which essentially rotates with the Earth and so is less sensitive to externalinfluence. We can immediately identify four general consequences.

(a). The high-latitude ionosphere is dynamic. It circulates in a pattern mainlycontrolled by the solar wind but which is also variable.

(b). The region is generally more accessible to energetic particle emissions fromthe Sun that produce additional ionization. Thus it is affected by sporadicevents, which can seriously degrade polar radio propagation. Over alimited range of latitudes the dayside ionosphere is directly accessible tomaterial from the solar wind.

(c). The auroral zones occur within the high-latitude region. Again, their loca-tion depends on the linkage with the magnetosphere, in this case into thedistorted tail of the magnetosphere. The auroral phenomena includeelectrojets, which cause magnetic perturbations, and there are “substorms”in which the rate of ionization is greatly increased by the arrival of ener-getic electrons. The auroral regions are particularly complex for radiopropagation.

(d). A “trough” of lesser ionization may be formed between the auroral andthe mid-latitude ionospheres. Although the mechanisms leading to the for-mation of the trough are not completely known, it is clear that one funda-mental cause is the difference in circulation pattern between the inner andouter parts of the magnetosphere.

This monograph is concerned mainly with the ionosphere at high latitudes, butbefore considering the special behavior which occurs in those regions we mustreview some processes affecting the ionosphere in general and summarize themore normal behavior at middle latitudes. In order to do that, we must first

1.1 Introduction 3

consider the nature of the neutral upper atmosphere in which the ionosphere isformed.

1.2 The vertical structure of the atmosphere

1.2.1 Nomenclature

A static planetary atmosphere may be described by four properties: pressure (P),density (), temperature (T ), and composition. Since these are not independent itis not necessary to specify all of them. The nomenclature of the atmosphere isbased principally on the variation of temperature with height, as in Figure 1.2.Here, the different regions are called “spheres” and the boundaries between themare “pauses”. The lowest region is the troposphere, in which the temperature fallsoff with increasing height at a rate of 10 K km1 or less. Its upper boundary is thetropopause at a height of 10–12 km. The stratosphere which lies above it was oncethought to be isothermal, but it is actually a region where the temperatureincreases with height. At about 50 km is a maximum due to the absorption of solarultra-violet radiation in ozone; this is the stratopause. Above that the temperatureagain decreases in the mesosphere (or middle atmosphere) and passes throughanother minimum at the mesopause at 80–85 km. At about 180 K, this is thecoldest part of the whole atmosphere. Above the mesopause, heating by solarultra-violet radiation ensures that the temperature gradient remains positive, andthis is the thermosphere. Eventually the temperature of the thermosphere becomes


Figure 1.2. Nomenclature of the upper atmosphere based on temperature, composition,mixing, and ionization. (J. K. Hargreaves, The Solar–Terrestrial Environment. CambridgeUniversity Press, 1992.)

10 000

3000

1000

300

100

30

10

3

1500 1000 1500

Temperature (K)

10 5 0

Electron density(105 cm–3)

Temperature Composition Gaseous escape Ionization

Thermosphere Protonosphere

Ionosphere

Heliosphere

Turbopause

Exobase or

baropause

Het

eros

pher

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pher

eor

hom

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Exo

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MesopauseMesosphereStratopauseStratosphereTropopauseTroposphere

Hei

ght (

km)

almost constant at a value that varies with time but is generally over 1000 K; thisis the hottest part of the atmosphere.

Though the classification by temperature is generally the most useful, othersbased on the state of mixing, the composition or the state of ionization are alsouseful. The lowest part of the atmosphere is well mixed, with a composition muchlike that at sea level except for minor components. This is the turbosphere or homo-sphere. In the upper region, essentially the thermosphere, mixing is inhibited bythe positive temperature gradient, and here, in the heterosphere, the various com-ponents separate under gravity and as a result the composition varies with alti-tude. The boundary between the two regions, which occurs at about 100 km, is theturbopause. Above the turbopause the gases separate by gaseous diffusion morerapidly than they are mixed by turbulence.

Within the heterosphere there are regions where helium or hydrogen may be themain component. These are the heliosphere and the protonosphere, respectively.From the higher levels, above about 600 km, individual atoms can escape from theEarth’s gravitational attraction; this region is called the exosphere. The base of theexosphere is the exobase or the baropause, and the region below the baropause isthe barosphere.

The terms ionosphere and magnetosphere apply, respectively, to the ionizedregions of the atmosphere and to the outermost region where the geomagneticfield controls the particle motions. The outer termination of the geomagnetic field(at about ten Earth radii in the sunward direction) is the magnetopause.

1.2.2 Hydrostatic equilibrium in the atmosphere

Between them the properties temperature, pressure, density, and compositiondetermine much of the atmosphere’s behaviour. They are not independent, beingrelated by the universal gas law which may be written in various forms, but for ourpurposes the form

PnkT, (1.1)

where n is the number of molecules per unit volume, is the most useful. The quan-tity n is properly called the concentration or the number density, but “density”alone is often used when the sense is clear.

Apart from its composition, the most significant feature of the atmosphere isthat the pressure and density decrease with increasing altitude. This height varia-tion is described by the hydrostatic equation, sometimes called the barometricequation, which is easily derived from first principles. The variation of pressurewith height is

PP0exp(h/H ), (1.2)

1.2 Vertical structure 5

where P is the pressure at height h, P0 is the pressure where h0, and H is the scaleheight given by

HkT/(mg), (1.3)

in which k is Boltzmann’s constant, T is the absolute temperature, m is the massof a single molecule of the atmospheric gas, and g is the acceleration due to gravity.

If T and m are constant (and any variation of g with height is neglected), H isthe vertical distance over which n falls by a factor e (2.718), and thus it servesto define the thickness of an atmosphere. H is greater, and the atmosphere thicker,if the gas is hotter or lighter. In the Earth’s atmosphere H varies from about 5 kmat height 80 km to 70–80 km at 500 km.

Using equation (1.1), the hydrostatic equation may be written in differentialform as

dP/Pdn/ndT/Tdh/H. (1.4)

From this, H can be ascribed a local value, even if it varies with height.Another useful form is

P/P0exp[(hh0)/H ]ez, (1.5)

where PP0 at the height hh0, and z is the reduced height defined by

z(hh0)/H. (1.6)

The hydrostatic equation can also be written in terms of the density () and thenumber density (n). If T, g, and m are constant over at least one scale height, theequation is essentially the same in terms of P, , and n, since n/n0/0P/P0.The ratio k/m can also be replaced by R/M, where R is the gas constant and M isthe relative molecular mass.

Whatever the height distribution of the atmospheric gas, its pressure P0 atheight h0 is just the weight of gas above h0 in a column of unit cross-section. Hence

P0NT mgn0kT0, (1.7)

where NT is the total number of molecules in the column above h0, and n0 and T0

are the concentration and the temperature at h0. Therefore we can write

NTn0kT0/(mg)n0H0, (1.8)

H0 being the scale height at h0. This equation says that, if all the atmosphere aboveh0 were compressed to density n0 (that already applying at h0), then it would


occupy a column extending just one scale height. Note also that the total mass ofthe atmosphere above unit area of the Earth’s surface is equal to the surface pres-sure divided by g.

Although we often assume that g, the acceleration due to gravity, is a constant,in fact it varies with altitude as g(h) 1/(REh)2, where RE is the radius of theEarth. The effect of changing gravity may be taken into account by defining a geo-potential height

h*REh/(REh). (1.9)

A molecule at height h over the spherical Earth has the same potential energy asone at height h* over a hypothetical flat Earth having gravitational accelerationg(0).

Within the homosphere, where the atmosphere is well mixed, the mean relativemolecular mass determines the scale height and the variation of pressure withheight. In the heterosphere, the partial pressure of each constituent is determinedby the relative molecular mass of that species. Each species takes up its own dis-tribution, and the total pressure of the atmosphere is the sum of the partial pres-sures in accordance with Dalton’s law.

1.2.3 The exosphere

In discussing the atmosphere in terms of the hydrostatic equation we are treatingthe atmosphere as a compressible fluid whose temperature, pressure, and densityare related by the gas law. This is valid only if there are sufficient collisions betweenthe gas molecules for a Maxwellian velocity distribution to be established. As thepressure decreases with increasing height so does the collision frequency, and atabout 600 km the distance traveled by a typical molecule between collisions, themean free path, becomes equal to the scale height. At this level and above we haveto regard the atmosphere in a different way, not as a fluid but as an assembly ofindividual molecules or atoms, each following its own trajectory in the Earth’sgravitational field. This region is called the exosphere.

While the hydrostatic equation is strictly valid only in the barosphere, it hasbeen shown that the same form may still be used if the velocity distribution isMaxwellian. This is true to some degree in the exosphere, and the use of the hydro-static equation is commonly extended to 1500–2000 km, at least as an approxima-tion. However, this liberty may not be taken if there is significant loss of gas fromthe atmosphere, since more of the faster molecules will be lost and the velocity dis-tribution of those remaining will be altered thereby. The lighter gases, helium andhydrogen, are affected most.

The rate at which gas molecules escape from the gravitational field in the exo-sphere depends on their vertical speed. Equating the kinetic and potential ener-gies of an upward-moving particle, its escape velocity (ve) is given by


v e22gr, (1.10)

where r is the distance of the particle from the center of the Earth. (At the Earth’ssurface the escape velocity is 11.2 km s1, irrespective of the mass of the particle.)

By kinetic theory the root mean square (r.m.s.) thermal speed of gas molecules( ) depends on their mass and temperature, and, for speeds in one direction, i.e.vertical,

m /23kT/2. (1.11)

Thus, corresponding to an escape velocity (ve) there can be defined an escape tem-perature (Te).

Te is 84000 K for atomic oxygen, 21000 K for helium, but only 5200 K for ato-mic hydrogen. At 1000–2000 K, exospheric temperatures are smaller than theseescape temperatures, and loss of gas, if any, will be mainly at the high-speed endof the velocity distribution. In fact, the loss is insignificant for O, slight for He, butsignificant for H. Detailed computations show that the resulting vertical distribu-tion of H departs significantly from the hydrostatic at distances more than oneEarth radius above the surface, but for He the departure is small.

1.2.4 The temperature profile of the neutral atmosphere

The atmosphere’s temperature profile results from the balance amongst sources ofheat, loss processes, and transport mechanisms. The total picture is complicated,but the main points are as follows.

Sources

The troposphere is heated by convection from the hot ground, but in the upperatmosphere there are four sources of heat:

(a). Absorption of solar ultra-violet and X-ray radiation, causing photodisso-ciation, ionization, and consequent reactions that liberate heat;

(b). Energetic charged particles entering the upper atmosphere from the mag-netosphere;

(c). Joule heating by ionospheric electric currents; and

(d). Dissipation of tidal motions and gravity waves by turbulence and molecu-lar viscosity.

Generally speaking, the first source (a) is the most important, though (b) and(c) are also important at high latitude. Most solar radiation of wavelength lessthan 180 nm is absorbed by N2, O2 and O. Photons that dissociate or ionize mole-cules or atoms generally have more energy than that needed for the reaction, andthe excess appears as kinetic energy of the reaction products. A newly created pho-toelectron, for example, may have between 1 and 100 eV of kinetic energy, which

v2

v2


subsequently becomes distributed throughout the medium by interactionsbetween the particles (optical, electronic, vibrational, or rotational excitation, orelastic collisions, depending on the energy.) Elastic collisions redistribute energyless than 2 eV, and, since this process operates mainly between electrons, theseremain hotter than the ions. Some energy is reradiated, but on average about halfgoes into local heating. It can generally be assumed that in the ionosphere the rateof heating in a given region is proportional to the ionization rate.

The temperature profile (Figure 1.2) can be explained as follows. The maximumat the stratopause is due to the absorption of 200–300 nm (2000–3000 Å) radia-tion by ozone (O3) over the height range 20–50 km. Some 18 W m2 is absorbedin the ozone layer. Molecular oxygen (O2), which is relatively abundant up to 95km, absorbs radiation between 102.7 and 175 nm, much of this energy being usedto dissociate O2 to atomic oxygen (O). This contribution amounts to some 30 mWm2. Radiation of wavelengths shorter than 102.7 nm, which is the ionization limitfor O2 (See Table 1.1 of Section 1.4.1), is absorbed to ionize the major atmosphericgases O2, O, and N2 over the approximate height range 95–250 km, and this iswhat heats the thermosphere. Though the amount absorbed is only about 3 mWm2 at solar minimum (more at solar maximum), a small amount of heat may raisethe temperature considerably at great height because the air density is small.Indeed, at the greater altitudes the heating rate and the specific heat are both pro-portional to the gas concentration, and then the rate of increase in temperature isactually independent of height.

At high latitude, heating associated with the aurora – items (b) and (c) – isimportant during storms. Joule heating by electric currents is greatest at 115–130km. Auroral electrons heat the atmosphere mainly between 100 and 130 km.

Losses

The principal mechanism of heat loss from the upper atmosphere is radiation,particularly in the infra-red. Emission by oxygen at 63 m is important, as arespectral bands of the radical OH and the visible airglow from oxygen and nitro-gen. The mesosphere is cooled by radiation from CO2 at 15 m and from ozoneat 9.6 m, though during the long days of the polar summer the net effect can beheating instead of cooling.

Transport

The thermal balance and temperature profile of the upper atmosphere are alsoaffected by processes of heat transport. At various levels conduction, convection,and radiation all come into play.

Radiation is the most efficient process at the lowest levels, and the atmosphereis in radiative equilibrium between 30 and 90 km. Eddy diffusion, or convection,also operates below the turbopause (at about 100 km), and allows heat to becarried down into the mesosphere from the thermosphere. This flow represents amajor loss of heat from the thermosphere but is a minor source for the mesosphere.


In the thermosphere (above 150 km) thermal conduction is efficient because of thelow pressure and the presence of free electrons. The large thermal conductivityensures that the thermosphere is isothermal above 300 or 400 km, though thethermospheric temperature varies greatly from time to time. Chemical transport ofheat occurs when an ionized or dissociated species is created in one place andrecombines in another. The mesosphere is heated in part by the recombination ofatomic oxygen created at a higher level. There can also be horizontal heat trans-port by large-scale winds, which can affect the horizonal distribution of tempera-ture in the thermosphere.

The balance amongst these various processes produces an atmosphere with twohot regions, one at the stratopause and one in the thermosphere. The thermo-spheric temperature, in particular, undergoes strong variations daily and with thesunspot cycle, both due to the changing intensity of solar radiation.

1.2.5 Composition

The upper atmosphere is composed of various major and minor species. Theformer are the familiar oxygen and nitrogen in molecular or atomic forms, orhelium and hydrogen at the greater heights. The minor constituents are othermolecules that may be present as no more than mere traces, but in some cases theycan exert an influence far beyond their numbers.

Major species

The constant mixing within the turbosphere results in an almost constant propor-tion of major species up to 100 km, essentially the mixture as at ground-levelcalled “air”, although complete uniformity cannot be maintained if there aresources and sinks for particular species. Molecular oxygen is dissociated to atomicoxygen by ultra-violet radiation between 102.7 and 175.9 nm:

O2h →OO, (1.12)

where h is a quantum of radiation. An increasing amount of O appears above 90km. The atomic and molecular forms are present in equal concentrations at about125 km, and above that the atomic form increasingly dominates. Nitrogen is notdirectly dissociated to the atomic form in the atmosphere, though it does appearas a product of other reactions.

Above the turbopause mixing is less important than diffusion, and then eachcomponent takes an individual scale height depending on its relative atomic ormolecular mass (HkT/(mg)). Because the scale heights of the common gasesvary over a wide range – H1, He4, O16, N228, O232 – the relative com-position of the thermosphere is a marked function of height, the lighter gasesbecoming progressively more abundant as illustrated in Figure 1.3. Atomicoxygen dominates at a height of several hundred kilometers. Above that is the


heliosphere, where helium is the most abundant, and eventually hydrogenbecomes the major species in the protonosphere. Because the scale height alsodepends on the temperature, so do the details of the composition. The protono-sphere starts much higher in a hot thermosphere, and the heliosphere may beabsent from a cool one.

Two of the important species of the upper atmosphere, helium and hydrogen,are no more than minor species in the troposphere. Helium comes from radioac-tive decay in the Earth’s crust. It diffuses up through the atmosphere, eventuallyescaping into space. The source of atomic hydrogen is the dissociation of watervapor near the turbopause from where it, also, flows constantly up through theatmosphere.

Minor species

Water, carbon dioxide, oxides of nitrogen, ozone, and alkali metals are all minorspecies of the atmosphere, but not all of them are significant for the ionosphere.

Water does not have the same dominating influence in the upper atmosphere asin the troposphere. It is important nevertheless, first as a source of hydrogen, andsecond because it causes ions to be hydrated below the mesopause. Carbondioxide, also, plays a part in the chemistry of the D region.


Figure 1.3. Atmospheric composition to 1000 km for a typical temperature profile. (USStandard Atmosphere, 1976.)

Nitric oxide (NO), on the other hand, makes an important contribution to thelower ionosphere since it is ionized by the intense Lyman- line of the solar spec-trum and is thereby responsible for much of the ionospheric D region at middlelatitudes (Section 1.4.3). The chemical story of NO is complicated because severalproduction and loss mechanisms are at work and the distribution is affected bythe dynamics of the mesosphere.

Nitric oxide in the mesosphere comes from two sources. One source is in thestratosphere and involves the oxidation of nitrous oxide (N2O) by excited atomicoxygen. The second one peaks in the thermosphere, at 150–160 km, and involvesa reaction with neutral or ionized atomic nitrogen, for example

N*O2→NOO, (1.13)

where the * indicates an excited state. The resulting NO diffuses down to the meso-sphere by molecular and then by eddy diffusion. Loss by photodissociation andrecombination, aided by the effect of the low temperature at the mesopause, issufficient to create a minimum at 85–90 km. The diffusion is weaker in the summer,and that is when the minimum is most marked. The depth of the minimum alsovaries with latitude.

The production of these atomic-nitrogen species is closely linked to ionizationprocesses, and it is estimated that 1.3 NO molecules are produced on average foreach ion produced. The concentration of nitric oxide therefore varies with time ofday, latitude, and season. It is 3–4 times greater at high latitude than it is at middlelatitude, and more variable. The production rate increases dramatically duringparticle precipitation events, and this is plainly an important mechanism in thehigh-latitude ionosphere.

The ozonosphere peaks between heights of 15 and 35 km, well below the ion-osphere. The small amounts of ozone that occur in the mesosphere are involvedin certain reacions in the D region, but we shall not be particularly concerned withthem in this monograph. It is, however, of some general interest that there is a reac-tion between ozone and nitric oxide that tends to remove ozone at mesosphericlevels. Thus,

O3NO→O2NO2

ONO2→O2NO

O3O→2O2. (1.14)

The net result, in the presence of atomic oxygen, is a catalytic conversion of ozoneback to molecular oxygen. In this way the ozone concentration is affected by thenatural production of nitric oxide discussed above.

Metallic atoms are introduced into the atmosphere in meteors, whose flux overthe whole Earth amounts to 44 metric tons per day. In the ionized state, metals


such as sodium, calcium, iron, and magnesium are significant to the aeronomy ofthe lower ionosphere in various ways, but they will not be of great concern to usat high latitudes.

1.3 Physical aeronomy

1.3.1 Introduction

The topic of physical aeronomy covers the physical considerations governing theformation and shape of an ionospheric layer. The detailed photochemical pro-cesses which are involved in a particular case are generally considered under chem-ical aeronomy; however, we shall include such chemical details as we require inSection 1.4 as part of our description of the actual terrestrial ionosphere.

Typical vertical profiles of the ionosphere are shown in Figure 1.4. The iden-tification of the regions was much influenced by their signatures on ionograms (seeSection 4.2.1), which tend to emphasize inflections in the profile, and it is not nec-essarily the case that the various layers are separated by distinct minima. The mainregions are designated D, E, F1, and F2, with the following daytime characteris-tics:

D region, 60–90 km: electron density 108–1010 m3 (102–104 cm3);

E region, 105–160 km: electron density of several times 1011 m3

(105cm3);


Figure 1.4. Typical vertical profiles of electron density in the mid-latitude ionosphere: ——,sunspot maximum; and – – –, sunspot minimum. (After W. Swider, Wallchart AerospaceEnvironment, US Air Force Geophysics Laboratory.)

F1 region, 160–180 km: electron density of several times 1011 to about 1012

m3 (105–106 cm3);

F2 region, height of maximum variable around 300 km: electron densityup to several times 1012 m3 (106 cm3).

All these ionospheric regions are highly variable, and in particular there is gener-ally a large change between day and night. The D and F1 regions vanish at night,and the E region becomes much weaker. The F2 region, however, tends to persist,though at reduced intensity.

The ionosphere is formed by the ionization of atmospheric gases such as N2,O2, and O. At middle and low latitude the required energy comes from solar radi-ation in the extreme ultra-violet (EUV) and X-ray parts of the spectrum. Oncethey have been formed, the ions and electrons tend to recombine and to react withother gaseous species to produce other ions. Thus there is a dynamic equilibriumin which the net concentration of free electrons (which, following standard prac-tice, we call the electron density) depends on the relative speed of the productionand loss processes. In general terms the rate of change of electron density isexpressed by a continuity equation:

N/tqLdiv(Nv) (1.15)

where q is the production rate (per unit volume), L is the rate of loss by recombi-nation, and div(Nv) expresses the loss of electrons by movement, v being theirmean drift velocity.

If we consider a representative ionization and recombination reaction andneglect movements,

Xh Xe. (1.16)

The “law of mass action” tells us that, at equilibrium,

[X][h]constant [X][e], (1.17)

where the square brackets signify concentrations. Thus, since [e][X] for electri-cal neutrality,

[e]2constant[X][h]/[X] (1.18)

During the day the intensity of ionizing radiation varies with the elevation of theSun, and the electron density responds to the variation of [h]. At night the sourceof radiation is removed and so the electron density decays. From this simple modelwe can also see that the electron density must vary with altitude. The intensity ofionizing radiation increases with height but the concentration of ionizable gas [X]


decreases. It is reasonable to expect from this that the electron density will passthrough a maximum at some altitude.

1.3.2 The Chapman production function

In 1931, S. Chapman developed a formula that predicts the form of a simple iono-spheric layer and how it varies during the day. Although it is only partly success-ful in explaining the observed behavior of the terrestrial ionosphere – and thisbecause of phenomena that it does not include – Chapman’s formula is at the rootof our modern understanding of the ionosphere and therefore it deserves a briefmention in this section.

At this stage we deal only with the rate of production of ionization (q), and theformula expressing this is the Chapman production function. In the simple treat-ment, which is sufficient for our purposes, it is assumed that

the atmosphere is composed of a single species, exponentially distributedwith constant scale height;

the atmosphere is plane stratified: there are no variations in the horizontalplane;

radiation is absorbed in proportion to the concentration of gas particles;and

the absorption coefficient is constant: this is equivalent to assuming thatwe have monochromatic radiation.

The rate of production of ion–electron pairs at some level of the atmospherecan be expressed as the product of four terms:

qnI. (1.19)

Here, I is the intensity of ionizing radiation and n is the concentration of atoms ormolecules capable of being ionized by that radiation. For an atom or molecule tobe ionized it must first absorb radiation, and the amount absorbed is expressed bythe absorption crossection, : if the flux of incident radiation is I (J m2 s1) thenthe total energy absorbed per unit volume of the atmosphere per unit time is nI.However, not all this energy will go into the ionization process, and the ionizationefficiency,, takes that into account, being the fraction of the absorbed radiationthat goes into producing ionization.

The Chapman production function is usually written in a normalized form as

qqm0exp(1zsec ez). (1.20)

Here, z is the reduced height for the neutral gas, z(hhm0)/H, H being the scaleheight. is the solar zenith angle, hm0 is the height of the maximum rate of pro-duction when the Sun is overhead (i.e. hm when 0), and qm0 is the production


rate at this altitude, also when the Sun is overhead. Derivations of equation (1.20)are given in many of the standard textbooks (see the list of further reading).Equation (1.20) can also be written

q/qm0eeze[sec .exp(z)], (1.21)

where the first term is a constant, the second expresses the height variation of thedensity of ionizable atoms, and the third is proportional to the intensity of the ion-izing radiation.

Figure 1.5 illustrates some general properties of the production-rate profile. Ata great height, where z is large and positive,

q→qm0eez. (1.22)

Thus the curves merge above the peak, becoming independent of and exhibit-ing an exponential decrease with height due to the decreasing density of the


Figure 1.5. The Chapman production function. (After T. E. VanZandt and R. W. Knecht,in Space Physics (eds. LeGalley and Rosen). Wiley, 1964.)

neutral atmosphere. In the region well below the peak, when z is large and nega-tive, the shape becomes dominated by the last term of Equation (1.21), produc-ing a rapid cut-off. Thus, as predicted in the previous section, the production rateis limited by a shortage of ionizable gas at the greater altitudes and by a lack ofionizing radiation low down. On a plot of ln(q) against z all the curves are thesame shape, but they are displaced upwards and to the left as the zenith angle, ,increases.

The intensity of radiation in an absorbing atmosphere may be written as

IIinfe (1.23)

where is the optical depth, which is equal to the absorption coefficient times thenumber of absorbing atoms down to the level considered:

NT; (1.24)

and Iinf is the intensity at great height. This leads to an important theorem:

The production rate is greatest at the level where the optical depth is unity.

From this general result there follow some particularly useful rules.

(1). The maximum production rate at a given value of is given by

qmIinf /(eHsec ). (1.25)

(2). The reduced height of the maximum depends on the solar zenith angle as

zmln(sec ). (1.26)

(3). The rate of production at this maximum is

qmqm0cos . (1.27)

These simple results are important in studies of the ionosphere because themaximum of a layer is the part most readily observed. From Equations (1.26) and(1.27) we see that a plot of ln(qm) against zm is effectively a plot of ln(cos ) againstln(sec ), which obviously gives a straight line of slope 1. This line is shown inFigure 1.5.

The Chapman production function is important because it expresses funda-mentals of ionospheric formation and of the absorption of radiation in any expo-nential atmosphere. Although real ionospheres may be more complicated, theChapman theory provides an invaluable reference point for interpreting observa-tions and a relatively simple starting point for ionospheric theory.


1.3.3 Principles of chemical recombination

Working out the rate of electron production is just the first step in calculating theelectron density in an ionized layer, and the next step is to reckon the rates at whichelectrons are removed from the volume under consideration. This is representedin the continuity equation (1.15) by two further terms, one for the recombinationof ions and electrons to reform neutral particles, and the other to account formovement of plasma into or out of the volume. We deal first with the principlesof chemical recombination. The question of which individual reactions are mostimportant in different parts of the ionosphere will be addressed in Section 1.4.

First we assume that the electrons recombine directly with positive ions andthat no negative ions are present: Xe→X. Then the rate of electron loss is

L[X]NeN e2 (1.28)

where Ne is the electron density (equal to the ion density [X]) and is the recom-bination coefficient. At equilibrium, therefore,

qN e2. (1.29)

The equilibrium electron density is proportional to the square root of the produc-tion rate, which may be replaced by the Chapman production function (1.20) toget the variation of electron density with height and solar zenith angle. In partic-ular, it is seen that the electron density at the peak of the layer varies as cos1/2 :

NmNm0cos1/2 . (1.30)

A layer with these properties is called an -Chapman layer.If one is concerned particularly with electron loss, then attachment to neutral

particles to form negative ions can itself be regarded as another type of electron-loss process. In fact, as we shall see, this becomes the dominant type at somewhathigher levels of the ionosphere (though by a different process). Without at thisstage specifying chemical details, we can see that the attachment type of reactioncan be written Me→M, and the rate of electron loss is LN, where is theattachment coefficient. The loss rate is now linear with N because the neutralspecies M is assumed to be by far the more numerous, in which case removing afew of them has no significant effect on their total number and [M] is effectivelyconstant.

At equilibrium,

qNe (1.31)


and taking q from the Chapman production function as before shows that thepeak electron density now varies as

NmNm0cos . (1.32)

Such a layer is a -Chapman layer.This simple formulation assumes that does not vary with height, though this

restriction does not affect the validity of Equation (1.31) at a given height.In fact is expected to vary with height because it depends on the concentra-

tion of the neutral molecules (M), and this has important consequences for theform of the terrestrial ionosphere. It is known that electron loss in the F regionoccurs in a two-stage process:

XA2→AXA (1.33)

AXe→AX (1.34)

in which A2 is one of the common molecular species such as O2 and N2. The firststep moves the positive charge from X to AX, and the second one dissociates themolecular ion through recombination with an electron, a dissociative-recombinationreaction. The rate of Equation (1.33) is [X] and that of (1.34) is [AX]Ne. Atlow altitude is large, (1.33) goes quickly and all X is rapidly converted to AX;the overall rate is then governed by the rate of (1.34), giving an -type processbecause [AX]Ne for neutrality. At a high altitude is small, and (1.33) is slowand controls the overall rate. Then [X]Ne and the overall process appears to beof -type. As height increases, the reaction type therefore alters from -type to -type. The reaction scheme represented by Equations (1.33) and (1.34) leads to equi-librium given by

, (1.35)

where q is the production rate as before. The change from - to -type behaviouroccurs at height ht where

(ht)Ne. (1.36)

In the lower ionosphere there are also significant numbers of negative ions.Electrical neutrality then requires NeN

N

, where Ne, N

and N

are, respec-tively, the concentrations of electrons, negative ions, and positive ions. Since thenegative and positive ions may also recombine with each other, the overall balancebetween production and loss is now expressed by

qeNeNiN

N

, (1.37)

1q

1

(h)Ne

1Ne

2


e and i being recombination coefficients for the reactions of positive ions withelectrons and negative ions, respectively. The ratio between negative-ion and elec-tron concentrations is traditionally represented by – which has nothing to dowith wavelength! In terms of , N

Ne and N

(1)Ne, and thus

q(1)(ei)N e2, (1.38)

which, in cases for which ie, becomes

q(1)eN e2. (1.39)

In the presence of negative ions the equilibrium electron density is still propor-tional to the square root of the production rate but its magnitude is changed. Theterm

(1)(ei)

is often called the effective recombination coefficient. As we shall see in Section1.4.3, the chemistry of the D region is complicated because of the presence ofmany kinds of positive and negative ions.

1.3.4 Vertical transport

Diffusion

The final term of the continuity equation (1.15) represents changes of electron andion density at a given location due to bulk movement of the plasma. Such move-ments can have various causes and can occur in the horizontal and the verticalplanes in general, but since our present emphasis is on the overall vertical struc-ture of the ionosphere, we shall concentrate here on the vertical movement of ion-ization, which, indeed, is very important in the F region. We assume now thatphotochemical production and loss are negligible in comparison with the effect ofmovements, and then the continuity equation becomes

, (1.40)

where w is the vertical drift speed and h is the height.We now suppose that this drift is entirely due to diffusion of the gas, and then

we can put

w , (1.41)

D being the diffusion coefficient. This equation simply states that the bulk drift ofa gas is proportional to its pressure gradient, and it effectively defines the diffu-

DN

Nh

dNdt

(wN )

h


sion coefficient whose dimensions are (length)2/time. From kinetic theory (equat-ing the driving force due to the pressure gradient to the drag force due to colli-sions as a minority gas diffuses through a stationary majority gas) the diffusioncoefficient may be derived in its simplest form as DkT/(m). Here k isBoltzmann’s constant, T the temperature, m the particle mass and the collisionfrequency.

In the present case the minority gas is the plasma composed of ions and elec-trons, and the majority gas is the neutral air. However, for drift in the verticaldirection the force of gravity also acts on each particle, adding to (or subtractingfrom) the drag force, and in this case we obtain

w(D/N )(dN/dhN/HN) (1.42)

for the upward speed instead of (1.41). Substitution into the continuity equationthen gives

. (1.43)

This is the basic equation that has to be satisfied by the time and height variationsof those regions (specifically the upper F region and the protonosphere) where ionproduction and recombination are both sufficiently small.

In this equation the scale height HN merely represents the value of kT/(mg), anddoes not necessarily describe the actual height distribution. This is given by thedistribution height, defined as

. (1.44)

Using Equations (1.43) and (1.44) we can easily see that is equal to the scaleheight at equilibrium.

A complication is introduced by the fact that a plasma is composed of twominority species, ions and electrons, which have opposite charges and very differ-ent masses. Initially the ions, being heavier, tend to settle away from the electrons,but the resulting separation of electric charge produces an electric field, E, and arestoring force eE on each charged particle. This electrostatic force also affects thedrift of the plasma. This problem is handled by writing separate equations foreach species and including the electrostatic force on each. We assume

(1). that the electron mass is small compared with the ion mass;

(2). that ion and electron number densities are equal; and

(3). that both species drift at the same speed;

and then it can be shown that Equations (1.42) and (1.43) are still valid for aplasma if one replaces D and H by

Dpk(TeTi)/(mii) (1.45)

1N

dNdh

1

dNdt

h DdN

dh

NHN


andHpk(TeTi)/(mig), (1.46)

respectively known as the ambipolar or plasma diffusion coefficient and the plasmascale height.

In that part of the ionosphere where plasma diffusion is important, the electrontemperature usually exceeds the ion temperature. However, taking TeTi by wayof illustration, we see that the plasma diffusion coefficient and scale height arethen just double those of the neutral gas at the same temperature. Effectively, thelight electrons have the effect of halving the ion mass since the two species cannotseparate very far. At equilibrium dN/dhN/Hp and the plasma is exponentiallydistributed as

N/N0exp(h/Hp) (1.47)

with scale height Hp. Note that this distribution has the same form as the upperpart of a Chapman layer but with (about) twice the scale height.

If the plasma is not in equilibrium the distribution changes with time at a ratedepending on the value of the diffusion coefficient, which, since it depends on therelevant collision frequency, increases with altitude. If H is the scale height of theneutral gas, then the height variation of the diffusion coefficient can be written as

DD0exp(hh0)/H (1.48)

where D0 is the value of D at a height h0. Thus, diffusion becomes ever more impor-tant at greater heights as the photochemistry becomes less important.

Another consequence of the height variation of D is that it leads to asecond solution of Equation (1.43) for the case dN/dt0. SubstitutingDD0exp(hh0)/H and NN0exp(hh0)/ into (1.43) and rearranging, gives

DN . (1.49)

If dN/dt0 this has two solutions. The first, Hp, is diffusive equilibrium as hasalready been pointed out, and in this case the vertical drift speed (Equation (1.41))is wD(1/1/Hp)0.

The second solution is H (H being the scale height of the neutral gas, gov-erning the diffusion coefficient). Here, dN/dt0 as before, but the drift speed is

wD D , (1.50)

which is not zero since HpH. The upward flow of plasma

NwND(1/H1/Hp), (1.51)

1H

1

Hp 1

1Hp

1

1

Hp 1

1HdN

dt


and in fact this is independent of height when H because the height variationsof D and of N cancel out. Thus, this second solution represents an unchangingdistribution of electron density and a constant outflow of plasma.

The effect of a neutral-air wind

Since the flow of ionospheric plasma is constrained by the geomagnetic field, theexact effect varies with latitude. One consequence is that, at middle latitudes, theheight distribution of ionization is affected by the neutral-air wind which flows inthe thermosphere. Suppose that the wind speed in the magnetic meridian is U andthe magnetic dip angle is I. Then the component of the neutral wind along thedirection of the magnetic field is U||UcosI, and the plasma tends to move in thesame way. This motion, along the magnetic field, has a vertical component

WU|| sinI . Usin(2I). (1.52)

Thus, a horizontal wind in the thermosphere tends to move the ionosphere up ordown depending on its direction of flow. The effect is greatest where the magneticdip angle is 45°. The consequences both for the height and for the magnitude ofthe peak of the F region can be significant (Section 1.4.5).

1.4 The main ionospheric layers

1.4.1 Introduction

The physical principles which govern the intensity and form of an ionospheric layerwere outlined in Section 1.3. To work out what the actual ionosphere should be likeon Earth or any other planet, we would have to consider the terms in Equation(1.19) (qnI ) in detail to get the ion production rate, specify the ion chemistryto obtain values for the loss coefficients in Equations (1.29) and (1.31) (qN e

2 andqNe), and, at the higher levels, consider the diffusion coefficient (Equation(1.46)) and take movements into account. We should then require to know aboutthe neutral atmosphere: its composition and physical parameters such as densityand temperature. Then we should need full information on the solar spectrum andany fluxes of energetic particles able to ionize the constituents of the atmosphere.

Knowing which gases could be ionized by the incident radiation, we could thendetermine the ionization rate of each species and sum over all wavelengths and allgases to get the total production rate in a given volume (q). If the loss processesindicated rapid attainment of equilibrium, the electron density (Ne) would begiven by Equation (1.29) or (1.31). Otherwise a more complex computation wouldbe required. (Mathematical modeling of the high-latitude ionosphere is discussedin Section 9.2.2.) There is no need to go into all these details here, but a few impor-tant points will be made.

12


Table 1.1 lists the ionization potentials of various atmospheric gases. To beionized a species must absorb a quantum of radiation whose energy exceeds theionization potential. Since the energy of a quantum of wavelength is Ehc/,there is a maximum wavelength of radiation that is able to ionize any particulargas. These values are included in Table 1.1. For easy reference the wavelengths aregiven both in ångström units and in nanometers.

These values of max immediately identify the relevant parts of the solar spec-trum as the X-ray (0.1–17 nm, 1–170 Å) and EUV, (17–175 nm, 170–1750 Å),emissions which come from the solar chromosphere and corona.

The value of the absorption cross-section, , generally increases with increas-ing wavelength up to max and then falls rapidly to zero. There is no ionization atall by any radiation with wavelength exceeding max, regardless of its intensity.

The ionization efficiency, , is such that, for atomic species, all the absorbedenergy goes into ion production at the rate of one ion–electron pair for every 34eV of energy. The energy is inversely proportional to the wavelength, and a con-venient formula in terms of wavelength is

360/ (Å). (1.53)

The Chapman theory (Section 1.3.2) shows that the production rate is amaximum at the level where the optical depth, nHsec , is unity. If the absorp-tion at a given wavelength is due to several species, then the condition formaximum production is

iniHi sec 1.i


Table 1.1. Ionization potentials

Maximum wavelength max

Species Ionization potential I (eV) (Å) (nm)

NO 9.25 1340 134.0O2 12.08 1027 102.7H2O 12.60 985 98.5O3 12.80 970 97.0H 13.59 912 91.2O 13.61 911 91.1CO2 13.79 899 89.9N 14.54 853 85.3H2 15.41 804 80.4N2 15.58 796 79.6A 15.75 787 78.7Ne 21.56 575 57.5He 24.58 504 50.4

The height of unit optical depth in a model terrestrial atmosphere is given as afunction of wavelength in Figure 1.6 and this, not the intensity of the ionizing radi-ation, is what determines the height of the ionospheric layers. This is an impor-tant point. It means, simply, that strongly absorbed radiation produces ionizationhigh up, and that low-level ionization must be due to radiation that is more weaklyabsorbed in the atmosphere.

The simple theory of Section 1.3.2 deals with the shape and intensity of an ion-osphere produced by monochromatic radiation acting on a single gas. On a realplanet the effect of all gases at a given wavelength has to be considered and then,since the ionosphere is in effect a number of overlapping Chapman layers, the pro-duction rate due to all relevant wavelengths has to be summed at each height. Thewavelength ranges giving the D, E, and F regions are summarized in Figure 1.6.


Figure 1.6. The height at which the optical depth reaches unity for radiation vertically inci-dent on the atmosphere. Ionization limits for common gases are marked. (J. D. Mathews,private communication.) The ranges responsible for the major ionospheric layers are indi-cated below.

200

150

100

50

F1

E

D

Hei

ght (

km)

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Wavelength (Å)

He N2 O, H O2 NO

Lyman α

1.4.2 The E and F1 regions

Aeronomy

The E region which peaks at 105–110 km, and the F1 region at 160–180 km, areboth fairly well understood. The F1 region is attributed to that part of the solarspectrum between about 200 and 900 Å, which is strongly absorbed in atomicoxygen, whose ionization limit is at 911 Å. The optical depth reaches unity fromabout 140 to 170 km. The band includes an intense solar emission line at 304 Å.The primary reaction products are O2

, N2, O, He, and N, but subsequent

reactions leave NO and O2 as the most abundant positive ions.

The E region is formed by the less strongly absorbed, and therefore more pen-etrating, parts of the spectrum. EUV radiation between 800 and 1027 Å (the ion-ization limit of O2) is absorbed by molecular oxygen to form O2

. The bandincludes several important emission lines. At the short-wavelength end X-rays of10–100 Å (1–10 nm) ionize all the atmospheric constituents. The main primaryions are N2

, O2, and O, but the most numerous are again observed to be NO

and O2. The intensity of solar X-rays varies over the solar cycle and they prob-

ably make little contribution to the E region at solar minimum.Direct radiative recombination of the type

eX→Xh (1.54)

is slow relative to other reactions and is not significant in the normal E and Fregions. Dissociative recombination, as

eXY→XY, (1.55)

is 105 times faster (with a reaction coefficient of 1013 m3 s1) and, both in the Eregion and in the F region, the electron and ion loss proceeds via molecular ions.The main recombination reactions of the E region are therefore

eO2→OO,

eN2→NN,

eNO→NO. (1.56)

In the F region the principal primary ion is O, which is first converted to amolecular ion by a charge-exchange reaction

OO2→O2O

orON2→NON. (1.57)


The molecular ion then reacts with an electron as in Equation (1.56), to give asthe net result

eOO2→OOOor

eON2→ONN. (1.58)

In the F1 region the overall reaction is controlled by the rate of the dissociativerecombination.

Observations show that both the E and the F1 layers behave like, or almost like,-Chapman layers (Equation (1.30)). On average the critical frequency, fOE orf0F1 (Section 3.4.2), varies with the solar zenith angle, , as (cos )1/4, which meansthat the peak electron density, Nm, varies as (cos )1/2. The exponent is subject tosome variation and ranges between about 0.1 and 0.4 for the E region.

Given that the E region is an -Chapman layer, the Chapman theory can beapplied to determine the recombination coefficient () from observations, and thismay be done using Equation (1.29):

(1). taking an observed electron density and an observed or computed produc-tion rate;

(2). by observing the rate of decay of the layer after sunset and assuming thatq0; or

(3). by measuring the asymmetry of the diurnal variation about local noon, aneffect sometimes called the sluggishness of the ionosphere, the time delaybeing given by

1/(2N). (1.59)

Such methods give values of in the range 1013–1014 m3 s1 (107–108 cm3 s1).

The night E layer

The E layer does not quite vanish at night, but a weakly ionized layer remains withelectron density about 5109 m3 (against 1011 m3 by day). One possible causeis meteoric ionization, though other weak sources might also contribute. Figure1.7 shows speciman electron-density profiles of the E region for day and night,measured by incoherent-scatter radar.

Sporadic-E

The most remarkable anomaly of the E region is sporadic-E, often abbreviated toEs. On ionograms sporadic-E is seen as an echo at constant height that extends toa higher frequency than is usual for the E layer; for example to above 5 MHz.Rocket measurements, and more recently incoherent-scatter radar, show that, atmid-latitude, these layers are very thin, perhaps less than a kilometer across.Examples are shown in Figure 1.8.


Figure 1.9 indicates the probability of occurrence of sporadic-E against time ofday and season in three latitude zones:

the equatorial zone, within 20° of the magnetic equator;

the high-latitude zone, poleward of about 60° geomagnetic;

and the temperate zone in between.

The high-latitude zone may be sub-divided into the auroral zone (approximately60°–70° magnetic) and the polar cap (poleward of the auroral zone). A full clas-sification of sporadic-E, particularly regarding its identification on ionograms, isgiven by Piggott and Rawer (1972). In general, sporadic-E exhibits little directrelationship with the incidence of solar ionizing radiation.

Sporadic-E tends to be particularly severe at low latitude. It occurs frequentlyduring the daytime hours, often with sufficient intensity to reflect radio waves upto 10 MHz. A major cause is the occurrence of instabilities in the equatorialelectrojet (Section 1.5.5).

The principal cause of sporadic-E at middle latitude is a variation of windspeed with height, a wind shear, which, in the presence of the geomagnetic field,


Figure 1.7. Speciman electron-density profiles of the E region for night and day, measuredby the incoherent-scatter radar at Arecibo, Puerto Rico (18° N, 67° W), in January 1981. (J.D. Mathews, private communication.)

acts to compress the ionization by a mechanism similar to that which allows theneutral-air wind in the thermosphere to raise or lower the F region (Section 1.3.4).The time scale of the process needs ions of relatively long life, and it is thoughtthat these are metallic ions of meteoric origin such as Fe, Mg, Ca, and Si.Being atomic, these cannot recombine dissociatively and therefore their recombi-nation coefficients are typical of the radiative process (1018 m3 s1), which givesthem relatively long lifetimes. Temperate sporadic-E occurs at heights of 95–135km, and the most probable height is 110 km. It occurs most frequently in summerdaytime, with maxima in mid-morning and near sunset. The seasonal variation iscomplex. Its character changes abruptly at about 60° magnetic latitude, the boun-dary of auroral Es.

The sporadic-E which occurs at high latitude is attributed to ionization byincoming energetic particles in the energy range 1–10 keV. It is mainly a night-timephenomenon, correlating to magnetic activity (Section 2.5.3), but not to sunspotactivity as such. Clouds of auroral Es drift at speeds between 200 and 3000 m s1,westward in the evening and eastward in the early morning, much like the aurora.The layer may be either “thick” or “thin”. Within the polar caps sporadic-E has


Figure 1.8. Some sporadic-E layers observed at Arecibo by incoherent-scatter radar,January 1981. (J. D. Mathews, private communication.)

a different character. It is weaker, and exhibits a negative correlation to magneticactivity. It takes the form of bands or ribbons extending across the polar cap in aroughly sunward direction. The properties and causes of sporadic-E have beenreviewed in detail by Whitehead (1970). The high-latitude E region is discussedfurther in Section 6.5.

Sporadic-E is significant in radio propagation because it may reflect signalsthat would otherwise penetrate to the F region, though in some cases (for example


Figure 1.9. Diurnal and seasonal occurrence patterns for three kinds of sporadic-E. (a) Theauroral kind maximizes at night but exhibits no seasonal variation. (b) The temperate kindpeaks near noon in summer. (c) The equatorial kind occurs mainly by day but has no sea-sonal preference. (After E. K. Smith, NBS Circular 582, US National Bureau of Standards,1957.)

the equatorial type) it is partly transparent. The irregularities within a sporadic-E layer can scatter radio waves if their dimensions are comparable to half a radiowavelength, and at times they may cause scintillation of trans-ionospheric signals,though F-layer irregularities are the more usual cause of this phenomenon.

The F1 ledge

The strange thing about the F1 region is that it does not always appear! In fact,real-height profiles show that it seldom exists as a distinct peak and for this reasonit is more correctly called the F1 ledge. The ledge is more pronounced in summerand at sunspot minimum, and it is never seen in winter at sunspot maximum. Theexplanation is to be found by comparing ht, the height at which transition between-type and -type recombination occurs, as discussed in Section 1.3.3, and hm, theheight of maximum electron-production rate. The F1 ledge appears only if hthm,and, since ht depends on the electron density (Equation (1.36)), the ledge vanisheswhen the electron density is greatest.

1.4.3 The D region

Aeronomy

The D region of the ionosphere does not include a maximumum but is that partbelow about 95 km which is not accounted for by the processes of the E region. Itis also the most complex part of the ionosphere from the chemical point of view.This is due, first, to the relatively high pressure, which causes minor as well asmajor species to be important in the photochemical reactions, and, second,because several different sources contribute to ion production.

The Lyman- line of the solar spectrum at 1215 Å penetrates below 95 km andionizes the minor species nitric oxide (NO), whose ionization limit is at 1340 Å.This is the main source at middle latitudes, though not necessarily at all heights.There is a smaller contribution from the EUV spectrum between 1027 and 1118Å, which ionizes another minor constituent, molecular oxygen in an excited state.At the higher levels ionization of O2 and N2 by EUV, as in the E region, makes acontribution. Hard X-rays of 2–8 Å ionize all constituents, the most effect beingtherefore from the major species O2 and N2. Since the intensity of the solar X-rayemissions varies considerably from time to time, this source is sometimes a majorone but at other times only minor. The lowest levels are dominated by cosmic-rayionization, which continues by night as well as by day and affects the whole atmos-phere down to the ground. The production rate due to cosmic rays increasesdownward in proportion to the total air density, and, since the production fromother sources is falling off, it is inevitable that the cosmic rays must come to dom-inate at some level. At high latitudes particles from the Sun or of auroral originionize the D region and at times they form the main source. We shall be particu-larly concerned with those sources and their effects later in the book.


Clearly, the relative contributions of these different sources vary with latitude,time of day, and level of solar activity. By way of example, theoretical profiles ofthe production rate (for solar zenith angle 42° and a 10-cm solar flux of 165 units)are given in Figure 1.10. Note that all the sources mentioned above are significantand that their relative importance depends on the altitude. At greater solar zenithangles the contributions from Lyman- and X-rays are reduced, and the cosmicrays become relatively more important below 70 km. The X-ray flux variesstrongly with solar activity (by a factor of a hundred to a thousand) and is prob-ably not significant in the D region at sunspot minimum.

These production-rate profiles are consistent with measurements of D-regionelectron densities (Figure 1.11). Friedrich and Torkar (1992) analyzed 164 elec-tron-density profiles of the D region measured by rocket-based wave-propagationtechniques (as in Section 4.3.4), to derive an empirical model covering a range ofsolar zenith angles. Figure 1.12 shows a set of profiles corresponding to a sunspotnumber of 60.

Following ionization, the primary ions in the D region are NO, O2, and N2

,but the latter are rapidly converted to O2

by the charge-exchange reaction

N2O2→O2

N2, (1.60)

leaving NO and O2 as the major ions. However, below 80 or 85 km, apparently

the level of the mesopause, are detected heavier ions that are hydrated species suchas H.H2O, H3O

.H2O, and hydrates of NO. These hydrates occur when theconcentration of water vapor exceeds about 1015 m3. The level at which hydra-tion first occurs is a natural boundary within the D region.


Figure 1.10. Calculated production rates at 42° due to extreme ultra-violet (EUV),Lyman- and nitric oxide (NO), X-rays (X), excited oxygen (O*

2), and galactic cosmic rays(GCR). (J. D. Mathews, private communication.)

Where simple ions dominate, the loss process is dissociative recombination asin the E region, with a recombination coefficient of about 51013 m3 s1, thereaction of NO being somewhat faster than that of O2

. In total the situation ismuch more complex, as illustrated in Figure 1.13. This scheme includes O2

, NO,O4

, hydrates and others, and has to be solved by means of a computer program.The hydrated ions, being larger molecules, have greater recombination rates thando the simple ions, of the order of 1012–1011 m3 s1, depending on their size.Thus the equilibrium electron density is relatively smaller in regions wherehydrates dominate.


Figure 1.11. Electron-density profiles observed at Arecibo for two solar zenith angles. (J. D.Mathews, private communication.)

120

100

80

60

1018

1019

1020

1021

1022

107 108 109 1010 1011 1012

ELECTRON DENSITY, m –3

NE

UT

RA

L D

EN

SIT

Y, m

–3

ALT

ITU

DE

, km

160°

90°

80°

70°

60°

40°

20°

Figure 1.12. Electron-density profiles in the Dregion derived fromrocket measurements fora range of solar zenithangles. The numberdensity of the neutral airis also shown. (M.Friedrich and K. M.Torkar, Radio Sci. 27,945, 1992. Copyright bythe AmericanGeophysical Union.)

Fig

ure

1.1

3.

A s

chem

e of

pos

itiv

e-io

n ch

emis

try

for

the

D r

egio

n. (

E. T

urun

en, p

riva

te c

omm

unic

atio

n.)

Thi

s m

odel

,de

velo

ped

at S

odan

kylä

Geo

phys

ical

Obs

erva

tory

, Fin

land

, inc

lude

s 24

pos

itiv

e an

d 11

neg

ativ

e io

ns, 3

5 in

all.

Lat

er v

er-

sion

s in

clud

e as

man

y as

55

ions

.

Below about 70 km by day or 80 km by night much of the negative charge is inthe form of negative ions. Their creation begins with the attachment of an elec-tron to an oxygen molecule, forming O2

. This is a three-body reaction involvingany other molecule, M, whose function is to remove excess kinetic energy from thereactants:

eO2M→O2M. (1.61)

This is followed by further reactions forming other and more complex negativeions such as CO3

, NO2, and NO3

(the most abundant negative ion in the Dregion) and clusters such as O2

.O2, O2.CO2, and O2

.H2O. Because the electronaffinity of O2 is small (0.45 eV), the electron may be removed by a photon of visibleor near infra-red light:

O2h→O2e. (1.62)

It may also be detatched through chemical reactions, such as with atomic oxygen(forming ozone), and with excited molecular oxygen. The effect of negative ionson the balance between electron production and loss was included in Equations(1.37)–(1.39). Variations of electron density in the D region can be due to changesin the negative-ion/electron ratio, , as well as to changes in production rate.

The complexity and uncertainty of D-region photochemistry is one reasonwhy, when one is relating electron-production rates to electron densities, it is usualto work with an “effective recombination coefficient” (Equation (1.38)), whichmay be either theoretically or experimentally determined.

Diurnal behavior

Although the mid-latitude D region is complex chemically, observationally itsbehavior may be deceptively simple. The region is under strong solar control andit vanishes at night. VLF ( f30 kHz) radio waves are, to a first approximation,reflected as at a sharp boundary in the D region because the refractive indexchanges markedly within one wavelength (Section 3.4.6). For VLF waves incidenton the ionosphere at steep incidence, the reflection height, h, appears to vary as

hh0H ln(sec ), (1.63)

where is the solar zenith angle. h0 is about 72 km, and H is about 5 km, whichhappens to be the scale height of the neutral gas in the mesosphere. This form ofheight variation is just what is predicted for a level of constant electron density inthe underside of a Chapman layer, and it is consistent with the ionization of NOby solar Lyman- radiation.

At oblique incidence, when the transmitter and the receiver are more thanabout 300 km apart, the height variation follows a quite different pattern. The


reflection level now falls sharply before ground sunrise, remains almost constantduring the day, and then recovers fairly rapidly following ground sunset. Thereason has to do with the formation and detachment of negative ions at sunset andsunrise, coupled with electron production by cosmic-ray ionization – a source withno diurnal variation. This lower part of the D region is sometimes called a C layer.These patterns of height variation are illustrated in Figure 1.14.

Radio absorption

The D region is the principal seat of radio absorption, and absorption measure-ments (Section 4.2.4) are one way of monitoring the region. The absorption perunit height depends both on the electron density and on the frequency of colli-sions between electrons and neutral particles, and the measurement gives the inte-grated absorption up to the reflection level. Multi-frequency absorptionmeasurements can provide some information about the height distribution.

Generally, the absorption varies with the solar zenith angle as (cos )n with n inthe range 0.7–1.0. However, the seasonal variation contains an intriguinganomaly, which is that, during the winter months, the absorption exceeds by afactor of two or three the amount that would be expected by extrapolation fromsummer. Moreover, the absorption is much more variable from day to day in thewinter. This phenomenon is the winter anomaly of ionospheric radio absorption.


Figure 1.14. Two kinds of diurnal behavior of the D region inferred from VLF radio prop-agation at vertical and oblique incidence. The regions originally called D

and D

are now

more usually called D and C. The evening recovery at oblique incidence tends to be moregradual than that in a simple D

pattern and similar to the dashed curve. (After R. N.

Bracewell and W. C. Bain, J. Atmos. Terr. Phys., 2, 216, Copyright 1952, with permissionfrom Elsevier Science.)

1.4.4 The F2 region and the protonosphere

The peak of the F2 layer

Compared with the good behavior of the lower layers of the ionosphere, the F2region, on first aquaintance, can be quite puzzling. In the first place it peaks at200–400 km, whereas Figure 1.6 shows no band of radiation producing amaximum ionization rate at any height above 180 km. The answer is to be foundin the height variation of the recombination rate, which forms the F2 region as anupward extension of F1 even though the production rate is now decreasing withheight.

Taking O as the major ion, the two-stage recombination process is

ON2→NON with rate [O]

followed by

NOe→NO with rate [NO]Ne.

As discussed in Section 1.3.3, the second reaction controls the overall rate at lowaltitude and the first is the rate-determining step at high levels, the transition beingwhere Ne(ht). The transition height, ht, is generally between 160 and 200 km.The F1 ledge can appear if ht is above the height of the maximum production rate,hm: that is, if there is a production maximum within an -type region.

To explain the F2 region we consider the upper part where the recombinationis of type, and where depends on the concentration of N2. On the other hand,the production rate depends on the concentration of O.Thus, at equilibrium,

Neq/ [O]/[N2]

Neq/ exp

where H(O) and H(N2) are the scale heights for O and N2. Since the masses of N2

and O are in the ratio 1.75: 1, this rearranges to give

Ne exp

exp . (1.64)

This is a layer whose electron density increases with height because the loss ratefalls off more quickly than does the production rate. It is often called a Bradburylayer.

0.75hH(O)

hH(O)

1 H(O)H(N2)

hH(O)

h

H(N2)


The Bradbury layer explains why the electron density increases with heightabove the level of maximum ion production, but it does not explain why the F2layer has a maximum. Here we have to invoke plasma transport. At the higherlevels, in situ production and loss are less important than diffusion, which hasbecome more important because of the decreasing air density. (That is, the right-hand side of Equation (1.15) is now dominated by the third term.) The F2 layerpeaks where chemical recombination and diffusion are equally important. Todecide the level at which this will occur, we regard the two loss processes – -typerecombination and transport – as being in competition, and compare their timeconstants for electron loss on the principle that the more rapid will be in effectivecontrol.

The characteristic time for recombination is

1/, (1.65)

and it may be shown that the corresponding time for diffusion is approximately

DH 12/D, (1.66)

where H1 is a typical scale height for the F2 region. Comparing these two equa-tions places the F2 peak at the level where

D/H 12. (1.67)

The electron density at the peak is given by

Nmqm/m. (1.68)

The protonosphere

At some level in the topside the ionosphere dominated by O gives way to theprotonosphere dominated by H. It so happens that the ionization potentials forthese two ions are almost the same (Table 1.1), and therefore the reaction

HO HO (1.69)

goes rapidly in either direction, and, around the transition level, the equilibriumis given by

[H][O](9/8)[H][O]. (1.70)

(The factor 9/8 arises for statistical reasons, and there is also a temperature depen-dence proportional to (Tn/Ti)

1/2.) Through this reaction ionization can move


readily between the ionosphere (as O) and the protonosphere (H). This is a veryimportant aspect of the behavior of the topside ionosphere.

The transition effectively defines the base of the protonosphere. Below that levelthe H distribution is determined by (1.71), and is related to the distribution ofO by

[H] [H][O]/[O]

exp[h/H(H)]exp[h/H(O)]/exp[h/H(O)]

exp[7h/H(H)]. (1.71)

There is a strong upward gradient in the H concentration below the transitionlevel. Above the transition the concentration of O decreases rapidly, and in thisregion the protonosphere, when it is in equilibrium, takes an exponential profilewith the appropriate scale height (Equation (1.47)). As for the F2 peak, the tran-sition level between ionosphere and protonosphere can be estimated by compar-ing time constants. If the rate constant of the reaction

HO→HO

is k, then the lifetime of a proton is (k[O])1. Taking the time constant for diffu-sion in the protonosphere as H2

2/D, the boundary occurs where

k[O]D/H 22. (1.72)

This occurs at 700 km or higher, which is always well above the peak of the F2layer.

1.4.5 Anomalies of the F2 region

The phenomena

The F2 region has the greatest concentration of electrons of any layer, and there-fore it is the region of greatest interest in radio propagation. Unfortunately, it isalso the region which is the most variable, the most anomalous, and the most diffi-cult to predict. From the point of view of the Chapman theory the F2 region’sbehavior is anomalous in several ways, and these are sometimes called the classi-cal anomalies of the F2 layer. Briefly, they are as follows.

(a). The diurnal variation may be asymmetrical about noon. There may be arapid change at sunrise but little or no change in the evening until wellafter sunset or even until just before the next sunrise (Figure 1.15). Thedaily peak may occur either before or after local noon in the summer,


though it is likely to be near noon in the winter (Figure 1.16). On somedays a secondary minimum appears near noon between the morning andevening maxima (Figure 1.16(a)).

(b). The daily pattern of variation often does not repeat from day to day. (If itdid, the next day could at least be predicted from the previous one.) Figure1.15 illustrates this point.

(c). There are several anomalous features in the seasonal variation. The mainone is that noon values of the F-layer critical frequency (see Equation(3.67)) are usually greater in winter than they are in summer, whereas theChapman theory leads us to expect the opposite. This is the seasonalanomaly, which is clear in Figure 1.16. The summer electron content (thesummation of electron density in a column through the ionosphere) isgreater than the winter value at some stations, but at others it is smaller orabout the same. The electron content is abnormally large at the equinoxes,giving the semi-annual anomaly. Some stations also show this anomaly inthe F-region critical frequency (Figure 1.17).

(d). The mid-latitude F2 region does not vanish at night, but remains throughto the next sunrise at a substantial level.

Although not all anomalies have yet been fully explained, it now appears thatthere are four main causes for this seemingly anomalous behaviour:


Figure 1.15. The diurnal behavior of f0F2 on successive days in December 1959 at a low-latitude station, Talara, Peru. Note, by contrast, the regularity of the E layer. (T. E.VanZandt and R. W. Knecht, in Space Physics (eds. Le Galley and Rosen), Wiley, 1964.)


Figure 1.16. (a) The diurnal behavior of f0F2 in summer and winter at a high-latitudestation in the northern hemisphere, Adak, Alaska. The F region is anomalous whereas theE layer behaves as expected according to the Chapman theory. (T. E. VanZandt and R. W.Knecht, in Space Physics (eds. Le Galley and Rosen), Wiley, 1964.) (b) Summer and winterelectron contents measured at Fairbanks, Alaska. (R. D. Hunsucker and J. K. Hargreaves,private communication.)

Fig

ure

1.1

7.

Var

iati

ons

of c

riti

cal f

requ

enci

es o

ver

seve

ral s

unsp

ot c

ycle

s. T

he t

hree

top

pan

els

show

the

sun

spot

num

ber,

the

10.7

-cm

sol

ar r

adio

flux

, and

the

mag

nitu

de o

f th

e in

terp

lane

tary

mag

neti

c fie

ld. (

Dia

gram

pro

vide

dby

M. W

ild, R

uthe

rfor

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pple

ton

Lab

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ory,

Chi

lton

, UK

.) N

ote

also

the

sea

sona

l mod

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ions

at

Slou

gh a

ndPo

rt S

tanl

ey. T

he E

and

F1

regi

ons

peak

in t

he s

umm

er w

here

as F

2 pe

aks

in t

he w

inte

r. T

he s

emi-

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y is

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ent

at P

ort

Stan

ley.

(a). reaction rates are sensitive to temperature;

(b). the chemical composition varies;

(c). there are winds in the neutral air that lift or depress the layer by the mech-anism indicated in Section 1.3.4; and

(d). the ionosphere is influenced by the protonosphere and by conditions in theconjugate hemisphere.

Reaction rates

Reaction rates are generally temperature sensitive. The rate for the reaction

ON2→NON,

the first step in an important two-stage loss process (Equations (1.57) and (1.56)),varies strongly with the temperature of neutral N2 and increases by a factor of 16between 1000 and 4000 K. This property obviously contributes both to the per-sistence of the night F region and to the seasonal anomaly.

Composition

Since the electron-production rate depends on the concentration of atomicoxygen, O, whereas the loss rate is controlled by the molecular species N2 and O2,increases in the ratios [O]/[O2] and [O]/[N2] will increase the equilibrium electrondensity. Satellite measurements have shown that such variations do occur. Theratio [O]/[N2] at 250–300 km is measured as about 6 in winter and about 2 insummer, a seasonal change amounting to a factor of three. The change of compo-sition is attributed to the pattern of global circulation in the thermosphere. Thisis plainly a factor in the seasonal anomaly.

Winds

Mathematical modeling has demonstrated how the meridional component of thethermospheric neutral wind, acting to depress the ionosphere when the wind isflowing equatorward and elevating it when it is flowing poleward (Section 1.3.4),exerts a major influence both on electron densities and on electron content. At 300km the neutral wind flows poleward by day and equatorward by night at speedsranging between tens and hundreds of m s1. Thus its effect is usually to depressthe ionosphere and thereby increase the rate of loss by day, but to lift the regionand reduce its rate of decay at night. It is estimated (taking H60 km for theneutral scale height, D2106 m2 s1 for the diffusion coefficient, and W30 ms1 as a typical vertical drift due to the poleward wind), that by day the peak ofthe layer is lowered by about 50 km.

The variability of the F region from one day to the next (e.g. Figure 1.15) is oneof its most remarkable and puzzling features. This might not be surprising in thepolar regions because of the sporadic nature of solar and auroral activity, but


these are not dominant influences at middle latitudes. Presumably the origin mustbe a source in the terrestrial atmosphere or in the solar wind. Variations of theneutral-air wind in the thermosphere are one possible cause.

The plasma temperature and the protonosphere

Variations in the temperature of the plasma affect its vertical distribution. Theheating comes from the excess energy of absorbed photons above that needed forionization. The excess energy is initially in the electrons and it is gradually sharedwith the positive ions, though transfer to the neutral species is less efficient.Consequently the plasma is hotter than the neutral air, and within the plasma theelectrons are hotter than the ions (TeTi). The electron temperature can be twoor three times the ion temperature by day, though by night the electron and iontemperatures are more nearly equal. These changes in temperature strongly affectthe distribution of F2-region plasma. When it is hotter, the plasma has a greaterscale height (Equation (1.46)) and so spreads to greater altitudes, where it tendsto persist for longer because the loss rate is smaller.

At the greater altitudes the positive ions are protons, and, as discussed inSection 1.4.4, the ionosphere and the protonosphere are strongly coupled throughthe charge-exchange reaction between protons and atomic oxygen ions (Equation(1.69)). As the F region builds up and is also heated during the hours after sunrise,plasma moves to higher altitudes where protons are created. These then flow upalong the field lines to populate the protonosphere. In the evening the proton pop-ulation flows back to lower levels, where it undergoes charge exchange to giveoxygen ions and so helps to maintain the F region at night.

Via the protonosphere the magnetically conjugate ionosphere may also have aneffect, since protonospheric plasma, coming mainly from the summer ionosphere,is equally available to replenish the winter ionosphere. Computations show thatthis is a significant source. Indeed, it is useful to treat the mid-latitude plasma-sphere as consisting of winter and summer ionospheres linked by a commonprotonosphere; the ionospheres act as sources to the protonosphere, which in turnserves as a reservoir to the ionospheres. Overall, the winter ionosphere benefitsfrom the conjugate region in the summer hemisphere. At sunrise, when electrondensities are low, the ionosphere may be significantly heated by photoelectronsarriving from the conjugate hemisphere. The effect may show up as an increase ofslab thickness (the ratio of electron content to maximum electron density) justbefore local sunrise.

It appears likely that the various classical anomalies of the F2 region arise fromcombinations of the factors outlined above, though the details might not be clearin any particular case.

1.4.6 The effects of the sunspot cycle

The varying activity of the Sun over a period of about 11 years, measured in termsof the number of sunspots visible on the disk, the rate at which flares occur, or the


intensity of the 10-cm radio flux, also affects the ionosphere because of variationsin the intensity of the ionizing radiations in the X-ray and EUV bands. The tem-perature of the upper atmosphere also varies with solar activity, approximately bya factor of two between sunspot minimum and maximum. Consequently, the gasdensity at a given height varies by a large factor.

The maxima of the E, F1, and F2 layers all depend on the number of sunspots,R. This influence can be seen in Figure 1.17. (The critical frequencies plotted there,fOE, fOF1, and fOF2, are proportional to the square root of the maximum electrondensity, and are defined as the highest radio frequencies reflected from the layerat vertical incidence – see Section 3.4.2.) We have seen that the E and the F1 layersboth behave as -Chapman layers. In such a layer (Equation (1.30)) the criticalfrequency varies with the solar zenith angle as (cos )1/4. Taking the number ofsunspots into account as well gives two empirical relations:

fOE3.3[(10.008R)cos ]1/4 MHz (1.73)

fOF14.25[10.015R)cos ]1/4 MHz. (1.74)

Note that the F1 layer is nearly twice as sensitive as the E layer to variations in thesunspot number.

From the status of the E and F1 as -Chapman layers it follows that the ratios(fOE)4/cos and (fOF1)4/cos are proportional to the ionization rates (q) in the Eand F1 layers, respectively. These ratios are called character figures. Taking R10for a typical solar minimum and R150 for a maximum, we see from Equation(1.73) that the E-region production rate varies by a factor of two over a typicalsunspot cycle.

The F2 layer does not behave like a Chapman layer but it nevertheless varieswith the sunspot number. The dependence may be seen by plotting the noonvalues of fOF2, and if these are smoothed over 12 months to remove the seasonalanomalies, a dependence such as

fOF2 (10.02R)1/2 MHz (1.75)

can be recognized.One measure of the strength of the D region is the radio absorption measured,

for example, by the pulsed sounding technique (Section 4.2.4). Other parametersbeing constant, it is observed that the absorption increases by about 1% for eachunit of sunspot number:

A(dB) (10.01R). (1.76)

At mid-latitude the absorption is expected to vary over a sunspot cycle by abouta factor of two.


1.4.7 The F-region ionospheric storm

From time to time the ionosphere suffers major perturbations called storms. Theylast from a few hours to a few days and tend to occur during times of geophysicaldisturbance resulting from increases in solar activity communicated via the solarwind. There are, on the face of it, connections with magnetic storms (Section 2.5),though some different mechanisms must be involved. Three phases may be iden-tified.

(a). In the initial or positive phase, which lasts for a few hours, the electrondensity and the electron content are greater than normal.

(b). Then follows the main or negative phase when these quantities are reducedbelow normal values.

(c). Finally, the ionosphere gradually returns to normal over a period of one toseveral days in the recovery phase.

The magnitude of the effect varies with latitude, being greatest at middle andhigh latitude, where the maximum electron density may be depressed by 30% in astrong storm. At latitudes below about 30° the effect is not likely to exceed a fewpercent. The beginning can be sudden or gradual, the term sudden commencementbeing used (as for magnetic storms) to describe the former. At middle latitudesionosondes show the apparent height of the maximium, h(F2), to be increased,though real-height analysis attributes this mainly to greater group retardation(Section 3.4.2) below the peak rather than to a genuine lifting of the region. Theslab thickness (the ratio of the electron content to Nmax) does increase, however,confirming that the F region broadens during the negative phase. Figure 1.18 com-pares electron content, electron density, and slab thickness in a typical mid-latitude storm.

The progress of the storm since its time of commencement is the storm-timevariation, but the time of day is also a significant parameter. Statistical studies, aswell as case histories of major storms, show that the magnitude and even the signof the effect depend on the time of day. The negative phase tends to be weaker inthe afternoon and evening, stronger in the night and morning. The positive phaseis often missing altogether at stations that were in the night sector at commence-ment. It has been suggested (Hargreaves and Bagenal, 1977) that the positivephase co-rotates with the Earth on the first day of the storm and does not reap-pear on the second day.

Seasonal and hemispheric effects are also marked. The negative phase is rela-tively stronger, and the positive phase relatively weaker, in the summer hemi-sphere. This holds both for the northern and for the southern hemisphere, thoughthe interhemispheric difference is such that Nm(F2) is actually increased during themain phase of storms occurring in the southern hemisphere during winter. Theinterhemispheric difference arises from the larger separation between the geo-graphic and the geomagnetic poles in the south.


The most likely cause of the main phase is abnormal heating at high latitude,which also alters the pattern of circulation of the thermospheric wind. The heatingreduces the ratio [O]/[N2] at given height in the F region, and the molecularlyenriched air is then convected down to the middle latitudes by the changed air cir-culation. As was pointed out in Section 1.4.5, the effect of a greater proportion ofmolecular species in the F region is to reduce the equilibrium electron density. Thismechanism has been verified by computer modeling (Rishbeth, 1991), thoughsome problems remain to be solved. There appears to be no generally agreed causeof the initial phase, though various mechanisms have been suggested.


Figure 1.18. The electron content, electron density, and slab thickness at a mid-latitudestation during an F-region storm. SC marks the time of sudden commencement. The 7-daymean is shown to indicate normal behavior. (M. Mendillo and J. A. Klobuchar, ReportAFGRL-TR-74-0065, US Air Force, 1974.)

1.5 The electrical conductivity of the ionosphere

1.5.1 Introduction

The presence of free electrons and ions allows the ionospheric layers to carryelectric currents. The conductivities of the ionosphere lie in the range 105–102

1 m1, a broad middle range between insulators (such as the tropospere,1014 1 m1) and good conductors like metals (6 107 1 m1 forcopper), being akin to that of the ground (107–1 1 m1) or a semiconductor(101–102 1 m1).

Radio propagation is generally considered in terms of the electron density ofan ionospheric layer rather than its conductivity, and we shall not need to dealwith conductivities very much, at least for propagation in the MF, HF, or VHFbands. However, the electric currents of the ionosphere and magnetosphere are amajor factor in the behavior of the ionosphere and in the way it is affected by geo-physical disturbances. These are particularly important at the high latitudes. Thesolar–geophysical environment, of which the ionosphere is a part, cannot beunderstood without including the several current systems that may exist within it.Hence, we give in this section the basis of ionospheric conductivity.

1.5.2 Conductivity in the absence of a magnetic field

If no magnetic field is present, the formula for the conductivity of an ionized gasis a simple one:

0Ne2/(m), (1.77)

where N is the number density of particles each with charge e and mass m, and is the collision frequency for collisions of a charged particle with neutral species(which are assumed to be in the majority). The formula is easily proved, remem-bering that the mobility of a charged particle (its velocity in a unit electric field) ise/(m), and the total charge per unit volume is Ne.

If more than one species of charge is present, for example electrons and posi-tive ions, the total conductivity is the sum of the conductivities for each speciesseparately.

1.5.3 The effect of a magnetic field

Unfortunately the Earth’s magnetic field permeates the ionosphere, and this com-plicates the conductivity enormously. A charged particle moving through a mag-netic field experiences a force (the Lorentz force) that acts at right angles both to


the direction of the magnetic field and to the direction of motion of the particle.If the particle is moving directly along the magnetic field, the Lorentz force is zero;the magnetic field has no effect and Equation (1.77) applies.

However, if the motion has a component at right angles to the magnetic field,the corresponding conductivity has two parts:

1 e2 (1.78)

2 e2. (1.79)

The subscript e here refers to electrons and i refers to positive ions. is the rele-vent gyrofrequency (eB/m, where B is the magnetic flux density). 1 is the Pedersenconductivity, which gives the current in the same direction as the applied electricfield, whereas 2 is the Hall conductivity giving the current at right angles to it – itbeing understood that the electric field and the currents are all in the plane normalto the magnetic field. Figure 1.19 may clarify the geometry.

Ni

mii

ii

( 2i 2

i ) Ne

mee

ee

( 2e 2

e )

Ni

mii

2i

( 2i 2

i ) Ne

mee

2e

( 2e 2

e )

1.5 Electrical conductivity 49

Figure 1.19. Currents due to the electric-field components parallel (E‖) and perpendicular(E

) to the magnetic field (B). The currents shown are those due to positive charges. The

direct and Pedersen currents due to negative charges are the same as those shown, but theHall current is opposite. The Hall current in the ionosphere is mainly due to electrons.

E||

E⊥

Dire

ct c

urre

nt

Pede

rsen

cur

rent

Hall current

B

1.5.4 The height variation of conductivity

It is clear from Equations (1.78) and (1.79) that the conductivity due to a singlespecies depends on the ratio /. Indeed, the ratio between the Hall and Pedersenconductivities for a given electron (or ion) density is just /, and is thereforestrongly height-dependent. Note, also, that, in Equation (1.79) the electron andion terms are of opposite sign, so the total Hall conductivity depends on the differ-ence between the electron and ion conductivities, not on their sum. Figure 1.20illustrates the height variations of the direct, Pederson, and Hall conductivities ina typical mid-latitude ionosphere. The Hall conductivity peaks in the E region, thePedersen conductivity peaks somewhat higher, and the direct conductivity contin-ues to increase with height. The Hall conductivity is very small in the F regionbecause the electron and ion components almost cancel out there.

Figure 1.21 indicates the motions of ions and electrons, and the resulting elec-tric current, at various key altitudes. In the upper panel the driving force is a windin the neutral air, which induces ion motion through collisions. The effect of anelectric field is shown in the lower panel.

1.5.5 Currents

For there to be an electric current there must also be a driving force (either a windor an electric field) and a path of conductivity providing a complete circuit. Where


Figure 1.20. Conductivity profiles calculated for middle latitude at noon. (S.-I. Akasofuand S. Chapman (after K. Maeda and H. Matsumoto), Solar–Terrestrial Physics, OxfordUniversity Press, 1972. By permission of Oxford University Press.). Multiply the conductiv-ity values by 1011 to convert them to the SI unit 1 m1.

the latter is not present, the flow of current is inhibited or modified by the electricpotentials created at the boundaries.

The geomagnetic equator is one interesting case. Here the magnetic field runshorizontally and therefore the current which would otherwise flow normal to thefield is inhibited in the vertical direction. Charges are created at the upper andlower boundaries, and the resulting electric field acts to increase the current in thehorizontal plane. It can be shown that, in this special situation, the conductivityacross the magnetic field and in the horizontal direction is given by

31 22/1, (1.80)

called the Cowling conductivity. The value of the Cowling conductivity is compar-able to that of the direct conductivity (Equation (1.77)), and therefore the currentover the magnetic equator is abnormally large. This is the equatorial electrojet.

The large value of the direct conductivity suggests that current should be ableto flow readily along the geomagnetic field direction. The existence of field-alignedcurrents was suggested by K. Birkeland in 1908, but the idea lay dormant for manyyears due to lack of evidence, and magnetic perturbations observed at the groundwere interpreted in terms of currents flowing purely horizontally. It was not until

1.5 Electrical conductivity 51

Figure 1.21. Ion and electron motions due to a neutral-air wind (top) and an electric field(bottom) at selected key altitudes. The current is proportional to the vector differencebetween the ion and electron velocities. (After H. Rishbeth. J. Inst. Electronic RadioEngineers 58, 207, 1988.)

B(in)

Height: 60 km 75 km 100 km 125 km >150 km

B E

Ion and electron motions due to an electric field E

Key:

IONS ELECTRONS CURRENT

νe >> ωe νe ∼ ωe νi >> ω i

νe << ωe

νi ∼ ω i νi << ω i

Ion and electron motions due to a wind U

V i Ve j ∝ (V i – Ve)

U

field-aligned currents were detected by satellite-borne magnetometers in the early1970s that the Birkeland current came into fashion and current systems becamethree-dimensional. Birkeland currents are particularly important in the auroralregions.

A fuller treatment of conductivity and the current systems of the solar–geophysical environment is given in several of the standard textbooks.

1.6 Acoustic-gravity waves and traveling ionosphericdisturbances

1.6.1 Introduction

The familiar acoustic wave, in which the compression of the gas provides the forcerestoring a displaced particle towards its original position, is actually the high-frequency limit of a more general class, the acoustic-gravity wave (AGW ). A parcelof air displaced vertically in a stratified atmosphere tends to be restored by buoy-ancy (due to gravity), and the AGW family results when both gravity and the com-pressional force are taken into account. We are here concerned mainly withatmospheric waves towards the low-frequency end of the AGW range, whoseperiods range from a few minutes to an hour or two. They have horizontal wave-lengths from several hundred to about a thousand kilometers. Gravity waves inthe atmosphere (which should not be confused with cosmological gravity waves,to which they have no connection whatsoever) are transverse waves, the displace-ment of the gas being normal to the direction of travel of the wave. Their proper-ties, in fact, are complex and in many respects not at all obvious.

Several sources of AGWs are known: the motion of the ground during an earth-quake, man-made explosions, weather systems, and ionospheric disturbances athigh latitude. Table 1.2 shows a classification based on period and wavelength.Waves of small scale come mainly from the troposphere; the medium-scale wavesmay be tropospheric or ionospheric in origin; and the large-scale events generallyhave their source in the high-latitude ionosphere – hence their appearance in thisopus! Some AGWs are, no doubt, a consequence of events in the solar–terrestrialsystem: for example, perturbations in the solar wind can produce magnetospheric


Table 1.2. A classification scheme for AGWs

Horizontal traceNomenclature velocity (m s1) Period (min) Wavelength

Large-scale 250–1000 70 1000 kmMedium-scale 90 to 250 15–70 Several hundred kilometersSmall-scale 300 2–5 —

effects, which couple to the high-latitude ionosphere as particle–precipitationevents and electric-field disturbances, which in turn generate medium- and large-scale AGWs. We shall meet other examples of solar–geophysical chains of eventslater in the book.

The ionospheric manifestation of AGWs is the traveling ionospheric distur-bance (TID), which is due to ion movement communicated from the motion of theneutral air through collisions. There are, however, some complications, the prin-ciple one being that, in the F region, the ion motion is constrained along the geo-magnetic field.

The generation of atmospheric waves at high latitude is discussed in Sections6.5.6 and 6.5.7.

1.6.2 Theory

Wave motions in the upper atmosphere have been known for over 100 years andTIDs have been noted in ionospheric observations since the 1940s, but not untilthe 1950s did adequate explanations start to emerge, the key theory being devel-oped by C. O. Hines (Hines, 1960). The underlying concepts of wave propagationare given in Sections 3.2.1 and 3.2.3 in the context of electromagnetic waves. Weoutline here some of the basic theory governing the properties and behavior ofAGWs.

In a planar, horizontally stratified, isothermal, single-species, windless, non-rotating atmosphere, the AGW obeys a dispersion relation

42s2(kx2kz

2)(1)g2kx222g2/(4s2)0. (1.81)

where

is the angular frequency of the wave,

kx is the horizontal wave number (2/x),

x being the wavelength in the horizontal,

kz similarly is the vertical wave number,

is the ratio of specific heats (constant pressure/constant volume),

s is the speed of sound, and

g is the acceleration due to gravity.

This equation states the relation between the frequency and the wavelength (orwave number) in the vertical and the horizontal directions for an AGW. ky doesnot appear in the equation because there is no asymmetry between the x and ydirections.

Two significant frequencies are the acoustic cut-off frequency,

ag/(2s) (1.82)

1.6 Acoustic-gravity waves 53

and the buoyancy or Brunt–Väisala frequency,

b(1)1/2g/s. (1.83)

a is the resonance frequency in the acoustic mode of a column of air extendingthrough the whole atmosphere, whereas b is the natural frequency of oscillationof a displaced parcel of air when buoyancy is the restoring force.

Substituting these frequencies into Equation (1.81) and rearranging gives

kz2 kx

2 . (1.84)

Putting 2b2 gives

kx2kz

2 [(2/)2], (1.85)

where is the wavelength. There is now no distinction between the x and z coordi-nates, and this is the acoustic regime. If we go a stage further by putting 2a

2,we get

s/(kx2kz

2)/(2). (1.86)

This represents a sound wave. In the acoustic regime the phase speed is indepen-dent of direction. Putting, now, 2s2kx

2, which removes the effect of compress-ibility, gives

kz2kx

2 , (1.87)

which represents a pure gravity wave.Since kx and kz must both be positive in a propagating wave, the frequency

must be either larger than a or smaller than b. These, the acoustic and thegravity regimes, are illustrated in Figure 1.22, which plots the regimes of AGW interms of the frequency () and the horizontal wave number (kx). Between theacoustic and the gravity regimes the waves are evanescent and do not propagate.

The angle of propagation with respect to the horizontal is

tan1(kz/kx). (1.88)

If 2 is small compared with b2, the ratio kz/kx is large and then the wave propa-

gates almost vertically. This is for the propagation of phase. The energy, on theother hand, travels at the group velocity, given (Equation (3.21)) by

u(dk/d)1,

b2

2 1

2

s21 2

a

2

1 2

b

22

s21 2

a

2


and, in a gravity wave, the energy flow is at right angles to the direction of phasepropagation. Figure 1.23 illustrates the relations amongst particle displacement,phase propagation, and group propagation in a gravity wave. Note that, if thesource is below, the energy flows upward (as it must) but the phase propagation isdownward. Furthermore, the amplitude of the air displacement increases withaltitude so that the energy flux may be constant (provided that there are no losses).

Figure 1.24 shows how the horizontal component of the group velocity varieswith the wave period (normalized by the Brunt frequency as b/) at fixed valuesof the ascent angle of the energy (i.e. the angle between the group velocity and thehorizontal). The energy flow approaches horizontal when the wavelength is verylarge. A distinction between the sections of the curves labeled “buoyancy” and“gravity” needs to be made when one is considering AGW propagation over largedistances (Francis, 1975).

For an AGW, the refractive index () is defined as the ratio between the speedof sound and the phase velocity of the wave. (Compare with Section 3.2.3.) Then


Figure 1.22. The acoustic, evanescent, and gravity regimes of acoustic-gravity waves. Thedashed lines show the effects of neglecting gravity and compressibility, repectively. At iono-spheric levels, waves with periods longer than 10–15 min are likely to be gravity waves, andany with periods of only a few minutes are probably acoustic. (After J. C. Gille, in Windsand Waves in Stratosphere, Mesosphere and Ionosphere (ed. Rawer). North-Holland, 1968.Elsevier Science Publishers.)


Figure 1.23. A simple gravity wave, showing the essential relations amongst phase propaga-tion, air displacement, and energy flow.

Figure 1.24. Contours of constant , the ascent angle of the group velocity from the hori-zontal, against the wave period and the horizontal component of the group velocity. Thedetails of the diagram depend on the values assumed for the acoustic (a) and Brunt (b)frequencies. (Reprinted from S. H. Francis, J. Atmos. Terr. Phys. 37, 1011, Copyright 1975,with permission from Elsevier Science.)

1

0.5

00.2 0.5 1 2 5 10

Φ = 0°

Φ = 20°

Φ = 40°

Φ = 60°

Φ = 80°

ACOUSTICWAVES

BUOYANCYWAVES

GRAVITYWAVES

Φ = 40°

Φ = 20°

Φ = 10°

Φ = 0°

Φ = 5°

a

b—–

NORMALIZED PERIOD ( b / )

NO

RM

ALI

ZE

D H

OR

IZO

NTA

L G

RO

UP

VE

LOC

ITY

k z

1 C—(

)∂ ∂k

x—

–

b

a—–

2 cos2 . 1.89

If a,b, →1, and if a,b,

→ .

In general the particle motion is elliptical in an AGW, combining the longitu-dinal displacement of an acoustic wave with the transverse displacement of agravity wave. There are alternate compressions and rarefactions at successive zero-displacement points in Figure 1.23. At extremely low frequency, the air motionand the group velocity would be horizontal, the phase propagation vertical, andthe compression and rarefaction zero.

Complexities neglected by the simple theory, but which affect AGWs in real life,are energy loss through the viscosity of the air, non-linear effects if the amplitudebecomes too large at the higher altitudes, reflection and ducting due to the changeof atmospheric properties with altitude, the curvature of the Earth’s surface, andwinds.

1.6.3 Traveling ionospheric disturbances

The mechanism by which AGWs produce ionospheric disturbances (TID) is col-lisional coupling between neutral and ionized particles. This force acts in thedirection of motion of the neutral air, but, in the ionospheric F region, the effectis strongly modified by the geomagnetic field which permits ion motion along thefield only. Thus, while there are several radio techniques able to measure proper-ties of a TID, to interpret these data as properties of the AGW causing it may beless than straightforward. This, however, hardly matters if propagation effects arethe principal concern.

Figure 1.25 is an elegant example of a TID observation by ionosonde (Section4.2.1). It shows the period of the wave and its wavelength, the latter derived usingthe velocity estimated from spaced observations. The downward phase propaga-tion is clearly seen.

1.6.4 The literature

The literature of published research on the topics of AGW and TID is very large.Surveys of the earlier work have been published by Yeh and Liu (1974) and Francis(1975). Studies performed from the mid-1970s up to 1981 have been reviewed byHunsucker (1982), and those between 1982 and 1995 by Hocke and Schlegel(1996). Work since then is addressed by Kirchengast (1996), Bristow andGreenwald (1997), Balthazor and Moffett (1997, 1999), Huang et al. (1998) andHall et al. (1999).

a

b

1cos

1 2

b

2/ 1 2

a

2


1.7 References and bibliography

1.2 The Vertical structure of the atmosphereHargreaves, J. K. (1992) The Solar–Terrestrial Environment. Cambridge UniversityPress, Cambridge.

Richmond, A. D. (1983) Thermospheric dynamics and electrodynamics.Solar–Terrestrial Physics (eds. R. L. Carovillano and J. M. Forbes), p. 523. Reidel,Dordrecht.

1.3 Physical aeronomyVanZandt, T. E. and Knecht, R. W. (1964) The structure and physics of the upperatmosphere. Space Physics (eds. D. P. LeGalley and A. Rosen), p. 166. Wiley, New York.

1.4 The main ionospheric layersBracewell, R. N. and Bain, W. C. (1952) An explanation of radio propagation at 16kc/sec in terms of two layers below E layer. J. Atmos. Terr. Phys. 2, 216.

Friedrich, M. and Torkar, K. M. (1992) An empirical model of the nonauroral Dregion. Radio Sci. 27, 945.

Hargreaves, J. K. and Bagenal, F. (1977) The behavior of the electron content duringionospheric storms: a new method of presentation and comments on the positivephase. J. Geophys. Res. 82, 731.


Figure 1.25. A train of gravity waves observed by ionosonde over Missouri, USA, inDecember 1966, identified from the virtual heights of echoes at frequencies between 1.6 and3.6 MHz. (T. M. Georges, Ionospheric Effects of Atmospheric Waves. Institutes forEnvironmental Research, report IER 57-ITSA 54, 1967, Boulder, Colorado, USA.)

Piggott, W. R. and Rawer, K. (1972) URSI Handbook of Ionogram Interpretation andReduction, Chapter 4. Report UAG-23A, World Data Center A, NOAA, Boulder,Colorado.

Rishbeth, H. (1991) F-region storms and thermospheric dynamics. J. Geomag.Geoelectr. 43 suppl., 513.

VanZandt, T. E. and Knecht, R. W. (1964) The structure and physics of the upperatmosphere. Space Physics (eds. D. P. LeGalley and A. Rosen), p. 166. Wiley, NewYork.

Whitehead, J. D. (1970) Production and prediction of sporadic E. Rev. Geophys. SpacePhys. 8, 65.

1.5 The electrical conductivity of the ionosphereAkasofu, S.-I. and Chapman, S. (after K. Maeda and H. Matsumoto) (1972)Solar–Terrestrial Physics, Oxford University Press, Oxford.

Kelley, M. (1989) The Earth’s Ionosphere. Academic Press, New York.

Rishbeth, H. (1988) Basic physics of the ionosphere – a tutorial review. J. Inst.Electronic Radio Engineers 58, 207.

1.6 Acoustic-gravity waves and traveling ionospheric disturbancesBalthazor, R. L. and Moffett, R. J. (1997) A study of atmospheric gravity waves andtravelling ionospheric disturbances at equatorial latitudes. Ann. Geophysicae 15, 1048.

Balthazor, R. L. and Moffett, R. J. (1999) Morphology of large-scale traveling atmos-pheric disturbances in the polar thermosphere. J. Geophys. Res. 104, 15.

Bristow, W. A. and Greenwald, R. A. (1997) On the spectrum of thermospheric gravitywaves observed by the Super Dual Auroral Radar Network. J. Geophys. Res. 102,11585.

Francis, S. H. (1975) Global propagation of atmospheric gravity waves: a review. J.Atmos. Terr. Phys. 37, 1011.

Gille, J. C. (1968) The general nature of acoustic-gravity waves. Winds and Turbulencein Stratosphere, Mesosphere and Ionosphere (ed. Rawer). Elsevier Science Publishers,Amsterdam.

Hall, G. E., MacDougall, J. W., Cecile, J.-F., Moorcroft, D. R. and St.-Maurice, J. P.(1999) Finding gravity wave positions using the Super Dual Auroral Radar network. J.Geophys. Res. 104, 67.

Hines, C.O. (1960) Internal atmospheric gravity waves at ionospheric heights. Can. J.Phys. 38, 1441.

Hocke, K. and Schlegel, K. (1996) A review of atmospheric gravity waves and travel-ling ionospheric disturbances: 1982–1995. Ann. Geophysicae 14, 917.

Huang, C.-S., Andre, D. A. and Sofko, G. (1998) High-latitude ionospheric perturba-tions and gravity waves: 1. Observational results. J. Geophys. Res. 103, 2131.

Hunsucker, R. D. (1982) Atmospheric gravity waves and traveling ionospheric distur-bances. Encyclopedia of Earth System Science, p. 217. Academic Press, New York.

Kirchengast, G. (1996) Elucidation of the physics of the gravity wave–TID relationshipwith the aid of theoretical simulations. J. Geophys. Res. 101, 13 353.


Yeh, K-C. and Liu, C-H. (1974) Acoustic-gravity waves in the upper atmosphere. Rev.Geophys. Space Phys. 12, 193.

General reading on the topics of Chapter 1

BooksAkasofu, S.-I. and Chapman, S. (1972) Solar–Terrestrial Physics. Oxford UniversityPress, Oxford.

Banks, P. M. and Kockarts, G. (1973) Aeronomy. Academic Press, New York.

Bauer, S. J. (1973) Physics of Planetary Atmospheres. Springer-Verlag, Berlin.

Brasseur, G. and Solomon, S. (1984) Aeronomy of the Middle Atmosphere. Reidel,Dordrecht.

Carovillano, R. L. and Forbes, J. M. (eds.) (1983) Solar–Terrestrial Physics. Reidel,Dordrecht.

Dieminger, W., Hartmann, G. K. and Leitinger, R. (eds.) (1996) The UpperAtmosphere – Data Analysis and Interpretion. Springer-Verlag, Berlin.

Hess, W. N. and Mead, G. D. (eds.) (1968) Introduction to Space Science. Gordon andBreach, New York.

Jursa, A. S. (ed.) (1985) Handbook of Geophysics and the Space Environment. Air ForceGeophysics Laboratory, US Air Force, National Technical Information Service,Springfield, Virginia.

Kato, S. (1980) Dynamics of the Upper Atmosphere. Center for Academic PublicationJapan, Tokyo.

Matsushita, S. and Campbell, W. H. (eds.) (1967) Physics of Geomagnetic Phenomena.Academic Press, New York.

Ratcliffe, J. A. (ed.) (1960) Physics of the Upper Atmosphere. Academic Press, NewYork.

Rawer, K. (1956) The Ionosphere. Frederick Ungar Publishing Co., New York.

Rees, H. M. (1989) Physics and Chemistry of the Upper Atmosphere. CambridgeUniversity Press, Cambridge.

Rishbeth, H. and Garriott, O. K. (1969) Introduction to Ionospheric Physics. AcademicPress, New York.

VanZandt, T. E. and Knecht, R. W. (1964) The structure and physics of the upperatmosphere. In Space Physics (eds. D. P. Le Galley and A. Rosen). Wiley, New York.

Whitten, R. C. and Poppoff, I. G. (1965) Physics of the Lower Ionosphere. Prentice-Hall, Englewood Cliffs, New Jersey.

Whitten, R. C. and Poppoff, I. G. (1971) Fundamentals of Aeromony. Wiley, NewYork.

Conference reportsMcCormac, B. M. (ed.) (1973) Physics and Chemistry of Upper Atmosphere. Reidel,Dordrecht.

McCormac, B. M. (ed.) (1975) Atmospheres of Earth and Planets. Reidel, Dordrecht.


Chapter 2

Geophysical phenomena influencing the high-latitudeionosphere

2.1 Introduction

Whereas the mid-latitude ionosphere is dominated by solar radiation and thechemistry of the upper atmosphere, modified by dynamic effects, the high-latitudeionosphere is, in addition, strongly affected by the nature of the geophysical envi-ronment and by various processes occurring within it. In particular, the form ofthe geomagnetic field connects the polar upper atmosphere to the magnetosphere.Thereby the polar ionosphere becomes accessible to particles that have been ener-gized within the magnetosphere or have come from the Sun; these provide anothersource of ionization. It is also affected by the dynamics of the magnetosphere andis thus subject to electric fields and currents generated by motions at high levels.At the highest latitudes the ionosphere is connected, via the field-lines, to the outermagnetosphere, giving it a ready response to variations in the flow of the solarwind.

The present chapter therefore summarizes the basic properties and behavior ofthe magnetosphere, which we must appreciate in order to understand the behav-ior of the ionosphere poleward of 60° latitude.

2.2 The magnetosphere

2.2.1 The geomagnetic field

To a first approximation the geomagnetic field at and close to the planet’s surfacecan be represented as a dipole field. The poles of the dipole are at geographic

61

latitudes and longitudes 79° N, 70° W, and 79° S, 70° E. The magnetic flux densityis given by

B(r, ) (13sin2)1/2, (2.1)

where M is the dipole moment, r the geocentric radial distance, and the mag-netic latitude. This is accurate to within about 30% at points within two or threeEarth-radii of the surface. Although it is not very accurate, the dipole form isuseful for making approximate calculations.

Figure 2.1 shows the lines of force, generally called field-lines, in a dipole field.Each line is the locus of the force on a single north pole and is represented by asimple equation,

rr0cos2. (2.2)

If 0, rr0; r0 is thus the radial distance to the field-line in the plane normal tothe axis of the dipole. There is a different value of r0 for each line of Figure 2.1,but, since the field is three-dimensional, each r0 actually describes a shell. Theother coordinate is provided by magnetic longitude.

In the magnetosphere it is convenient to use the radius of the Earth, RE, as theunit of distance. Then, putting r/RER,

B(R, ) (13sin2)1/2 G. (2.3)

0.31 G (3.1105 Wb m2) is the flux density at the magnetic equator on theEarth’s surface. In these terms, the field-line equation becomes

RR0cos2, (2.4)

0.31R3

Mr3

62 Geophysical phenomena

Figure 2.1. Dipolar field-lines. (D. L. Carpenter and R. L. Smith, Rev. Geophys. 2, 415,1964, copyright by the American Geophysical Union.)

where both R and R0 are measured in Earth-radii. The latitude where the field-lineintersects the Earth’s surface is given by

cosER01/2. (2.5)

The dipole form is convenient for its mathematical simplicity, but for many pur-poses it is not sufficently accurate. A closer approximation is the displaced-dipolemodel, in which the dipole is displaced by 400 km from the center of the Earth.However, for accurate work (not too far above the surface) it is usual to derive thefield from the magnetic potential expressed as a series of spherical harmonics –the flux density being the gradient of the magnetic potential. The dipole form cor-responds to the first term of the expansion.

The coefficients are derived by fitting the expression to measurements of themagnetic elements on the global scale, using magnetometers both on the groundand on satellites. Because the geomagnetic field changes with time – the secularvariation – a fresh set of coefficients, relating to a specific epoch, is published fromtime to time. Such representations are accurate to within about 0.5% at and nearthe surface. The terms of higher order become less important at greater distancesand the field tends to become more dipolar. However, beyond three or four Earth-radii the distortion due to the solar wind has to be taken increasingly into account.

The pressure of the solar wind confines the geomagnetic field on the sunwardside and forms the geomagnetic cavity.

2.2.2 The solar wind

The solar wind was first observed directly by space probes in the early 1960s,though its existence had previously been proposed in theoretical work and someof its properties had been deduced from studies of comets.

There have been many observations of the solar wind since that time. It is basi-cally an outflow driven by the continual expansion of the solar corona and it istherefore composed of solar material. Most of the ions are protons (H) but thereis also an -particle (He2) component typically amounting to 5% though excep-tionally up to 20%. Still heavier atoms amount to perhaps 0.5% in total, though,in contrast to the light ions, these are not fully ionized. The concentration of pos-itive ions varies between 3 and 10 cm3 (3106 to 107 m3), the most typical valuebeing 5 cm3, and there is a similar number of electrons for bulk neutrality. Themean mass of solar-wind particles is therefore about half that of the proton, about1027 kg. There are fluctuations as large as by a factor of ten over times of minutesand hours, implying irregularities within the solar wind over distances of 105 kmand more.

At the distance of the Earth’s orbit the speed of the solar wind is usuallybetween 200 and 700 or 800 km s1 (Figure 2.2), on which is superimposed arandom component of temperature 105 K. The solar wind is not very hot by solar



Figure 2.2. (a) The speed of the solar wind: a histogram of measurements between 1962and 1970. (J. T. Gosling, in Solar Activity Observations and Predictions (eds. McIntosh andDryer), by kind permission of The MIT Press, 1972.) In (b) (c) are shown the distributionsof the magnitude and components of the interplanetary magnetic field, 1988–1990. By iseast–west and Bz is north–south. (F. J. Rich and M. Hairston. J. Geophys. Res. 99, 3827,1994, copyright by the American Geophysical Union.)

(a)

(b)

(c)

standards, the energy being more directed than random. It carries an energy fluxof about 104 W m2, which is approximately a tenth of that in the EUV regionof the solar spectrum.

The solar wind is the principal medium by which the activity of the Sun iscommunicated to the vicinity of the Earth, and it is extremely important insolar–terrestrial relations and in the behavior of the high-latitude ionosphere.

The interaction depends on a weak magnetic field, the interplanetary magneticfield (IMF), which is carried along by the plasma. This field amounts only to afew nanoteslas (a few ) and it is “frozen in” to the plasma because of the largeelectrical conductivity. The magnitude of the IMF varies slightly with the sunspotcycle (Figure 1.17). The kinetic energy of the solar-wind particles exceeds theenergy density of the magnetic field by a factor of about eight, and therefore themotion of the total magnetoplasma is governed by the motion of the particlesrather than by the magnetic field.

Although the solar wind flows out almost radially from the Sun, the solar rota-tion gives the magnetic field a spiral form, as in Figure 2.3. This is sometimesknown as the garden-hose effect since it may be simulated by turning round whilewatering the garden and noting that the jet of water follows a spiral path althoughthe trajectories of individual drops are radial. It so happens that, at the orbit ofEarth, the IMF field-lines run at about 45° to the radial direction: the radial andthe east–west components of the IMF are therefore about equal in magnitude.

One of the most remarkable of the early results, and a fact of great significance,is that distinct sectors may be recognized within the solar wind, the field beinginward and outward in alternate sectors. Figure 2.3 shows some of the originalmeasurements, in which four sectors – two inward and two outward – werepresent. However, this is not always the case because the sector structure evolveswith time. Sometimes there are only two sectors, and sometimes the sectors arenot all of the same width.

The proton density can vary by more than a factor of ten and the speed of thesolar wind by a factor of two during one solar rotation as the sectors go by, witha degree of anticorrelation.

At first sight the form of the IMF appears anomalous. Although there may bea north–south component, it is equally likely to be northward or southward; thusit seems that a spiral in the ecliptic plane is indeed the basic form of the IMF. Howis this to be reconciled with an origin in the solar magnetic field which we expectto be essentially dipolar? The problem is that the early observations were confinedto the ecliptic plane and there is still not much knowledge of its form at highersolar latitudes. It is now thought that there is a current sheet in or near the equa-torial plane that effectively divides the outward field (above the plane) from theinward field (below it) as in Figure 2.4. If the solar magnetic dipole is tilted fromthe rotation axis, the current sheet will be tilted from the ecliptic plane and a space-craft near the Earth will observe a two-sector structure as the Sun rotates. Whenmore than two sectors are seen, it is thought that the current sheet has developed



Figure 2.3. (a) The form of the interplanetary magnetic field (IMF) in the solar equatorialplane, corresponding to a solar-wind speed of 300 km s1. (T. E. Holzer, Solar SystemPlasma Physics, Vol. I, North-Holland, 1979, p. 103, Elsevier Science Publishers.) (b) Thesector structure of the solar wind in late 1963, showing inward () and outward () IMF.(J. M. Wilcox and N. F. Ness, J. Geophys. Res. 70, 5793, 1965, copyright by the AmericanGeophysical Union.)

undulations as in the skirt of a pirouetting ballerina; hence the concept of Figure2.4 is often known as the ballerina model. Spacecraft venturing out of the eclipticplane have observed that the sector structure disappears – which is consistent withthe ballerina model.

A link between the solar wind and a particular feature of the corona was dis-covered by the Skylab missions between May 1973 and February 1974. A so-calledcoronal hole emits less light at all wavelengths than do adjoining regions, but it ismost marked in an X-ray photograph, on which it appears as a black area.Coronal holes are regions with abnormally low density where the magnetic fieldhas a single polarity – all inward or all outward. This is an open magnetic fieldthat goes out into interplanetary space rather than returning to the Sun. The holeis the source of fast solar-wind streams in which the speed exceeds 700 km s1. Thespeed is greater from a larger hole. Less than 20% of the solar surface is composedof coronal holes, and they are more numerous during the declining phase of thesunspot cycle.

The fast streams interact with the slower solar wind as in Figure 2.5(a), com-pressing the magnetic field and the plasma ahead and sometimes, though notalways, creating a shock front. The compressed plasma is heated, and a rarefac-tion follows. Within the stream the magnetic field maintains the same polarity(inward or outward) and is the same as in the corresponding coronal hole. Thefast streams from coronal holes co-rotate with the Sun and can persist for severalrotations. They are the probable cause of recurring geomagnetic storms (Section2.5.4).

Intermittant perturbations of the solar wind can be caused by specific solarevents, particularly the coronal mass ejection (CME). This is not the same as a


Figure 2.4. The ballerina model of the current sheet in the solar wind. M is the axis of thecurrent sheet and is the Sun’s rotation axis. (E. J. Smith et al., J. Geophys. Res., 83, 717,1978, copyright by the American Geophysical Union.)

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solar flare, though in some instances a flare occurs at about the same time orshortly afterwards. The CME travels away from the Sun at a speed that may beless than 50 km s1 or greater than 1200 km s1, and the speedier examplesproduce a shock front in the solar wind. The typical structure of such a distur-bance is illustrated by Figure 2.5(b), (except that “flare” should be replaced by“CME”). The IMF is compressed by the shock, and a turbulent region is formedbetween the shock and the ejected matter. Within the CME the magnetic field isstrong and well ordered, possibly as a closed loop. These magnetic structures,sometimes called magnetic clouds, are about 0.25 AU across at the orbit of Earth.

The form of the cavity formed by the interaction of the solar wind and the geo-magnetic field is illustrated in Figure 2.6. Because it has very high electrical con-ductivity, the solar wind is not able to penetrate the geomagnetic field but is sweptaround it. Pressure is exerted against the magnetic field, which is distorted andconfined within a large but nevertheless limited region around the Earth. Thiskind of behavior was foreseen by Chapman and Ferraro as long ago as 1930 intheir pioneering study of the cause of magnetic storms (Section 2.5.2). (In modernterms the solar wind is said to be “frozen out” of the geomagnetic field.)

The magnetosphere has a complex structure. In the rest of this section we willdescribe some of its main features: the magnetopause, the magnetosheath and theshock, the polar cusps, and the magnetotail. To begin with they will be treated asthough they were essentially static. Dynamic aspects will be introduced in Section2.4.

2.2.3 The magnetopause

To a first approximation the form of the boundary between the geomagnetic fieldand the incident solar wind can be deduced by considering the pressure balanceacross the boundary. We assume that, when the system is in equilibrium, the pres-sure of the solar wind outside is at every point of the surface equal to that of themagnetic field inside. If the solar wind contains N particles m3, each of massm kg, traveling at velocity v m s1 and striking the surface at angle from thenormal, then it can be shown that the total rate of change of momentum dueto the flux of solar wind particles is 2Nmv2cos2 N m2. This has to be equatedto the magnetic pressure B2/(20). All species within the solar wind contribute butthe protons have greatest effect.

A simple calculation along these lines readily gives a realistic distance for theposition of the boundary (approximately 10RE) along the Earth–Sun line, andallows one to estimate how it varies if the solar wind changes. We assume that0, and the magnetic flux density B varies as (distance)3. Then the distance tothe magnetopause is

lm , (2.6) B2E

40Nmv21/6



Figure 2.6. Two sketches of the geomagnetic cavity in north–south cross-section.(a) Showing the external flow of solar-wind plasma and the principal features of the dis-torted geomagnetic field. Note the tapering of the inner field-lines, suggesting that there is aneutral line further down the tail. (Adapted from V. M. Vasyliunas, in Solar–TerrestrialPhysics, Reidel, 1983, p. 243, with kind permission from Kluwer Academic Publishers.)(b) Showing the main plasma regions in relation to the magnetic structure. Note the north-ward displacement of the plasma sheet during summer in the northern hemisphere. (W. J.Raitt and R. W. Schunk, in Energetic Ion Composition in the Earth’s Magnetosphere (ed. R.G. Johnson), Terra Scientific Publishing Co., Tokyo, 1983, p. 99. After Bahnsen 1978).)

(a)

(b)

where BE is the geomagnetic flux density at the Earth’s surface at the magneticequator.

A full computation is more complicated since the orientation of the boundaryat each point is not known at the outset and an iteration proceedure is required.There is also a degree of coupling between the interplanetary and geomagneticfields, which affects the location of the boundary. Spacecraft find the magneto-pause about 0.5RE closer to the Earth when the IMF has a southward componentrather than a northward one. According to Petrinec and Russell (1993) the boun-dary moves closer by one RE for every 7.4 nT of southward IMF component, butthe distance is not much affected by varying amounts of northward component.

Another approximate method, which is fairly successful over the sunward sideof the magnetopause only, is the image-dipole method, which replaces the dynamicpressure of the solar wind by the magnetic pressure of an image dipole of momentMI placed parallel to and at distance d from the Earth’s dipole, ME. The fields dueto these two dipoles are added and the distorted field-lines associated with ME –the two fields do not interconnect – are taken to represent the geomagnetic fieldwithin the magnetopause. Those associated with the image have no physical sig-nificance. A satisfactory model is given by MI28ME; d40RE. This method isnot valid down the sides of the magnetosphere and in the anti-sunward direction.

The resulting boundary, the magnetopause, is indicated in Figure 2.6. The geo-magnetic field is severely distorted within the magnetosphere. Note in particularthe following points.

(a). Field-lines originating at low latitude form closed loops between northernand southern hemispheres, though there can be some distortion from thedipole form.

(b). Lines emerging from the polar regions are swept back, away from the Sun;in a dipole field some of these would have connected on the day side.

(c). Intermediate between these regions are two lines, one in each hemisphere,that go out and meet the magnetopause on the day side, though in facttheir flux density falls to zero as they reach it; here, neutral points areformed.

The magnetopause has a finite thickness, though it is thin (approximately 1 kmthick) in comparison with the size of the magnetosphere. Figure 2.7 (in Section2.3.1) gives some idea of the form of the magnetosphere in three dimensions.

2.2.4 The magnetosheath and the shock

A shock front is formed in the solar wind two or three RE upstream of the mag-netopause (Figure 2.6). The region between the shock front and the magneto-pause is the magnetosheath, and here the plasma, composed mainly of solarmaterial but in other respects not typical of the solar wind, is turbulent.

Tenuous though it may be by any ordinary standard, the magnetosphere is a


relatively solid object in comparison with the solar plasma. Furthermore, the solarwind is “supersonic” at the orbit of Earth, meaning that its velocity exceeds thatof any waves that can propagate within it. In the solar wind the speed of hydro-magnetic waves, that is, the Alfvén speed, given by

vA , (2.7)

where B is the magnetic flux density and the particle density (in kg m3), is about50 km s1. For a solar wind speed of 400 km s1, therefore, the Alfvén Machnumber is 8. We therefore have the conditions for a shock front, a discontinuitycreated when information about an approaching obstruction is not transmittedahead into the medium.

The existence and location of the shock were predicted from theory in the early1960s and subsequently verified by observation. On crossing the shock, solar-wind plasma is slowed down to about 250 km s1 and the corresponding loss ofdirected kinetic energy is dissipated as thermal energy, increasing the temperatureto 5106 K. Magnetosheath plasma is therefore slower than the solar windproper but 5–10 times hotter.

2.2.5 The polar cusps

The simple models of the magnetosphere predict two neutral points on the mag-netopause where the total field is zero. These points connect along field-lines toplaces on the Earth’s surface near 78° magnetic latitude. These are in fact theonly points on the Earth’s surface which connect directly to the magnetopause,and all the field from the magnetopause converges into those two points. They aretherefore regions of great interest where solar-wind particles (from the magneto-sheath) can enter the magnetosphere without having to cross field-lines.

Measurements reveal regions that are more extended than points, and they arenow called the polar cusps or clefts. Particles with energies typical of those in thesheath are observed over some 5° of latitude around 77°, and over 8 h of local timearound noon. The cusps are funnels of weak magnetic field filled with magneto-sheath plasma, and they have significant effects on the high-latitude ionosphere.The ionospheric effects of particle entry provide “signatures” of the cusp, indicat-ing its location – see Section 5.2.2 and Figure 5.7.

2.2.6 The magnetotail

In the anti-sunward direction the magnetosphere is extended into a long tail, themagnetotail. As is shown by spacecraft magnetometers, the geomagnetic fieldbeyond about 10RE on the night side of the Earth tends to run in the Sun–Earthdirection, and there is a central plane within which the field reverses direction.This is the neutral sheet. The field points towards the Earth in the northern lobe,

B(0)1/2


and away from the Earth in the southern. The tail is roughly circular, some 30RE

(2105 km) across, and of uncertain length, though it has been detected down-wind beyond 107 km. Its significance for the high-latitude ionosphere is that itmaps into the polar caps at its earthward end, and thus the polar ionosphere canbe affected directly by events in the tail.

The basic form of the magnetotail in the plane containing the magnetic polesis shown in Figure 2.6. The flux density is about 20 (20 nT) in the tail lobes,but the field is much weaker in the neutral sheet where the reversal occurs. In thisregion the magnetic pressure of the tail lobes (BT

2/(20) is more or less balancedby an enhancement of the plasma density, the plasma sheet (to be consideredfurther in Section 2.3.3). However, in fact the tail, like the whole magnetosphere,is dynamic and it forms an essential part of the magnetospheric circulation, to beconsidered in Section 2.4.

2.3 Particles in the magnetosphere

2.3.1 Principal particle populations

The geomagnetic field holds within it several distinct populations of charged par-ticles.

(a). Deep within the magnetosphere (in the region often known as the innermagnetosphere) is the plasmasphere, closely linked to the mid-latitude ion-osphere and comprising electrons, protons, and some heavy ions, allhaving energies in the thermal range.

(b). Also trapped on closed field-lines are the energetic particles generallyknown as the Van Allen particles after their discoverer. Apart from cosmicrays and solar protons, which are merely passing through, the Van Allenparticles are the most energetic particles in the magnetosphere and theymake some contribution to the ionization of the upper atmosphere whenthey are precipitated out of the trapping region.

(c). The plasma sheet is associated with the magnetotail, essentially with thecentral region where the magnetic field reverses direction. Plasma-sheetparticles are energized within the magnetotail and they are important inauroral activity and the behavior of the high-latitude ionosphere. Theirenergy is intermediate between those of the plasmasphere and the VanAllen belt. The inner edge of the plasma sheet supports the ring currentthat flows in the magnetosphere during magnetic storms.

(d). At the edges of the magnetosphere, and obviously connected with thephysics of the magnetopause, are boundary layers. Their composition andenergy are governed by the solar wind and plasma in the magnetosheath.


The locations of these particle populations are indicated in Figure 2.7. They arenot merely incidental to the magnetosphere, but are in fact essential to its proper-ties and behavior. Except for the boundary layers, each of these populations is dis-cussed in a following section. In most of the magnetosphere the ratio of the energydensity of the particles to that of the magnetic field () is less than unity, but thereare exceptions.

Originally it was thought that most of the particles in the magnetosphere comefrom the solar wind, but, on the evidence of heavy ions observed in the magneto-sphere, it is now recognized that there are also major sources in the ionosphere:specifically the auroral zones, the clefts, and the polar caps. It is thought that thesolar wind is the dominant source in the distant magnetotail, but the ionosphericsources are important during storms and are sometimes dominant. (See also thediscussion of the polar wind, Section 5.2.3.)

2.3.2 The plasmasphere

Ionized particles in the upper ionosphere (F region and topside) have tempera-tures up to several thousand degrees Kelvin, and electron energies are thereforeseveral tenths of an electron volt. The particle density is typically 1010 m3 at 1000km altitude, decreasing with increasing height – though not very rapidly becauseof the large scale height when atomic hydrogen is the principal atom. The theoryof the protonosphere (Section 1.4.4) shows how ionospheric plasma flows up thefield-lines to populate the protonosphere as far as the equatorial plane, provided


Figure 2.7. Plasma populations and current systems of the magnetosphere in three dimen-sions. (T. A. Potemra, Johns Hopkins APL Tech. Digest 4, 276, 1983. © The Johns HopkinsUniversity Applied Physics Laboratory, 1983. All rights reserved. Reproduced by permis-sion.)

that the field-lines are closed. Some of this plasma flows back to lower levels atnight, where it helps to maintain the ionosphere during hours of darkness, but theplasmasphere nevertheless persists as a permanent feature of the inner magneto-sphere. The outer boundary of the plasmasphere is called the plasmapause.

The plasmapause was discovered by a ground-based technique based on thereception of VLF whistlers. The whistler is a naturally occurring radio signal thatpropagates along the geomagnetic field between the northern and southern hemi-spheres. If the travel time of a whistler is displayed against frequency, it is seen thatthere is one frequency at which the travel time is a minimum. This is a character-istic of all whistlers. Not all show it clearly but those which do are called nose whis-tlers. The frequency corresponding to the minimum travel time indicates whichfield-line the whistler has traveled along, and the time taken can be interpreted togive the minimum electron density encountered along that field-line. A detaileddiscussion of whistlers and other magnetospheric noises would be beyond ourscope; the reader who wishes to persue that interesting topic is referred to thebook by Helliwell (1976).

By means of this technique it is possible to determine the variation of electrondensity in the equatorial plane, as in Figure 2.8. The remarkable feature of suchplots is that they often exhibit a sudden drop in the electron density near 4RE. Thedecrease may be of a factor of ten or more within a distance of 0.5RE or less. Thisedge is the plasmapause, sometimes also known as the knee. If it is traced inwardalong the geomagnetic field, it is found to correspond approximately to theionospheric main trough which effectively marks the poleward extent of the mid-latitude ionosphere (Section 5.4). The plasmasphere thus occupies a doughnut-shaped region of the inner magnetosphere where the field-lines are not too


Figure 2.8. Electron density in the equatorial plane determined from whistlers. (J. A.Ratcliffe (after D. L. Carpenter), An Introduction to the Ionosphere and Magnetosphere.Cambridge University Press, 1972.)

distorted from the dipolar form, and the mid-latitude ionosphere is part of it.The plasmapause is dynamic and variable. Its position varies during the day,

the most notable feature being a bulge in the evening sector (Figure 2.9). In addi-tion, the whole region contracts when geomagnetic activity increases, and there isthen a gradual recovery over the next few days. Figure 2.10 shows measurementsof the plasmapause position as a function of the global magnetic activity index Kp

(described in Section 2.5.4). For most of the time it occurs between three and sixRE, though it has been detected as close to the Earth as 2RE (i.e. only one RE abovethe surface), and satellite data show occasions when no plasmapause was detectedwithin seven or eight RE.

According to whistler observations and in situ data (Carpenter and Anderson,1992), the average geocentric distance to the plasmapause in Earth-radii (Lpp) isrelated to the greatest preceeding value of Kp (Kp) by the empirical relation

Lpp5.60.46Kp. (2.8)

For this purpose Kp is derived over the previous 24 h, except that, for the hours06–09, 09–12, and 12–15, respectively, the Kp values for the preceeding one, two, andthree periods (of 3 h) are omitted. Carpenter and Anderson also give formulae for


Figure 2.9. Plasma flow in the equatorial plane and the daily variation of the plasmapause.(After J. L. Burch, in The Upper atmosphere and Magnetosphere. National Academy ofSciences, Washington DC, 1977.)


Figure 2.10. Variations of the plasmapause with the magnetic activity index, Kp. (a)Satellite observations of ion density, showing the plasmapause at several levels of Kp. (b)The relation between the plasmapause distance, Lpp, and Kp. (After C. R. Chappell et al., J.Geophys. Res. 75, 50, 1970, copyright by the American Geophysical Union.)

the electron density in the plasmasphere inside the pause and its regular variations,the thickness of the pause, and the distribution in the “trough” beyond it.

2.3.3 The plasma sheet

Beyond the plasmapause the electron density is much smaller and the temperatureis much higher. Clearly this is a different population of particles. The electron andion densities are each only about 0.5 cm3. Particle energies are generally 102–104

eV. The average energy of the electrons is about 0.6 keV and that of the protonsabout 5 keV. The total energy density of the particles is about 3 keV cm3.

The particular importance of the plasma sheet lies in its association with thecentral plane of the magnetotail where the magnetic field reverses. To a firstapproximation, the pressure of the particles in the sheet balances the magneticpressure in the tail lobes. Thus,

nkTBT2/(20), (2.9)

where BT is the tail magnetic field outside the plasma sheet. As indicated in Figure2.6, the plasma sheet follows the magnetic field down to lower altitudes in thevicinity of the auroral zone. It also continues round to the day side of the Earth.In the equatorial plane there is an identifiable, though variable, inner edge near7RE at midnight. The sheet is several Earth-radii thick (also variable) and itextends across the tail between the dusk and dawn sides. As the Earth’s magneticaxis tilts seasonally and diurnally with respect to the Sun, the tail plasma sheetand neutral sheet oscillate north and south of the solar-ecliptic plane.

The magnetic field runs in opposite directions in the two lobes of the magnet-otail, and the existence of a sheet of plasma between them creates an unusual phys-ical situation. The configuration being far from dipolar, it represents a store ofenergy that could be released in the right circumstances. There is good evidencethat a neutral line forms some 50RE down the tail. Here the magnetic field islocally collapsed and plasma is accelerated both towards and away from theEarth. It is also known that events in the magnetotail are closely related to the phe-nomena of the aurora and the substorm, and it is thought that, at such times, aneutral line forms closer to the Earth, matter in the plasma sheet being then accel-erated to higher energies. (This topic is discussed further in Section 6.4.)

2.3.4 Trapped particles

The discovery that there are energetic particles trapped in the magnetosphere wasmade early in the satellite era by Van Allen and colleagues at the University ofIowa. The information came from a Geiger counter, which they had built for thefirst successful satellite launched by the USA, Explorer 1, with the intention ofstudying cosmic rays. The cosmic rays were detected, but the high counting rates


which were recorded in parts of the orbit indicated something much more excit-ing. Apart from its scientific value, this discovery was important in a politicalsense since it showed that the “space” near the Earth was not an empty void butcontained at least some matter and energy and would, very likely, repay closerinvestigation. Figure 2.11 reproduces what is probably the most famous illustra-tion of that period, showing the double structure deduced from the passage ofPioneer 3, an eccentric-orbit spacecraft that went out 107400 km in 1958. Thuswere discovered the “inner” and “outer” Van Allen belts.

The division into two belts is something of an over-simplification because thestructure of the trapping region depends on the nature and energy of the particles.The original discovery concerned protons with energy exceeding 30 MeV. Figure2.12 shows a modern version of the trapped-particle distribution.

The mechanism of particle trapping is of interest. A trapped particle moves inthree ways (Figure 2.13). It gyrates around a line of the geomagnetic field, bouncesback and forth along the line of force between mirror points, and gradually driftslongitudinally around the Earth. The motions are based on a set of adiabaticinvariants:

(1). the magnetic moment is constant;

(2). the integral of the parallel momentum over one bouce between mirrorpoints is constant; and

(3). the total geomagnetic flux enclosed by the drift orbit is constant.

The first holds if the magnetic field does not change during a gyration period, thesecond if it does not change during a bounce period, and the third if it does notchange during the time taken for the particle to encircle the Earth. Hence they areprogressively less stringent.

The basic trapping mechanism is determined by the first invariant. Consider acharged particle gyrating in the geomagnetic field but also with a component ofmotion along the field. As the particle spirals from the equator towards the pole it


Figure 2.11. Van Allen’sfirst map of the radiationbelts, showing countingrates of the Geigercounter on Pioneer-3. (J.A. Van Allen and L. A.Frank, J. Geophys. Res.64, 1683, 1959, copyrightby the AmericanGeophysical Union.)

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moves into a region of stronger field. A consequence of the first invariant is that thecomponent of kinetic energy perpendicular to the field is proportional to the mag-netic flux density. Hence that component of the particle’s energy increases, and theparallel component decreases by the same amount. Eventually, provided that theparticle does not run into the atmosphere first, all the energy is transverse, forwardmotion stops, and, at that point, the mirror point, the particle is reflected backtowards the equator and then into the other hemisphere. Since no energy is being lostor gained, the particle can continue thus for ever or until something else happens toit. The mirror point does not depend on the energy of the particle, but it is directlyrelated to the particle’s direction of travel (the pitch angle) as it crosses the equator.

If the mirror point is deep enough to be within the atmosphere, those particleswill be lost. Correspondingly, there is, at any point along the path, some pitchangle within which all particles will be lost to the atmosphere at the next bounce.This defines the loss cone, which is generally a small angle of only a few degrees atthe equator but increases progressively towards 90° as the Earth’s surface isapproached.

A trapped particle drifts longitudinally due to the forces which arise from thecurvature and the radial change of intensity of the geomagnetic field. Electronsdrift eastward and protons westward, and the rate of drift depends (more or lesslinearly) on the energy of the particle. The times which electrons of various ener-gies and pitch angles take to orbit the Earth are illustrated in Figure 2.14.

The longitudinal drift path is determined by the second invariant. The princi-pal population of trapped particles lives in the region of the magnetosphere wherethe field-lines are closed and almost dipolar. To remove particles from such orbitsit is necessary to infringe one of the invariants. These particles are stably trapped.However, owing to the distorted form of the magnetosphere, some drift pathsstarting in the outer zone take particles into the magnetotail or into the solarwind. These particles cannot complete a full circuit of the Earth, and are trappedonly temporally, in pseudo-trapping regions.


Figure 2.13. The motions of a particle trapped in the geomagnetic field.

longitudinal drift

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The Van Allen particles do not have significant effects on the high-latitude ion-osphere while they remain stably trapped. However, it is almost certain that theprocesses of trapping and loss, because they may transport energetic particlesbetween different regions of the magnetosphere and then deposit them in the ion-osphere, are important at high latitudes.

2.3.5 The ring current

One significant consequence of particle trapping is the formation of a ring currentin the magnetosphere at or near the inner edge of the plasma sheet. The westwarddrift of trapped protons and the eastward drift of trapped electrons both contrib-ute to a ring current directed clockwise as viewed from over the north pole. Thiscurrent increases under disturbed conditions and may be detected with a magnet-omenter at the ground as the main phase of a magnetic storm (Section 2.5.2).

These particles are not the energetic ones typical of the Van Allen belts, buthave been shown by direct measurement to be mainly protons of energy 10–100keV. The current is generally located at a distance between four and six RE (Figure2.15) and its existence indicates that there is a concentration of charged particlesin that region. Since the drift rate of a trapped particle is proportional to its energy,and all protons carry the same charge, it is easily shown that the total current inthe ring is proportional to the total energy of the contributing protons.


Figure 2.14. The time taken for trapped electrons to make one circuit of the Earth at theorbit of a geostationary satellite (6.6RE): (a) as a function of pitch angle for various ener-gies; (b) as a function of energy for pitch angle 90°. (a) Also gives the velocity of the foot-print (at 67° latitude) 100 km above the Earth’s surface. (P. N. Collis, privatecommunication.)

It is possible to have a ring current, or a component of a ring current, whichgoes only part way around the Earth: a partial ring current.

2.3.6 Birkeland currents

Since current has to be continuous, we may well ask how the circuit of a partialring current is completed. Since the early 1970s it has become generally acceptedthat currents may flow along the magnetic field-lines between the magnetosphereand the ionosphere. Such currents had first been suggested in 1908 by K.Birkeland, but direct measurements in situ were required in order to prove theirexistence. Typical distributions of Birkeland currents, sometimes called field-aligned currents, are illustrated in Figure 2.16.

The currents fall into several distinct regions, and by convention the polewardone is “region 1” and the equatorward one is “region 2”, irrespective of the


Figure 2.15. Radial profiles of various heavy ions during an inbound pass of the AMPTEsatellite on 5 September 1984. (a) The number density. (b) The energy density of particles ofenergy 5–315 keV per unit electronic charge. (D. J. Williams, Space Sci. Rev. 42, 375, 1985.With kind permission from Kluwer Academic Publishers; after G. Gloeckler et al., privatecommunication.)

direction of the current. The intensity varies during the day in each region and isgenerally up to 1 or 2 A m2. The total field-aligned current is 106–107 A.

The concept of the Birkeland current has profoundly affected ideas aboutcurrent systems in the ionosphere and magnetosphere. Magnetic observationsmade at the surface of the Earth may always be interpreted as a two-dimensionalcurrent system flowing horizontally at some (unspecified) height in the iono-sphere, and the earlier analyses were always performed in this way. These are actu-ally equivalent current systems, which are mathematically correct but are not theonly solutions if vertical current is also allowed. The inclusion of Birkelandcurrent has led to distributions that include both ionospheric and magnetosphericparts, and are physically more instructive.

2.4 The dynamics of the magnetosphere

2.4.1 Circulation patterns

A static description of the magnetosphere is alright as far as it goes, but there arecertain facts and phenomena that it cannot hope to explain. If we perform asimple calculation of the shape of the magnetopause from the pressure of the solarwind (as suggested in Section 2.2.3), we are using just the component of forcenormal to the boundary. However, if the solar wind also exerts a force tangentialto the boundary, energy will be transfered into the magnetosphere from the solarwind, and we have the possibility that material within the magnetosphere will beset in motion. The situation is then somewhat akin to that of a falling raindrop,


Figure 2.16. Distributions of Birkeland currents during (a) weak and (b) active distur-bances. (T. Iijima and T. A. Potemra, J. Geophys. Res. 83, 599, 1978, copyright by theAmerican Geophysical Union.)

in which liquid is swept back at the surface and returns down the middle of thedrop.

The concept of a circulating magnetosphere driven by viscous interaction at thesurface was put forward by Axford and Hines in 1961. The nature of the viscousinteraction was not specified but was thought to be effectively some kind of fric-tion. Experimental support for circulation came from a study of the Sp

q currentsystem, whose existence may be inferred from observations with magnetometersat medium and high latitudes. The current system causing Sp

q – the term means thepolar part (p) of the magnetic variation related to the solar day (S) which isobserved under magnetically quiet conditions (q) – is illustrated in Figure 2.17.The current flows over the poles from night to day, and there are return currentsat lower latitudes. We show the pattern here because it is one that will prove basicin later considerations of the high-latitude ionosphere.

Since electrons are tied to the magnetic field in the dynamo region (whereas thepositive ions move with the neutral air), the Sp

q current flow can be interpreted asa motion of magnetic field-lines in the opposite direction; that is, over the polefrom the day to the night side of the Earth. In the magnetosphere, therefore, thefield-lines circulate over the poles from the day to the night sectors of the Earthwith a return flow around the dawn and dusk sides.

Figure 2.18(a) shows the basic pattern of magnetospheric circulation in a sectionthrough the equatorial plane. Figure 2.18(b) includes the distortion due to theEarth’s rotation which carries the inner part of the magnetosphere with it. Section2.4.4 indicates how the combined effect of two circulation patterns may be handled.


Figure 2.17. The Spq current system in the polar regions due to the circulation of magnetos-

pheric field-lines. (J. A. Ratcliffe, An Introduction to the Ionosphere and Magnetosphere,Cambridge University Press, 1972.)

Present evidence, however, is that, although viscous interaction plays some partin driving the magnetosphere, it is not the major cause. One reason is that the solarwind is so tenuous (having a mean free path between collisions of perhaps 109 km!)that it is hard to believe in a sufficient amount of friction at the magnetopause.Attention therefore moved to an alternative mechanism, based on the work of J.W. Dungey concerning interconnection between the interplanetary magnetic fieldand the geomagnetic field. The various field configurations that arise when adipole is placed in a uniform magnetic field are easily illustrated in simple labor-atory experiments using a bar magnet situated in an external field. When the fieldsare parallel there are neutral points on the equator and connections between thetwo fields. When they are antiparallel, the neutral points are over the poles andthere is no interconnection. Figure 2.19 depicts a distorted dipole field represent-ing the geomagnetic field in polar section, with the addition of (a) a northwardIMF and (b) a southward IMF. In the second case, neutral points are formed inthe equatorial plane and some lines of the IMF connect to geomagnetic lines. Thisis not so in the first case. We have seen that the IMF tends to lie in the solar-eclipticplane, oriented at the “garden-hose” angle, but there is usually a north–southcomponent as well and it is this, when it is directed southward, which connectswith the geomagnetic field.

The IMF is frozen into the solar wind and is therefore carried along with it.When geomagnetic field-lines are connected to those from the IMF they aredragged over the poles from the sunward neutral point, as in Figure 2.19(c), andthereby transported from the day to the night side. While they are over the polarcaps the field-lines are open in the sense that they do not connect back to the otherhemisphere in any simple or obvious manner. In the tail these lines reconnect andmove back towards the Earth. The above picture is of course a simplified one.Detailed consideration taking account of the three-dimensional form of the mag-netosphere shows how it is possible to have a degree of connection when the IMF


Figure 2.18. Patterns of magnetospheric circulation in the equatorial plane: (a) if due to“friction” of the solar wind at the magnetopause; and (b) if the Earth’s rotation is included.(After A. Nishida, J. Geophys. Res. 71, 5669, 1966, copyright by the American GeophysicalUnion.)

is northward, and how the east–west component affects the connection point andthe resulting circulation pattern. Also, it is thought that viscous interaction doesmake some contribution; a minor one when the IMF has a southward component,but perhaps the main one when the IMF is northward (and the circulation is thenmuch reduced).

The details and mechanisms of magnetic connection and magnetospheric cir-culation continue to be topics for research, but there is little doubt that the IMFplays an important role. Signatures indicating magnetic connection are observed


Figure 2.19. The interaction of terrestrial and interplanetary magnetic fields seen in polarsection: (a) northward IMF; (b) southward IMF; and (c) circulation due to the flow of thesolar wind. (After C. T. Russell, Critical Problems of Magnetospheric Physics, 1972, after J.W. Dungey, 1963, and R. H. Levy et al., Am. Inst. Aeronaut. Astronaut. J. 2, 2065, 1964.)A: Interplanetary field-line. B: Interplanetary field-line connecting to, or disconnectingfrom, a geomagnetic field-line. C: Open geomagnetic field-line. D: Closed geomagneticfield-line. N: Neutral point. 0–7: Successive positions of a selected interplanetary field-line.

by spacecraft passing through the magnetopause, and the level of geophysicalactivity increases when the IMF has a southward component. At such times (a)substorms (Section 6.4) occur more frequently; (b) the magnetic flux in the taillobes is increased; (c) the auroral zone and the dayside cusps are displaced equat-orward; and (d) the dayside magnetopause moves inward – all suggesting that asouthward IMF increases the rate of magnetospheric circulation.

Magnetospheric circulation is a concept of great significance, not only in mag-netospheric theory but also, as we shall see, for the high-latitude ionosphere.

2.4.2 Field merging

Magnetospheric circulation requires that, on both the day and the night sides ofthe Earth, magnetic field-lines are broken and then reconnected in a different con-figuration. The simplest model of such a process, the X-type neutral line, is illus-trated by Figure 2.20. This shows a situation in the central region of the tail. Theconfiguration cannot be static because the tension in the field-lines will producenet forces towards the Earth and into the tail. However, there can be dynamicequilibrium, in which the depletion is replaced by other field-lines moving in fromthe lobes. Those lines are, of course, replaced by others moving over the poles fromthe day side of the Earth. There can also be a Y-type neutral line, where the fieldcontinues to converge on the tailward side; in that case all the reconnected linesmove towards the Earth.

The theories of magnetic reconnection came originally from studies of solarflares. In the magnetosphere the process is thought to be that of fast reconnection,first proposed by Petschek (1964) more than 30 years ago. This mechanisminvokes an Alfvén wave, which allows reconnection to proceed more rapidly thanwould diffusion only. The velocity of reconnected field-lines towards the Earth isestimated as about 100 km s1, and the drift towards the neutral sheet as about 10


Figure 2.20. An X-type neutral line in the magnetotail. Plasma flows in from the north andsouth lobes, and leaves Earthward and tailward along the plasma sheet.

km s1. Particles on the field-lines passing through the reconnection region areaccelerated in the direction of the contraction. It is likely that reconnection in thetail occurs not steadily but intermittently in limited regions, and this is probablyimportant in the causes of the aurora.

While a pattern of circulation must plainly include reconnection on the nightside, it is the connection between the IMF and geomagnetic field-lines on the dayside which drives the circulation. Though various ideas have been suggested, thedetails of this process have not been finally decided. Obviously, a geomagnetic fluxtube has to break and connect with an IMF tube, and this is the event which hasbeen identified from the magnetic signature recorded by a nearby spacecraft, as aflux-transfer event (FTE). The newly connected tube of plasma then moves pole-ward into the boundary layer and joins the general circulation. FTEs are frequent,though individually of short duration and limited spatial extent ((0.5–1)RE).There are more FTEs when the IMF has a strong southward component, andnone when it is northward. Presumably, details also vary with the direction of theeast–west component. “Quasi-steady” connection is also a possibility.

2.4.3 Magnetospheric electric fields

It is sometimes helpful to regard the dynamic magnetosphere in terms of an elec-tric dynamo and a motor. The magnetosphere may be treated as a magneto-hydrodynamic generator, in which a jet of plasma (the magnetosphere) is forcedthrough a static magnetic field (the IMF) and an electric potential is developed bydynamo action. The total potential difference across the magnetosphere is

VTvLBn, (2.10)

where v is the solar-wind speed, L is the width of the magnetosphere, and Bn is themagnetic flux density normal to the boundary. Its value is estimated as about60 kV, equivalent to an electric field of about 0.3 mV m1. The electric field isdirected from the dawn to the dusk side of the magnetosphere. The same poten-tial difference appears across the open field region of the high-latitude ionosphere,the field again being directed from dawn to dusk.

The motion of magnetoplasma within the magnetosphere can now be regardedas the effect of this electric field on the geomagnetic field as in an electric motor,according to

vEB/|B|2, (2.11)

where v is the velocity, E the electric field, and B the magnetic field. The magni-tude is simply

vE/B. (2.12)


The potential distribution across the magnetotail maps along the field-linesinto the polar caps, where it is more accessible to direct measurement, and rela-tionships have been found with the speed of the solar wind and the magnitude anddirection of the IMF (Section 5.1.2). If the potential difference across the polarcap is 60 kV, the field-line velocity is about 300 m s1.

2.4.4 The dynamics of the plasmasphere

A good example of treating the dynamics of the magnetosphere in terms of elec-tric fields is the question of the location of the plasmapause, the boundary betweenthe plasmasphere and the outer magnetosphere. The higher levels of the plasma-sphere are created by ionospheric plasma moving up and down closed geomag-netic field-lines. However, to explain the dynamics of the plasmasphere as a whole,it is also necessary to take account of the circulation of the magnetosphere. Theinner magnetosphere co-rotates with the Earth while the outer magnetospherefollows its own circulation pattern under the control of the solar wind. Generallyspeaking, the plasmasphere exists on the co-rotating field, and the plasmapausemarks the boundary between the inner and outer regions.

If we imagine that the plasmasphere is observed by a person fixed in space (i.e.not rotating with the Earth), we can show that its motion in the equatorial planemay be ascribed to a co-rotation electric field of magnitude

ErBLRE, (2.13)

where B is the geomagnetic flux density, L is the observer’s geocentric distance inEarth-radii, RE is the radius of the Earth and is the Earth’s angular velocity. Theplasmapause occurs approximately where the cross-tail and co-rotation fields areequal:

ET LRE, (2.14)

where BE is the geomagnetic flux density at the surface of the Earth at the equator,and we have used the radial variation of flux density in a dipole field, BBE/L3.The condition expressed by Equation (2.14) marks the transition between the cir-culation regimes of the inner and outer magnetosphere. Putting in numericalvalues gives:

L214.4/ET (mV m1). (2.15)

If the tail field is 1 mV m1 we expect to find the plasmapause at about 4RE.A computation of plasma convection about the Earth was shown in Figure 2.9.

In general these flow lines are also equipotentials. The bulge in the plasmaspherein the evening sector, a well-established feature, occurs because the co-rotationand cross-tail fields are in opposite directions on the evening side.

BE

L3


We now see that the principal dynamics of the plasmasphere are (1) filling andemptying along the tubes of force from the ionosphere, which depends on the timeof day; and (2) rotation about the Earth in a pattern that is also affected towardsits outer edge by the circulation of the magnetosphere. The second factor explainswhy the location of the plasmapause varies with geomagnetic activity. An increasein the circulation of the magnetosphere implies that condition (2.15) is satisfiedcloser to the Earth and the plasmasphere must then be smaller. It is thought thatthe change of circulation peels off layers of plasma, which may exist as detachedregions before becoming lost to the outer magnetosphere or into the solar wind.

When activity returns to normal the magnetospheric circulation and electricfields return to their previous state, but now the outer tubes of magnetic flux aredevoid of plasma. These gradually refill from the ionosphere over a period ofseveral days. The rate of filling is determined by the rate of diffusion of protonsfrom the upper ionosphere (where they are formed by charge exchange betweenhydrogen atoms and oxygen ions – Section 1.4.4), and by the volume of the fluxtube to be filled. Since the latter varies as L4 it takes much longer to refill tubesoriginating at higher latitude, and, since active periods may recur every few days,there will be periods when the outer tubes are never full. It is probably safe to saythat the plasmasphere always suffers from some degree of depletion.

2.5 Magnetic storms

2.5.1 Introduction

The ionospheric storm was introduced in Chapter 1, but the magnetic storm, themain part of which is due to the ring current, is probably the more fundamental.Like the ionospheric storm, it may last from a few hours to several days and itoften exhibits three phases. It has been known – though not by its present name –since the eighteenth century from its effect on a compass needle, but progress inunderstanding any of the storm phenomena dates only from modern times.Because magnetic storms can be monitored without great difficulty using a mag-netometer, and long runs of such measurements exist, the magnetic storm hascome to be a common reference point in geophysical studies.

Although there are superficial similarities between magnetic and F-regionstorms, the physical connections are not so obvious. These are phenomena thathave not proved amenable to simple explanations, and some major questionsremain. Part of the problem is that a chain of events is involved. The primary causeis almost certainly the solar wind, affecting the magnetosphere. The magnetos-pheric consequences then affect the upper atmosphere, and, in some cases, theremight even be contributions ascending from the troposphere or stratosphere.


2.5.2 The classical magnetic storm and the Dst index

The typical magnetic storm is illustrated in Figure 2.21. Like the ionosphericstorm, this classical magnetic storm consists of three phases:

(a). an increase of magnetic field lasting a few hours only, called the initialphase;

(b). a large decrease in the H component building up to a maximum in about aday, the main phase; and

(c). a slow recovery to normal over the next few days, the recovery phase.

The initial phase is caused by the compression of the front of the magnetospherewith the arrival of a burst of solar plasma, as in Chapman and Ferraro’s theory of1930 (Section 2.2.2). The main phase is due to the ring current which was intro-duced in Section 2.3.5. The recovery phase is simply a recovery to the pre-stormcondition as the ring current decays.

The Dst index of magnetic storms is derived from low-latitude magnetograms.In units of nanoteslas (), it simply expresses the reduction of the magnetic Hcomponent at the equator due to the ring current, and it serves as a useful indica-tor of the intensity and duration of individual storms. If we assume a distance forthe ring current, its magnitude may be derived from the equation

B 3I/(10r)Ir, (2.16)

where B is the change of magnetic flux (in nanoteslas), I is the current (inamperes) and r is the assumed distance (in kilometers). Equation (2.16) is derivedfrom the standard formula for the flux density at the center of a current loop, butwith a correction for currents induced in the ground.


Figure 2.21. Typical magnetic storms registered by an equatorial magnetometer. (After M.Sugiura and S. Chapman, Abhandl. Akad. Wiss. Gottingen Math.-Phys. Kl., Special Issue 4,1960.)

2.5.3 Magnetic bays at high latitude; the auroral electrojet

The magnetic storm appears in a different guise at high latitude. By contrast withrecords from low latitudes, where the effects are due to the growth and decay ofthe ring current in times measured in hours and days, magnetograms recorded inand near the auroral zone show more rapid changes. The typical pattern there isa series of bay events with typical durations of tens of minutes to an hour or two,such as those illustrated in Figure 2.22. The magnitude of the perturbation in thehorizontal component (H ) can be as much as 1000 nT, and its sign tends to bepositive before midnight and negative afterwards. Where the sign changes is calledthe Harang discontinuity.

The magnetic bay is caused by an electric current, the auroral electrojet, flowingnot in the magnetosphere but in the auroral E region. To explain the sign of thebay, the current flow must be eastward before midnight and westward afterwards:i.e. converging on the midnight meridian. Obviously there must also be return cur-rents for continuity. Chapman’s original interpretation assumed that the currentsonly flowed only horizontally and this gives a pattern called the SD current system,which is composed of two electrojets with return currents at higher and lowerlatitudes as in Figure 2.23. This pattern was obtained by averaging the daily


Figure 2.22. Examplesof positive and negativemagnetic bays recordedat College, Alaska. Thetime zone is that of the150° W meridian. (AfterS.-I. Akasofu, Polar andMagnetosphericSubstorms, Reidel, 1968,with kind permissionfrom Kluwer AcademicPublishers.)

magnetic variations during the first two days of a number of storms, a proceedurethat concentrates the inferred current into the auroral zone. If a three-dimensionalcurrent system is allowed, other distributions become possible; modern interpre-tations include Birkeland currents (Sections 2.3.6 and 6.4.4).

2.5.4 Magnetic indices

The magnetic bays of the kind illustrated in Figure 2.22 are the basis of severalmagnetic indices, which are regularly compiled and published. The primarypurpose of these indices is to quantify the intensity of geomagnetic disturbanceand thereby provide a common reference point and a basis for comparisonbetween different observations. The bays, and of course the electrojets causingthem, are part of the substorm phenomenon – see Section 6.4 – and as such maybe expected also to be related to the intensity of substorms and their frequency ofoccurrence. The most useful indices are probably those known as Kp, Ap, and AE.

Kp and Ap

Kp is based on the range of variation within 3-h periods of the UT day observedin the records from about a dozen selected magnetic observatories. After localweighting, and averaging, the Kp value for each 3 h of the day is obtained on ascale from 0 (for “very quiet”) to 9 (for “very disturbed”). The scale is quasi-logarithmic, and the integer values are sub-divided into thirds by use of thesymbolsand : thus, 2, 2, 3, 3, 3, etc.

Ap is a daily index, obtained from the same basic data, but converted to a linearscale (the 3-h ap) and then averaged over the U. T. day. The value of the interme-diate ap is approximately half the range of variation of the most disturbed mag-


Figure 2.23. Chapman’soriginal SD currentsystem. The SD analysistakes magnetic distur-bance vectors observedsimultaneously at anumber of stations andinfers a current systemthat could give rise tothem. (S.-I. Akasofu,Polar andMagnetosphericSubstorms, Reidel, 1968,with kind permissionfrom Kluwer AcademicPublishers.)

netic component measured in nanoteslas. The relationship between Kp and ap isgiven in Table 2.1.

It is convenient to present Kp as a Bartels musical diagram, as in Figure 2.24, inwhich form it often shows how the activity tends to recur with the 27-day solarrotation. In such cases the diagram has some predictive value, but the 27-dayrecurrence is not always in evidence, being more apparent during the decliningphase of the solar cycle.

Early measurements of the solar wind led to an empirical relation between thespeed of the solar wind and Kp:

v (km s1)(8.440.74)Kp(33017), (2.17)

where Kp is the sum of the eight Kp values over a U. T. day. This was an impor-tant early result in that it demonstrated a relationship between the solar wind anddisturbances of the geomagnetic field.

AU, AL, and AE

The magnetic observatories which contribute to Kp and Ap are situated at variouslatitudes and longitudes, but with a preponderance in the higher middle latitudes,i.e. the equatorward side of the auroral zone. To achieve an index more tightlyrelated to the auroral regions and to provide better time resolution, AU, AL, andAE were invented by Davis and Sugiura (1966). These indices are obtained by arather different proceedure. Magnetograms from observatories at several differentlongitudes around the auroral zone are superimposed and the upper and lowerenvelopes are read. The upper envelope is AU, the lower envelope is AL and thedifference between the envelopes is AE. AL indicates the greatest positive excur-sion in the auroral zone (probably a pre-midnight bay), AU the greatest negative


Table 2.1. The relationshipbetween Kp and ap

Kp ap

0 01 32 73 154 275 486 807 1408 2409 400

excursion (probably post-midnight), and AE, which is the most widely used of thethree, is taken as a general indicator of auroral-zone activity irrespective of thelocal time. Figure 2.25 shows an example. The mean of AU and AL is plotted asA0. The values are published at an hourly interval in printed reports, and they areavailable at a 2-min interval by special request.

In principle, AU is a measure of the eastward auroral electrojet and AL ameasure of the westward electrojet. However, both of these indices may be affectedby the ring current. The advantage of AE, being their difference, is that it depends


Figure 2.24. A Bartels diagram of Kp. (Solar–Geophysical Data. National GeophysicalData Center, NOAA, Boulder, Colorado, February 1987.)

solely on the eastward and westward electrojets and should be independent of thering current and any other zonal current. AE is particularly valuable for indicat-ing when a magnetic substorm occurs. It is also well correlated to the energycoupled into the magnetosphere from the solar wind (Section 6.4.6).

Indices such as AE are obviously more sophisticated than Kp, and their prep-aration requires a correspondingly greater effort, so that the values may notbecome generally available for a year or more. (Kp, on the other hand, can beobtained through the World Data Centers within a few days.)


Figure 2.25. AU, AL,AE, and AO indices for12 and 14 June 1988, thefirst a quiet day (Ap4)and the second a dis-turbed one (Ap20).(Data Book 23,September 1994, WorldData Center-C2, KyotoUniversity, Kyoto.)

The history of magnetic indices and their derivations, advantages, and disad-vantages are discussed in detail by Maynaud (1980). Some of the older indicescontinue to be of interest and are still produced to maintain continuity. The Cindex, one of the first, is a simple character index in which 0, 1, and 2 mean simply“quiet”, “moderately disturbed” and “disturbed”, respectively. The R and Qindices are range indices like K, but are derived at hourly and 15-min intervals,respectively, instead of every 3 h.

We shall refer frequently to magnetic indices of one variety or another whendealing in later chapters with the behavior of the disturbed ionosphere at high lat-itude. They are the common currency of geophysical disturbance, and are usefulbeyond the limited topic of magnetic disturbance because of their ease of meas-urement and the long runs of values accumulated over the years.

2.5.5 Great magnetic storms and a case history

Some storms are so intense and their effects so dramatic that they attract both sci-entific and popular attention. Though such storms are rare, they serve to illustratehow great the effects can be in extreme cases. Table 2.2 gives the top ten magneticstorms of modern times, ranked in order of the maximum Ap occurring during thestorm. In terms of the equatorial index Dst, which measures the strength of thering current (and comes out with a negative value, since the H component isreduced), the greatest storm was that of 13 March 1989. (Values are not availablefor the earliest storms of Table 2.2 because Dst has been derived only since 1957.)

From more extended lists of great storms it has been noted that (1) most ofthem occurred after solar maximum rather than at the maximum or before it; and(2) more than half occurred during the four months of the year nearest the equi-noxes: that is, during March, April, September, and October.


Table 2.2. The top ten magnetic storms of modern times (after J. A. Joslyn,private communication)

Order Date Maximum Dst Ap Solar cycle

1 13 Nov 1960 301 280 192 13 Mar 1989 599 246 223 1 Apr 1960 327 241 194 15 Jul 1959 429 236 195 18 Sep 1941 230 176 5 Jul 1941 220 177 28 Mar 1946 215 188 1 Mar 1941 205 179 6 Oct 1960 287 (7th) 203 19

10 8 Feb 1986 307 202 21

The storm of 13 March 1989

The storm of March 1989 (Joslyn, 1990), the largest or next to largest on record –depending on the criterion applied – had some quite remarkable effects. It wasrelated to the largest group of sunspots to be seen on the solar disk since 1982, andthe effects were not only magnetic but were also detected in the neutral atmos-phere, in the ionosphere and on radio communications, as auroral displays inunusual places, and on electric-power transmission.

Magnetic effects

The storm began with a sudden commencement at 0127 UT on 13 March, andlater that day the magnetic deviation at one mid-latitude station (Boulder,Colorado) amounted to more than 1300 nT. This is nearly three times the typicaldeviation for a K index of 9, and clearly this storm went well beyond the normalrange of measurement for magnetic storms. The Ap index for 13 March was 246(which is the second largest value recorded during the 57 years since that indexwas commenced in 1932), and the Dst index determined from equatorial iono-grams reached almost 600 nT at one time. The storm continued for about twodays.

The magnetic variations were large enough to have serious effects on magneticprospecting. Whereas geophysical exploration techniques are concerned withvariations of half a degree, in Alaska the magnetic declination varied by as muchas 5°. Analysis of magnetometer data showed that, towards the end of 13 March,the electrojet (normally considered an auroral electrojet) was flowing south ofFredericksburg, Virginia, whose geomagnetic latitude is 49° N.

It was reported from Alaska that the flight of homing pigeons was affected.

The aurora, magnetosphere, and solar wind

Aurorae were reported at unusually low latitude in several countries. Over thewestern hemisphere, a red aurora was observed as far south as Florida, Mexico,and Grand Cayman Island. The key to the apparent displacement of the auroralzone, evidenced by the electrojet as well as by the luminosity, is provided bymagnetic-field measurements on geosynchronous satellites. GOES-6 and GOES-7, both at 6.6RE, left the magnetosphere and entered the solar wind between 0700and 0800 LT on 13 March. Making a reasonable assumption about the shape ofthe magnetosphere, it was deduced that the magnetopause at noon was at 4.7RE

instead of the usual 10RE. Clearly the magnetosphere was so compressed that itsvarious internal as well as external boundaries moved to unusual places.

Unfortunately, there were no direct measurements in the solar wind during thisstorm. Section 2.2.3 showed that the position of the magnetopause is related tothe pressure of the solar wind (Equation (2.6)); in the simplest case the distancedepends on the inverse sixth root of the pressure (2Nmv2). Therefore, if the sub-solar magnetosphere moved in from 10RE to 4.7Re, the solar-wind pressure would


have had to increase by a factor of 60. Some one RE of the movement might beattributable to the southward component of the IMF (Section 2.2.3), but even soone can say with reasonable certainty that the solar-wind pressure must haveincreased by at least a factor of (10/5.7)630 on 13 March.

The ionosphere

There was also a severe ionospheric storm on that day. The mid-latitude electroncontent in the night sector was unusually low immediately after the commence-ment. After sunrise it remained at essentially night-time values for most of the dayand then it returned rapidly to a daytime value just before sunset. The equatorialionosphere virtually disappeared, and HF radio communications were practicallyimpossible over many circuits, particularly those involving high latitudes.However, VHF communication, which is normally restricted to line-of-sight prop-agation, was achieved over remarkably long distances due to high-latitude spo-radic-E. An analysis of the remarkable world-wide ionospheric effects waspresented by Yeh et al. (1992).

Satellite drag

The main effect on the neutral upper atmosphere was an increase in air density(and thus an increase in satellite drag) resulting from the heating of the atmos-phere. Those whose work it is to track satellites found themselves with many moreexamples than usual of objects that could not immediately be identified becausethey were not in the places where they were expected. (In general, magnetic stormsincrease the rate of decay of satellites in orbit and cause them to re-enter theatmosphere sooner than predicted.)

Electric-power distribution

Perhaps the most serious consequence of this storm, however, was its effect onelectric-power distribution. It is known that fluctuations in the geomagnetic fieldinduce currents in long metallic lines (both power lines and oil pipelines). Inpower-distribution systems these may cause the voltage to surge, saturating trans-formers and tripping protective relays. The electric-power system of Québecsuffered a power black-out lasting 9 h on 13 March. Users in the north-easternUSA were also affected. There was a loss of voltage on several power-distributionlines in Sweden.

Great storms may be infrequent, but they attract interest as extreme cases ofthe storm phenomena requiring scientific explanation, and because they may haveserious effects on a number of practical activities.


2.5.6 Wave phenomena of the magnetosphere

Hydromagnetic and magnetosonic waves

In a sound wave the restoring force is due to the compressibility of the medium;in the case of a gas, its pressure. At the lowest frequencies gravity can also be sig-nificant, giving the acoustic-gravity wave (Section 1.6). In a magnetic field anotherrestoring force comes into play, and that is the magnetic pressure across the fieldand the magnetic tension along the field-lines.

The situation in the magnetosphere is that the ionization may not cross field-lines, and therefore in transverse motions they must move together. Ordinarysound waves are allowed along the field-lines because the gas displacement is lon-gitudinal, but in waves transverse to the field both the gas pressure and the mag-netic pressure must be included. This combination makes possible a range ofhydromagnetic waves.

The basic hydromagnetic wave is the Alfvén wave, which propagates along themagnetic field but whose displacement is transverse. The Alfvén wave is analogousto the transverse wave on a taut string, the tension being the magnetic tension(B2/0), and the mass per unit length being simply the mass density of the plasma.The speed of the transverse wave is then given by Equation (2.7):

vA ,

where B is the magnetic flux density, 0 the permeability of free space, and thedensity of the plasma in kg m3.

The Alfvén wave and the sonic wave are independent when they are travelingalong the field direction, but at other angles they interact to give mixed magneto-sonic waves. There are two such waves in general, except that perpendicular to thefield there is only one, having speed (v2

As2)1/2, s being the speed of sound.

Micropulsations

If the sensitivity of a magnetometer is increased sufficiently, small fluctuations ofthe geomagnetic field with periodicites of minutes and seconds can be detected.These are micropulsations. They are due not to electromagnetic waves but tohydromagnetic waves in the magnetosphere, and in magnitude they are less than104 of the total geomagnetic field. Their connection with ionospheric radio issomewhat indirect, but an introduction is in order because they comprise a signifi-cant phenomenon of the high-latitude ionosphere. In some cases they are con-nected with auroral activity, and there are also some diagnostic applicationsindicating conditions in the magnetosphere.

Micropulsations are classified according to period and duration, as in Table2.3. The impulsive variety (Pi) occurs mainly in the evening, whereas the more

B(0)1/2


regular and persistent regular pulsations (Pc) prefer the morning and the daylighthours.

The magnetospheric origin of micropulsations is demonstrated by their simi-larity in magnetically conjugate regions, but a variety of mechanisms is involvedin their generation. Pc pulsations are generated either at the surface of the mag-netosphere or within it, and they propagate in a hydromagnetic mode. Pc1 areattributed to bunches of protons (probably) traveling back and forth betweenmirror points (Section 2.3.4). A resonance between protons and ion-cyclotronwaves, which rotate in the same sense in the geomagnetic field, is probablyinvolved. Pc2–5 are explained as various modes of oscillation within the magnet-osphere, some propagating across and some along the field lines. The period ofPc3 and 4 may be interpreted in terms of Alfvén waves, whose speed depends onthe plasma density and the magnetic field strength. The characteristic frequencychanges across the plasmapause (Section 2.3.2) due to the sharp change of parti-cle density.

The topic of micropulsations was discussed in detail by Jacobs (1970).

Instabilities

The interaction of magnetospheric waves with the particle population of the mag-netosphere is a complex subject that we can no more than indicate here. For iono-spheric physics its basic importance is that waves and particles may exchangeenergy, and that this process can become unstable. For example, trapped electronsgenerate whistler-mode waves in the VLF band (Section 3.4.7), which, under theright conditions, may then interact with the population of trapped electrons, scat-tering them into the loss cone (Section 2.3.4). This is a mechanism that, thereby,may cause the spontaneous precipitation of trapped electrons into the atmos-phere. There is a large literature on wave–particle interactions in the magneto-sphere. The interested reader might like to start with Lyons and Williams (1984),Chapter 5.

One purely ionospheric instability is the two-stream, or Farley–Buneman,instability, which produces electrostatic waves in the E-region electrojet when


Table 2.3. Micropulsations

Continuous and regular Irregular

Type Period (s) Type Period (s)

Pc1 0.2–5 Pi1 1–40Pc2 5–10 Pi2 40–150Pc3 10–45Pc4 45–150Pc5 150–600

streams of ions and electrons differ in velocity by more than a critical amount.This is the mechanism causing the irregularities in ionization that make auroralradar possible. That topic is discussed in Sections 3.5.1, 4.2.2, and 6.5.5.

The Kelvin–Helmholz instability is a commonplace phenonenon, being thecause of waves on the surface of a pond on a windy day. It works because any pro-jection above the level surface alters the air flow in such a way as to increase theperturbation – a simple case of positive feedback. The magnetosphere also hasinterfaces, the most obvious being that with the solar wind at the magnetopause,and Kelvin–Helmholtz waves and vortices may be produced there.

A slightly less superficial introduction to magnetospheric waves can be foundin Hargreaves (1992), Chapter 9.

2.6 Ionization by energetic particles

The main source of ionization in the upper atmosphere is solar radiation in the X-ray and EUV bands. There is, however, another source of a quite different kind,namely energetic particles. Although they are not entirely absent from middle lat-itudes, these are much more important at high latitudes, where they may at timesbecome the main source of ionization. As we shall see in later chapters, two verysignificant sources at high latitude are electrons associated with the aurora, andprotons (plus some -particles) emitted from the Sun during some solar flares.

2.6.1 Electrons

Various methods have been used to calculate the rate of ion production by astream of energetic electrons arriving from some source above the atmosphere.The most generally useful one relies on laboratory measurements of the range ofelectrons in air. An electron loses energy to the neutral gas particles with which itcollides, and the rate of loss obviously depends on the number of gas particlesencountered. Thus, in a uniform atmosphere the distance traveled varies in inverseproportion to the gas pressure. The unit of range (r0) is therefore [pressure] [dis-tance]: atm cm, for example. The energy goes into exciting and ionizing the neutralparticles. In this instance we are interested in the ionization.

An energetic particle entering the atmosphere from above travels into amedium of increasing density, and the altitude, hp, to which it penetrates is suchthat the product of pressure and distance, integrated above hp, is equal to the ranger0. Obviously, this particle will ionize only above height hp, and the total numberof ion–electron pairs produced will depend on E/E, where E is the initial energyof the particle and E is the energy required for each ionization (generally takenas 34 or 35 eV).

The third fact which has to be taken into account is that the rate of energyloss, and therefore of ion production, along the path is a function of the particle’s


velocity. To quantify this, laboratory measurements are again brought to bear inthe form of an “efficiency”, which is a function of the atmospheric depth at a pointalong the track divided by that at the point where the particle eventually stops; i.e.s/sp. (The atmospheric depth is the total mass of gas in a column of unit cross-section along the path of the particle.) The efficiency is normalized to unity overthe whole path (from s/sp0 to s/sp1), and it comes to a maximum at s/sp0.4for a monoenergetic electron beam traveling directly along the magnetic field. Ifthe electrons arrive over a range of angles, as would be the case in nature, someparticles travel in spiral paths and so cover a greater distance; in this case the effi-ciency peaks at a smaller value of s/sp such as 0.1 or 0.2. These factors clearly influ-ence the distribution of ion production in the atmosphere, and the height of themaximum production rate in particular.

Figure 2.26 shows calculated production rates in a model atmosphere due tomonoenergetic electrons of various initial energies. Note that the production ratepeaks at lower altitude and the distribution is narrower for higher initial enery. Toget the effect of a more realistic spectrum, the production rate must be integratedover energy.

2.6.2 Bremsstrahlung X-rays

When energetic electrons collide with neutral gas particles a small amount of theirenergy is converted to X-rays through the Bremsstrahlung process – literally“braking radiation” – as they are rapidly decelerated. The X-rays penetrate deeperinto the atmosphere than do the primary electrons and may be observed byballoon-borne detectors at heights of 30–40 km. Some X-rays are also scatteredback out of the atmosphere and may be detected on satellites.


Figure 2.26. Productionrates due to monoener-getic electrons of variousinitial energies. (After M.H. Rees, Planet. SpaceSci. 11, 1209, Copyright1963, with permissionfrom Elsevier Science.)

The computation of the Bremsstrahlung X-ray flux is fairly complicated, sincean electron of energy E can produce photons of energy E or less, and the X-raysare emitted over a wide range of angles. Inverting an observed X-ray spectrum togive the spectrum of the primary electrons is even more difficult. The usual prac-tice is to draw on a set of computations giving the Bremsstrahlung due to singleelectrons of specified energy, but even so it is usually neccessary to assume a formfor the spectrum (e.g. exponential).

When the X-rays are stopped by the atmosphere they create ionization at thatlevel. The ionization rate due to Bremsstrahlung is several factors of ten smallerthan that due to the primary electrons higher up, but, at the height concerned, pos-sibly 50 km or below, it is the major source of ionization at times of auroral elec-tron precipitation. Table 2.4 compares the heights and maximum production ratesdue to direct and Bremsstrahlung ionization for several initial electron energies.

Figure 2.27 illustrates the relative altitude and magnitude of direct and X-rayionization (actually the rate of deposition of energy) due to a spectrum of incidentelectrons with characteristic energy 10 keV.

2.6.3 Protons

Significant ionization may also be caused by energetic protons, especially at highlatitudes during polar-cap events, which are due to fluxes of protons released fromthe Sun at the time of a solar flare. A lesser flux of -particles will generally arrivesimultaneously. These particles, which are significantly more energetic than theauroral electrons discussed above, lose energy in colliding with the atmospheric gasand leave ionized trails. The gas concerned is principally that of the mesosphere,whose composition is essentially like that of the troposphere, and therefore the rateof energy loss is well known from laboratory measurements. A graph showing therate of energy loss against the distance traveled is called a Bragg curve. In the energyrange of interest to us the loss rate increases as the proton slows down, and, overthe range 10–200 MeV, the loss rate is almost inversely proportional to the energy,a typical value being 0.8 MeV per meter of path in air at standard temperature and


Table 2.4. Direct and X-ray ion production

Height of Maximummaximum production production rate

(km) (ion pairs cm3 per electron)

E (keV) Direct X-ray Direct X-ray

3 126 88 2.5105 5.91010

10 108 70 1.4104 1.3108

30 94 48 5.6104 2.3107

100 84 37 1.9103 1.3105

pressure when the energy is 100 MeV. The energy may be assumed to be usedentirely in creating ion–electron pairs, each requiring about 35 eV.

The nature of the Bragg curve, combined with the density distribution of theatmosphere, means that the ionization due to a proton entering the atmospherefrom space is very concentrated towards the end of the path. A vertically incident50 MeV proton, for example, loses half its energy over the last 2.5 km of the pathand the last 10% over only 100 m. One consequence is that the penetration leveldoes not depend strongly on the angle of incidence except near 90°. Production-rate profiles for protons of various initial energies are given in Figure 2.28. Notethe low altitudes which may be reached by the more energetic particles. For a spec-trum of proton energies the total effect would be calculated by appropriatesumming over these curves at each height.

There is a similar procedure for dealing with the ionization by -particles.


Figure 2.27. A comparison of direct and X-ray energy-deposition rates from an incomingelectron flux with an exponential spectrum of characteristic energy 10 keV. The solid line (J.G. Luhmann, J. Atmos. Terr. Phys. 38, 605, 1976) shows the deposition by electron impact.The dashed line (M. J. Berger et al., J. Atmos. Terr. Phys. 36, 591, 1974) and the circles (J.G. Luhmann, J. Atmos. Terr. Phys. 39, 595, 1977) include the energy deposited byBremsstrahlung X-rays. (After Luhmann 1977, Copyright, with permission from ElsevierScience.)

140

130

120

110

100

90

80

70

60

50

40

30

20

10–12 –11 –10 –9 –8 –7 –6 –5

Log of Energy Deposition Rate (in keV cm–3 s–1)

Alti

tude

(km

)


2.2 The magnetosphereBurlaga, L. F. (1982) Magnetic fields in the interplanetary medium. Solar SystemPlasmas and Fields (eds. J. Lemaire and M. J. Rycroft), p. 51. Pergamon Press, Oxford.Carpenter, D. L. and Smith, R. L. (1964) Whistler measurements of electron density inthe magnetosphere, Rev. Geophys. 2, 415.Gosling, J. T. (1972) Predicting the solar wind speed. Solar Activity Observations andPredictions (eds. P. S. McIntosh and M. Dryer), p. 231. MIT Press, CambridgeMassachusetts.Holzer, T. E. (1979) The solar wind and related astrophysical phenomena. SolarSystem Plasma Physics (eds. C. F. Kennel, L. J. Lanzerotti and E. N. Parker), Vol. I, p.103. Elsevier Science Publishers, Amsterdam.Pertinec, S. M. and Russell, C. T. (1993) External and internal influences on the size ofthe dayside terrestrial magnetosphere. Geophys. Res. Lett. 20, 339.Raitt, W. J. and Schunk, R. W. (1983) Composition and characteristics of the polarwind. Energetic Ion Composition in the Earth’s Magnetosphere (ed. R. G. Johnson), p.99. Terra Scientific Publishing Co., Tokyo.Rich, F. J. and Hairston, M. (1994) Large-scale convection patterns observed byDMSP. J. Geophys. Res. 99, 3827.Smith, E. J., Tsurutani, B. T. and Rosenberg, R. L. (1978) Observations of the inter-planetary sector structure up to heliographic latitudes of 16°: Pioneer 11. J. Geophys.Res. 83, 717.Vasyliunas, V. M. (1983) Large-scale morphology of the magnetosphere.Solar–Terrestrial Physics (eds. R. L. Carovillano and J. M. Forbes), p. 243. Reidel,Dordrecht.


Figure 2.28. Production rates due to incident monoenergetic protons. The initial energiesare given in MeV, and in each case the flux is 1 proton cm2 s1 sr1. (G. C. Reid,Fundamentals Cosmic Phys. 1, 167, 1974. Copyright OPA (Overseas PublishersAssociation) NV, with the permission of Gordon and Breach Publishers.)

Wilcox, J. M. and Ness, N. F. (1965) Quasi-stationary corotating structure in the inter-planetary medium. J. Geophys. Res. 70, 5793.

2.3 Particles in the magnetosphereBurch, J. L. (1977) The magnetosphere. Upper Atmosphere and Magnetosphere, p. 42.National Academy of Sciences, Washington DC.Carpenter, D. L. and Anderson, R. R. (1992) An ISEE/whistler model of equatorialelectron density in the magnetosphere. J. Geophys. Res. 97, 1097.Chappell, C. R., Harris, K. K. and Sharp, G. W. (1970) A study of the influence ofmagnetic activity on the location of the plasmapause as measured by OGO-5.J. Geophys. Res. 75, 50.Helliwell, R. A. (1976) Whistlers and Related Ionospheric Phenomena. StanfordUniversity Press, Stanford, California.Iijima, T. and Potemra, T. A. (1978) Large-scale characteristics of field-aligned cur-rents associated with substorms. J. Geophys. Res. 83, 599.Potemra, T. A. (1983) Magnetospheric currents. Johns Hopkins APL Tech. Digest 4, 276.Ratcliffe, J. A. (1972) An Introduction to the Ionosphere and Magnetosphere.Cambridge University Press, Cambridge.Van Allen, J. A. (1959) The geomagnetically trapped corpuscular radiation. J.Geophys. Res. 64, 1683.Walt, M. (1994) Introduction to Geomagnetically Trapped Radiation. CambridgeUniversity Press, Cambridge.Williams, D. J. (1985) Dynamics of the Earth’s ring current: theory and observation.Space Sci. Rev. 42, 375.

2.4 Dynamics of the magnetosphereDungey, J. W. (1963) The structure of the exosphere or adventures in velocity space. InGeophysics, The Earth’s Environment (eds. C. De Witt, J. Hieblot, and A. Lebeau).Gordon and Breach, New York.Levy, R. H., Petschek, H. E., and Siscoe, G. L. (1964) Aerodynamic aspects of themagnetospheric flow. Am. Inst. Aeronaut. Astronaut. J. 2, 2065.Nishida, A. (1966) Formation of plasmapause, or magnetospheric plasma knee, by thecombined action of magnetospheric convection and plasma escape from the tail.J. Geophys. Res. 71, 5669.Petschek, H. E. (1964) Magnetic field annihilation. The Physics of Solar Flares (ed. W.N. Hess), Report SP-50, p. 425. NASA, Washington DC.Ratcliffe, J. A. (1972) An Introduction to the Ionosphere and Magnetosphere.Cambridge University Press, Cambridge.Russell, C. T. (1972) The configuration of the magnetosphere. Critical Problems ofMagnetospheric Physics, p.1. IUCSTP Secretariat, National Academy of Science,Washington D.C.

2.5 Magnetic stormsAkasofu, S.-I. (1968) Polar and Magnetospheric Substorms. Reidel, Dordrecht.Davis, T. N. and Sugiura, M. (1966) Auroral electrojet activity index AE and its uni-versal time variations. J. Geophys. Res. 71, 785.Hargreaves, J. K. (1992) The Solar–Terrestrial Environment. Cambridge UniversityPress, Cambridge.


Jacobs, J. A. (1970) Geomagnetic Micropulsations. Springer-Verlag, Berlin.Joslyn, J. A. (1990) Case study of the great magnetic storm of 13 March 1989.Astrodynamics (eds. Thornton, Proulx, Prussing and Hoots).Lyons, L. R. and Williams, D. J. (1984) Quantitative Aspects of MagnetosphericPhysics. Reidel, Dordrecht.Maynaud, P. N. (1980) Derivation, Meaning and Use of Geomagnetic Indices.American Geophysical Union, New York.Sugiura, M. and Chapman, S. (1960) The average morphology of geomagnetic stormswith sudden commencement. Abhandl. Akad. Wiss. Göttingen Math.-Phys. Kl. SpecialIssue 4, 53.Yeh, K. C., Lin, K. H. and Conkright, R. O. (1992) The global behavior of the March1989 ionospheric storm. Can. J. Phys. 70, 532.

2.6 Ionization by energetic particlesBerger, M. J., Seltzer, S. M. and Maeda, K. (1974) Some new results on electron trans-port in the atmosphere. J. Atmos. Terr. Phys. 36, 591.Luhmann, J. G. (1976) Auroral electron spectra in the atmosphere. J. Atmos. Terr.Phys. 38, 605.Luhmann, J. G. (1977) Auroral bremsstrahlung spectra in the atmosphere. J. Atmos.Terr. Phys. 39, 595.Rees, M. H. (1963) Auroral ionization and excitation by incident energetic electrons.Planet. Space Sci. 11, 1209.Reid, G. C. (1974) Polar-cap absorption – observations and theory. FundamentalsCosmic Phys. 1, 167.

General reading on the topics of Chapter 2

BooksAkasofu, S.-I. and Chapman, S. (1972) Solar–Terrestrial Physics. Oxford UniversityPress, Oxford.Baumjohann, W. and Treumann, R. A. (1996) Basic Space Plasma Physics. ImperialCollege Press, London.Carovillano, R. L. and Forbes, J. M. (eds.) (1983) Solar–Terrestrial Physics. Reidel,Dordrecht.Carovillano, R. L., McClay, J. F. and Radoski, H. R. (1968) Physics of theMagnetosphere. Springer-Verlag, New York.Hess, W. N. (1968) The Radiation Belt and Magnetosphere. Blaisdell, Waltham,Massachusetts.Hess, W. N. and Mead, G. D. (eds.) (1968) Introduction to Space Science. Gordon andBreach, New York.Hundhausen, A. J. (1972) Coronal Expansion and Solar Wind. Springer-Verlag, NewYork.Jacobs, J. A. (1970) Geomagnetic Micropulsations. Springer-Verlag, New York.Jursa, A. S. (ed.). (1985) Handbook of Geophysics and the Space Environment. AirForce Geophysics Laboratory, US Air Force, National Technical Information Service,Springfield, Virginia.Kamide, Y. (1988) Electrodynamic Processes in the Earth’s Ionosphere andMagnetosphere. Kyoto Sangyo University Press, Kyoto.


Le Galley, D. P. and Rosen, A. (eds) (1964) Space Physics. Wiley, New York.Lyons, L. R. and Williams, D. J. (1984) Quantitative Aspects of MagnetosphericPhysics. Reidel, Dordrecht.Nishida, A. (ed.) (1982) Magnetospheric Plasma Physics. Reidel, Dordrecht.Parks, G. K. (1991) Physics of Space Plasmas. Addison-Wesley Publishing Co.,Redwood City, California.Roederer, J. G. (1974) Dynamics of Geomagnetically Trapped Radiation. Springer-Verlag, Berlin.Schulz, M. and Lanzerotti, L. J. (1974) Particle Diffusion in the Radiation Belts.Springer-Verlag, New York.Treumann, R. A. and Baumjohann, W. (1997) Advanced Space Plasma Physics.Imperial College Press, London.

Conference reportsAkasofu, S.-I. (ed.) (1980) Dynamics of the Magnetosphere. Reidel, Dordrecht.Beynon, W. J. G., Boyd, R. L. F., Cowley, S. W. H. and Rycroft, M. J. (1989) TheMagnetosphere, the High-Latitude Ionosphere, and their Interactions. The RoyalSociety, London.Johnson, R. G. (ed.) (1983) Energetic Ion Composition in the Earth’s Magnetosphere.Terra Scientific Publishing Co., Tokyo.Kamide, Y. and Slavin, J. A. (eds.) (1986) Solar Wind–Magnetosphere Coupling. TerraScientific Publishing Co., Tokyo.King, J. W. and Newman, W. S. (eds.) (1967) Solar–Terrestrial Physics. AcademicPress, London.Lemaire, J. F., Heynderickx, D. and Baker, D. N. (eds.) (1996) Radiation Belts: Modelsand Standards. American Geophysical Union, Washington DC.McCormac, B. M. (ed.) (1966) Radiation Trapped in the Earth’s Magnetic Field.Reidel, Dordrecht.McCormac, B. M. (ed.) (1968) Earth’s Particles and Fields. Reinhold, New York.McCormac, B. M. (ed.) (1970) Particles and Fields in the Magnetosphere. Reidel,Dordrecht.McCormac, B. M. (ed.) (1972) Earth’s Magnetospheric Processes. Reidel, Dordrecht.McCormac, B. M. (ed.) (1974) Magnetospheric Physics. Reidel, Dordrecht.McCormac, B. M. (ed.) (1976) Magnetospheric Particles and Fields. Reidel, Dordrecht.Olsen, W. P. (ed.) (1979) Quantitative Modeling of Magnetopheric Processes. AmericanGeophysical Union, Washington DC.Potemra, T. A. (ed.) (1984) Magnetospheric Currents. American Geophysical Union,Washington DC.Song, P., Sonnerup, B. U. O. and Thomsen, M. F. (eds.) (1995) Physics of theMagnetopause. American Geophysical Union, Washington DC.Tsurutani, B. T., Gonzalez, W. D., Kamide Y. and Arballo, J. K. (1997) MagneticStorms. American Geophysical Union, Washington DC.


Chapter 3

Fundamentals of terrestrial radio propagation

3.1 Introduction

Since we are concerned with the propagation of radio waves over the entire radiospectrum at high latitudes, it should be useful to review the basic physics and ter-minology of the propagation of radio waves in general. The radio spectrumextends from the extra-low-frequencies (ELF) band through microwaves andmillimeter waves. Table 3.1 shows the radio spectrum from 30 Hz to 30 GHz,along with the International Telecommunications Union (ITU) band designa-tions.

3.2 Electromagnetic radiation

3.2.1 Basics of line-of-sight propagation in vacuo

An example of line-of-sight (LOS) propagation is that between two spacecraft indeep space where the medium is virtually a vacuum. The refractive index is unityand the speed of an electromagnetic (EM) wave is independent of its frequencyand equal to the speed of light.

By definition, an isotropic radiator is one that radiates equally in all directions.If power P is radiated, the power density S (the power crossing unit area) at a dis-tance d from the source is

SP/(4d 2), (3.1)

113

From EM theory, the vector S is the Poynting flux, given by

SEH, (3.2)

114 Fundamentals of propagation

Table 3.1. The radio spectrum (as defined by the InternationalTelecommunications Union (ITU)), primary modes of propagation, and effects ofthe terrestrial ionosphere

Principal ITU designation Frequency range modes of propagation Principal uses

Extra-low- 30–300 Hz Groundwave and Submarine frequency (ELF) Earth–ionosphere communication

waveguide mode

Very-low- 3–30 kHz Same as above Navigation, frequency (VLF) standard frequency

and timedissemination

Low-frequency 30–300 kHz Same as above Navigation (LF) LORAN-Ca

Medium- 300–3000 kHz Primarily groundwave, AM broadcasting, frequency (MF) but skywaveb at night maritime,

aeronauticalcommunication

High-frequency 3–30 MHz Primarily skywave, Shortwave (HF) some groundwave broadcasting,

amateur, fixedservices

Very-high- 30–300 MHz Primarily LOS, some FM broadcasting, frequency (VHF) skywave at lower VHF television,

aeronauticalcommunication

Ultra-high-(UHF) 300–3000 MHz Primarily LOS, some Television, radar, frequency refraction and navigationc,

scattering by the aeronautical ionosphere communication

Super-high-(SHF) 3–30 GHz Same as above Radar, spacefrequency communication

Notes:a The LORAN-C system will probably be superseded by the GPS system.b “Skywave” denotes the Earth–ionosphere–Earth-reflected mode.c Global Positioning System of satellite constellation.

E being the electric vector and H the magnetic vector. Since

E /H120 (3.3)

for an EM wave, E and H being, respectively, the electric and magnetic fieldstrengths,

SE2/(120). (3.4)

Therefore,

E . (3.5a)

In SI units, P is in watts, d in meters, S in W m2, and E in V m1. It may be morepractical to express d in kilometers, P in kilowatts, and E in mV m1, in which case

E (mV m1)173 /d (km). (3.5b)

If the antenna does not radiate isotropically, it is said to have a gain (G), given bythe ratio of the Poynting flux at a point on the axis divided by the flux that wouldbe received at the same point if the same power were radiated instead from an iso-tropic radiator. If an antenna with gain Gt transmits power Pt and the receivingantenna has aperture Ar (m

2 ), the power received is

PrAr SAr Gt Pt/(4d 2) (3.6)

and

Er /d. (3.7)

Antenna theory shows that gain and aperture are related by

G4A/2, (3.8)

in which A is the true aperture if the antenna has the form of an efficient dish,but may be an effective area otherwise. An isotropic radiator (which is hypotheti-cal in any case for an EM wave) has unity gain and effective area 2/(4). Fora half-wave dipole, which may be taken as the reference, G1.64 and

A1.642/(4)0.13052.

In a point-to-point link it is often convenient to represent the reduction ofsignal due to the separation (d ) between transmitting and receiving antennas asthe free-space attenuation,

Lb20 log(4d /), (3.9)

which follows from Equations (3.6) and (3.8) assuming that both antennas areisotropic radiators. The gain (sometimes called directivity) is given approximatelyby

G30000/(), (3.10)

30PtArGt

P (kW)

30P/d


where and are the half-power beamwidths (in degrees) in the E and H planes,respectively, assuming that there are no sidelobes. The formula applies only up to20° beamwidth.

Although it is an important topic in radio propagation, a full discussion ofantennas would be outside the scope of this book. Some of the many treatmentsare listed in the references. Information on radiation patterns and advice on sitingare contained in the publications of the American Radio Relay League (ARRL)in the references. For detailed discussions of Fresnel-zone siting fundamentals,bandwidth, and terrain effects see Appendix A7 of Hunsucker (1991), Freeman(1997), or Wolff (1988). Computer programs for antenna design and performanceanalysis are listed in Table 3.2 and in Balanis (1997).

3.2.2 Principles of radar

In radar a transmitted signal is reflected from a target and then detected by areceiver, which may but need not be co-located with the transmitter. These aremonostatic and bistatic systems, respectively. The target may be a solid object(Figure 3.1) or a distributed scattering medium (as in coherent and incoherent


Table 3.2. Antenna design and performance-analysis programs

Name of software Description Source

NEC Numerical electromagnetic code

NEC/WIRES 1.5 One version of NEC Brian Beezley3532 Linda Vista Dr., San Marcos, CA 92069, USA

NEC/Yagis 2.0 Uses NEC to model Yagis and ″ ″arrays of Yagis

YO 6.0 Optimizes Yagi–Uda designs ″ ″

AO 6.0 Optimizes antenna designs for ″ ″any wire-or tubing-type antenna

ELNEC

MININEC

GAP, BIA, ACP, and General Antenna COMSAT antenna lab suitePhased Array Program, Beam IntermodulationProgram Analyzer, Antenna Coverage http://www.comsat.

Program, and Phased Array Com/Corp/lab/labs.htmlProgram

XFDTD 4.0 User-friendly electromagnetics REMCOM Inc.software, covers more esoteric http://www.remcominc.comantennas, scattering, etc.

scatter radars – Sections 4.2.2 and 4.2.3). A treatment of radar begins with theradar equation.

If power Pt is radiated by the transmitter using an antenna with gain G, thepower density at a target at distance R is (Equation (3.6))

SGPt/(4R2). (3.11)

If the target has cross-section , and the power intercepted is scattered equally inall directions, the power received back at the radar is

PrGPt/(4R2)Ae/(4R2)

PrGPt Ae /[(4)2 R4] (3.12)

where Ae is the effective area of the radar antenna. (If the scattering is not omni-directional, this is taken into account in the value of .) From Equation(3.8) wemay also write the radar equation as

G 22 / (642 R4). (3.13)

The distance beyond which the target cannot be detected is the maximum radarrange, Rmax, and the limit is when the received echo power, Pr, just equals theminimum detectable signal, Smin. Hence (from Equation (3.12)),

Pr

Pt

G 2

4

2

4R2


Figure 3.1. A schematic diagram of the radar principle.

R

∆R

Ae

RmaxPtGAe/[(4)2 Smin]1/4, (3.14a)

which is the most common form of the radar range equation.Using Equation (3.8) gives the alternative forms

RmaxPt G2 2 /[(4)3 Smin]1/4 (3.14b)

and

Rmax[Pt Ae2 /(42 Smin )]

1/4. (3.14c)

The foregoing discussion applies to situations like that in Figure 3.1, where thetarget is smaller than the transmitter beamwidth. The larger the target, the morepower is returned. If the scattering region is larger than the beamwidth (a beam-filling target), as may happen when the ionosphere is the target, all the incidentpower is intercepted and then the expression for the echo power received back atthe radar has the form

PrPt Ae/(4 R2), (3.15)

where represents the scattering property of the target medium. The echo powernow varies as R2 instead of R4. If the ionosphere is the target, the return wouldprobably come from a large number of individual scatterers, and the wouldinclude the number of scatterers within the radar pulse and beamwidth at any onetime, as well as their directional properties.

The physical length of the transmitted pulse (Figure 3.1) is

Rc, (3.16)

c being the speed of light in vacuo and the pulse duration. The resolution in rangeis R /2.

Discussions of the various forms of the radar equation and their implications,including theorems applicable to “soft targets,” are given by Skolnik (1980) andby Hunsucker (1991; pp. 38–39).

3.2.3 The significance of the refractive index

A simple propagating wave

If a radio source generates an electric field EE0cos(t), which propagates atspeed v in the z direction, the field at a distance z from the source is


EE0cos[(tz/v)]

E E0cos[(tkz)]

E E0cos[2(t /T z/)] (3.17)

since 2f, vf, and k2/ by definition. T is the period, and f are thefrequency in radians s1 and hertz, respectively, and k is the wave number, propa-gation constant, or phase-shift factor. E is the instantaneous value of the electricfield at (t, z) and E0 is the amplitude of the electric field. Plainly, the same phaserepeats itself every T (1/f ) in time and every 2/k in distance. For the prop-agation of a plane wave in three dimensions, k can be regarded as a vector alongthe propagation direction, having components kx , ky, and kz that give the wave-lengths in the x, y, and z directions and thus the phase velocities vxx f, vyy f,and vzz f.

The refractive index

We will use vp to denote the phase velocity, and for an EM wave its value dependson the nature of the medium;

vp1/( )1/2, (3.18)

where and are the permeability and permittivity of the medium. In free spacethis becomes

c1/(0 0)1/23108 m s1, (3.19)

where 0 and 0 are, respectively, the permeability and permittivity of free space.The ratio nc/v is the refractive index of the medium, and the propagating wavemay then be written

EE0cos(tnz/c). (3.20)

If the refractive index varies with the wave frequency, the medium is said to be dis-persive. A modulated wave is not monochromatic, and in a dispersive medium themodulation travels not at the phase velocity but at the group velocity (u), which isrelated to the phase velocity by

u(k/)1. (3.21)

Only if vp is independent of , so that k/vp, does uvp.

Propagation in a lossy medium

If the medium absorbs energy from the wave, the amplitude decreases with dis-tance as exp(!z), where ! is the absorption coefficient, and the amplitude


decreases by a factor of e over a distance 1/!. It is convenient here to use the j nota-tion, writing

EE0exp[ j(tnz/c)], (3.22)

where j√1, and it is understood that the real part is taken (since e jcos

j sin). Taking a complex refractive index

nj (3.23)

( has been added to to avoid confusion with the permeability), gives

EE0exp[ j(tz/c)] exp( z/c). (3.24)

Hence, the real part of the refractive index determines the velocity of the wave, andthe imaginary part gives the absorption coefficient

! /c2 /0, (3.25)

0 being the free-space wavelength.Alternatively, we may introduce a complex propagation constant,

jkj (3.26)

giving

EE0exp[ j(tjz)

EE0exp[ j(tz)]exp(z). (3.27)

Thus, comparing with Equations (3.24) and (3.25),

/c; /c!.

Conductivity

For a partial conductor the absorption is related to the conductivity, , and it canbe shown (Hunsucker, 1991, pp. 25–31) that

. (3.28)

Squaring, and equating real and imaginary parts, gives

(3.29)

2 1

2

2 21/2

11/2

( j / )


and

. (3.30)

The units of and are nepers m1 and radians m1, respectively.If, in (3.28), /, the medium approximates a pure dielectric. If /,

the medium approximates a conductor. There is a cross-over frequency given by

/ . (3.31)

Evanescent waves

Going back to Equation (3.23), it is possible for the refractive index to be purelyimaginary, so that nj , and then

EE0exp( jt)exp( z/c). (3.32)

This is an evanescent wave, which extends into the medium by about c/( ) butdoes not propagate because its phase does not vary with distance. When a propa-gating wave is totally reflected at the interface between two media, an evanescentwave exists just inside the second medium.

3.2.4 Interactions between radio waves and matter

The basic interactions are reflection, refraction, dispersion, diffraction, scattering,change of polarization, and attenuation; and these – singly or in combination –are the processes which underlie the various phenomena of terrestrial radio prop-agation. They have also provided us with a number of well-proven techniques forthe investigation of the propagation media and their behavior, knowledge ofwhich is essential to the understanding of radio communication and its problems.

Reflection occurs at the boundary between two media, returning energy backtowards the source in the case of normal incidence, whereas refraction causes anytransmitted ray to emerge at an angle to the incident ray. These effects are dis-cussed in the context of ionospheric reflection in Section 3.4.3, and of the partialreflection technique in Section 4.2.4.

Dispersion, the variation of velocity with frequency, has consequences for thetransmission of information (Section 3.4.1).

Diffraction phenomena occur when there are irregularities in the propagatingwavefront, causing the wavefront to evolve as the wave travels on. It is the basis ofradio scintillation (Section 3.4.5).

Scattering from structures in the medium that are small relative to the wave-length of the incident wave diverts some fraction of the incident signal over a widerange of directions. It is the basis of communication over scatter links (Section

2 1

2

2 21/2

11/2


8.5). Also, a (usually weak) echo may be detected at the transmitter site, which isutilized in the techniques of coherent and incoherent scatter radars described inSection 3.5 and in Sections 4.2.2 and 4.2.3.

Polarization changes occur in an ionized medium in the presence of a geomag-netic field. There are consequences for the design of transmitting and receivingantennas and polarization may be exploited in ionospheric measurements(Sections 3.4.1 and 3.4.4).

Attenuation is, of course, undesirable in communications, often setting the lowerlimit to the usable frequency band. Measurements of absorption may give usefulinformation, particularly about the lower ionosphere (Sections 3.4.4 and 4.2.4).

3.3 Propagation through the neutral atmosphere

3.3.1 The refractivity of the neutral atmosphere

Although this book is primarily concerned with the high latitude ionosphere and itseffects upon radio propagation, there are some tropospheric effects peculiar to highlatitudes that affect radio propagation in the line-of-sight (LOS) and earth-to-satel-lite modes. For that reason, we will briefly discuss some of the fundamentals of thesemodes. We will exclude the troposcatter propagation mode in which forward scatterin the troposphere (3–8 km height) permits communication over path-lengths from300 to 600 km, using frequencies from 200 MHz to 10 GHz (see Norton andWiesner, 1955; and Collin, 1985). Radio waves propagating in the troposphere areaffected by the refractive index, n – which is a function of atmospheric pressure, tem-perature, and humidity, and, near the Earth’s surface at VHF/UHF, n is approxi-mately 1.0003. It is convenient to define a radio refractive index, N, as

N(n1)106. (3.33)

Since the terrestrial atmosphere varies exponentially with height, we may expressit as

N(h)Nsexp(ch), (3.34)

where Ns is the surface refractivity, h is the height above the surface in kilometers,Cln(N/Ns)N, and N is the difference between the values of N at a height of1 km above the surface and at the surface.

Ns may be estimated from

N7.32exp(0.005577Ns). (3.35)

A useful parameter, the effective Earth-radius (the actual Earth-radius correctedfor “normal” atmospheric refraction) for radio propagation is given by


K 1 cNs106 (3.36)

where ns1Ns106, r0 is the Earth-radius6373.02 km, Ns289, andc0.136, so K1.3332410, or 4/3.

The basic exponential reference atmosphere is defined by the relation

N(h)289exp(0.136h), (3.37)

where h is the height above the surface in kilometers.Table 3.3 shows the CRPL (the old Central Radio Propagation Lab – now the

Institute for Telecommunication Sciences (ITS) – in Boulder, Colorado) exponen-tial radio refractivity atmosphere. The standard model of the atmosphere isobtained by assuming that N decreases linearly over the first kilometer above thesurface:

NNsN(hhs ); hsh(hs1), (3.38)

where N is from Equation (3.34), h is the height above sea level, hs is the heightof the surface above mean sea level in kilometers and N is the difference betweenNs and N 1 km above the Earth’s surface. The constants adopted for the standardatmosphere are given in Table 3.4. N can be calculated from radiosonde data:

N77.6P/T3.73105e/T 2“dry term”“wet term”, (3.39)

where P is the atmospheric pressure in millibars, e is the vapor pressure in milli-bars, and T is the temperature in kelvins.

A set of “standard atmospheres” showing the height dependence of radio

1

r0

ns

3.3 Through the neutral atmosphere 123

Table 3.3. CRPL exponential radio-refractivity atmospheres,NNsexp(ch)

Ns N K C

200 22.33177 1.17769 0.118399250 29.33177 1.25016 0.125626289 36.68483 1.33324 0.135747300 39.00579 1.36280 0.139284320 43.60342 1.42587 0.146502350 51.55041 1.55105 0.159332400 68.12950 1.90766 0.186719450 90.010 56 2.77761 0.223256

refractivity as a function of its value at the surface, Ns, has been defined.. Near theground the following empirical relationship between Ns and the difference inrefractivity, N, between Ns and N at 1 km above the Earth’s surface is valid:

N (km)7.32exp(0.005577Ns). (3.40)

inverting Equation (3.37), we can obtain Ns as a function of the refractory gradi-ent N:

Ns412.87log|N |356.93. (3.41)

Figures 3.2 and 3.3 show estimates of Ns for winter afternoons in the northerntemperate zone and global variations. Charts similar to Figure 3.2 applicable forhigh latitudes may be obtained from the appropriate national meteorological depart-ments. Radio-refractivity values at high latitudes are sometimes radically differentfrom those in temperate zones. For example, Fairbanks, Alaska has some of thesteepest temperature inversions in the world, causing anomalous refraction on someVHF/UHF radio paths. These effects will be described in Chapters 8 and 9.

3.3.2 Terrain effects

The most obvious feature of the Earth affecting terrestrial radiowave propagationis its curvature. The troposphere of the Earth refracts radiowaves on LOS paths


Table 3.4. Constants for the standard reference atmosphere

Ns hs (ft) a (miles) N K ae (miles) c (km)

0 0 3960.0000 0.3318 1.00000 3960.00 0200 10000 3961.8939 22.3318 1.16599 4619.53 0.106211250 5000 2960.9470 29.5124 1.23165 4878.50 0.114559301 1000 3960.1894 39.2320 1.33327 5280.00 0.118710313 900 3960.1324 41.9388 1.36479 5403.88 0.121796350 0 3960.0000 51.5530 1.48905 5896.66 0.130579400 0 3960.0000 68.1295 1.76684 6996.67 0.143848450 0 3960.0000 90.0406 2.34506 9286.44 0.154004

Note:ae is the effective Earth-radius and is equal to the product aK.aahs, where hs is the height of the Earth’s surface above sea level.a3960 miles.

c .1

8 hs ln N1

105

in such a way that one can use a modified Earth-radius when planning these prop-agation paths and, from Section 3.3.1, we use the “4/3 Earth-curvature” as shownin Figure 3.4.

Topographical features such as mountain ranges and deep valleys will, ofcourse, also affect the propagation of radio waves – especially if they block off lowtakeoff angles for HF paths or if the ground-reflection areas of a skywave modeoccur where are large topographic features. A “rule of thumb” is that the radiohorizon should be no higher than about 5° in the desired direction of propagationfor a long-haul HF skywave circuit. For LOS propagation, one usually takesadvantage of mountains to site either active or passive repeaters for VHF throughmicrowave frequencies.

Theoretical calculations of antenna patterns usually assume that one has a per-fectly conducting reflecting plane, when in reality the conductivity and permittiv-ity of the Earth’s surface exhibit great variation – as illustrated in Table 3.5. Thevertical radiation pattern of a practical antenna depends upon the electrical char-acteristics of the ground plane of the antenna. For antennas that use the Earth’ssurface as their ground plane, in addition to the electrical properties of the Earth,the relative “smoothness” of the Earth is also important. The concept of theFresnel zone is invaluable in calculating the relation of the propagation path to theterrain in the context of engineering the best path characteristics. Extensive treat-ments of Fresnel zones applied to radio propagation may be found in standardelectrical engineering textbooks and Handbooks (see Jordan and Balmain, 1968,


Figure 3.2. Minimum surface-refractivity values (Ns) referred to mean sea level for anaverage winter afternoon, continental U. S. A. (from Freeman, 1997).

Fig

ure

3.3

.M

inim

um m

onth

ly s

urfa

ce-r

efra

ctiv

ity

valu

es (

Ns)

ref

erre

d to

mea

n se

a le

vel.

pp. 498–503; Hall and Barclay, 1989, pp. 38–42 and Hunsucker, 1991, pp.258–266). Several computer programs which treat terrain effects and LOS linkperformance have recently become available (Table 3.6). It should also be men-tioned that certain atmospheric and ionospheric conditions could produce signalsover the LOS distance.

3.3.3 Noise and interference

Electrical noise is one of the limiting factors in radio communication and must beconsidered in the design of communications circuits. The three components ofelectrical noise are cosmic noise, atmospheric noise, and manmade noise. There areextensive discussions of the noise figure and noise temperature of receivers, andcosmic, atmospheric, and manmade noise in Collin (1985), Kraus (1988),


Figure 3.4. Earth-curvature-correction curves for D1 from 0.5 to 7 miles (from Freeman,1997).

Spaulding and Washburn (1985), and CCIR Report 322; and a shorter descrip-tion in Hunsucker (1991, pp. 15–20 and Appendix A5).

Cosmic noise emanates from sources of extraterrestrial origin, such as our Sun,galactic radio sources, and the extragalactic sources, and its dependences on fre-quency and antenna-pointing direction are shown in Figure 3.5. From Figure 3.5we see that cosmic radio noise decreases with increasing frequency and varieswith the antenna-pointing direction. The terrestrial ionosphere acts as a “high-pass” filter, attenuating or refracting cosmic noise in the ELF through low-HFbands.

Solar radio noise varies in frequency, intensity, and time. An example of thebehavior of the quiet Sun of large bursts, storms, and plages for frequencies from15 MHz to microwave frequencies is shown in Figure 3.6. Good representationsof the dynamic behavior of solar radio burst frequency and intensity are shown inFigures 3.7 and 3.8. An example of radio noise from a galactic source is shown inFigure 3.9. Examples of the variation with frequency of some extragalactic radiosources are illustrated in Figure 3.10.

Atmospheric noise originates in atmospheric electrical discharges like lightningand precipitation static, etc., and may reach the receiving antenna either by a LOSpath or via propagation by the ionosphere. The most intense thunderstorms on


Table 3.5. Electrical conductivities and permitivities for various types of terrain

Permitivity ( ) Conductivity, (relative dielectric)

Type of surface (1 m1) constant

Coastal dry sand 0.002 10.0Flat, wet coastal 0.01–0.02 14.0–30.0Rocky land (steep hills) 0.002 10.0–15.0Highly moist soil 0.005–0.02 30.0Marshy 0.1 30.0Hills (to 1000 m) 0.001 15.0Freshwater 0.001 80.0–81.0Sea water 3.0–5.0 80.0–81.0Sea ice 0.001 14.0Polar ice (free) 0.000025 13.0Polar ice (cap) 0.0001 11.0Arctic land 0.0005–0.001 23–34 for silts

12 for dry sandTundra underlain by permafrost surfacea 103 to 102 5–70

Note:a Acquired in 1988/1989 in Central Alaska from 2–30 MHz by G. Hagn of SRIInternational.

earth occur in the tropics and this HF noise is propagated by LOS modes and bythe ionosphere to distances of thousands of kilometers. The areas of lowest prop-agated atmospheric noise are at high northern and southern latitudes (55° geo-graphic latitude).


Table 3.6. Computer programs for diffraction/terrain predictions

Name Description Source Reference

EREPS Engineer’s http://trout.nosc. Patterson Refractive Effects mil/NraDMosaic (1994), Proc. of Prediction System Home.html the BLOS

Conference

IFDG/GTD *Finite Difference.../ Anderson et al. Generalized Theory (1993) Marcus of Diffraction (1994)

GELTI/ATLM GTD Estimated Dr Kent Chamberlain Loss due to Terrain Chamberlain and Luebbers Interaction/ Department of (1992)Automated Terrain Electrical and Linearization Model Computer

Engineering,University of NewHampshire,Durham, NH03824-3591

HARPO Hamiltonian Brent and equations in Ormsby (1994)sphericalcoordinates,modified by usingGaussian beams

EFEPE/SSP Institut für Dr R. Großkopf Ditto IRT Rundfunktechnik Institut für Grosskopf

propagation model Rundfunktechnik (1994)for digital broadcast Münchensystems in urbanareas

VTRPE Variable Terrain Dr Frank Ryan Ryan (1991)Radio Parabolic NCCOSC/RDT&E Equation microwave Division, propagation in San Diego, CA complex real-world 92152-6435environments

Several reports and papers deal with the global levels of atmospheric noise, themost cited being Spaulding and Washburn (1985) and the CCIR Report 322-3c(1988). Sailors (1993) has noted some major problems in CCIR Report 322-3, andconcludes that “the model should be used with caution, especially in the northernand southern high latitudes, the Arabian Peninsula, northern Africa and the mid-Atlantic areas. In these areas, consider using the original CCIR Report 322model.” He also suggests serious modifications to the development of the modeland using correction factors for certain locations. Figures 3.11–3.15 give examplesof atmospheric models and noise as a function of frequency.

Manmade noise usually originates from rotating electrical machinery, high-current switching circuits, and arcing power-line components. It is obviously mostintense in industrial areas and problems from this type of noise need to beresolved on a case-by-case basis as outlined in a report by Vincent and Munsch(1996).

Interference from other transmitters sometimes dominates portions of the spec-trum, such as the HF band – where frequency assignments seem to be largelyignored. Interference can be minimized by maintaining the frequency stability of


Table 3.6. (cont.)

Name Description Source Reference

MSITE, Two- and three- EDX Engineering,TPATH, MCS dimensional plots of Inc., P. O. Box 1547,

signal levels from Eugene, OR 97440multiple transmitters, Ph. (541)345-0019microwave-link Fax (541)345-8145studies andinterference http://www.edx.comprediction, ray-tracing for urbanand indoorenvironments,wireless, etc.

TIREM/DUCTAPE Terrain Integrated Dr Homer Riggins Eppink and

Rough-Earth Model/ and Dr David Kuebler (1994)Ducting and Eppink, IIT Anomalous Research Institute, Propagation 185 Admiral Environment Cochrane Drive,

Annapolis, MD

the transmitter and maximizing the selectivity of the receiver and by makingrather extensive interference measurements at the receiver site before finalizing theoperating frequency and time slots.

The basic theorems governing vertical and oblique HF propagation are givenin the following section.


Figure 3.5. Variations of “antenna temperature” as a function of frequency from 10 MHzto 100 GHz (from Freeman, 1997).

Figure 3.6. A typicalradio spectrum from theSun (after Hey, 1983).

Figure 3.7. Dynamic spectra of solar radio bursts (from Hey, 1983, p. 100).

Figure 3.8. The power variation of solar radio bursts (from Hey, 1983, p. 100).

Figure 3.9. The spectrum of radio sources in the Orion Nebula compared with a curve cal-culated for an electron temperature of 10000 K (from Hey, 1983).

Figure 3.10. Spectra of radio galaxies Cygnus A, Virgo A, and Hercules A, compared withthe supernova remnants in Cassiopeia (dashed curve) (from Hey, 1983).

Fig

ure

3.1

1.

Rad

io-n

oise

-rec

ordi

ng s

tati

ons

used

to

obta

in d

ata

used

to

deve

lop

the

orig

inal

CC

IR R

epor

t 32

2(f

rom

Sai

lors

, 199

3).

Fig

ure

3.1

2.

A t

ypic

al fi

gure

fro

m C

CIR

Rep

ort

322

(fro

m S

ailo

rs, 1

993)

.

Fig

ure

3.1

3.

Rad

io-n

oise

-rec

ordi

ng lo

cati

ons

(fro

m S

ailo

rs, 1

993)

.


Figure 3.14. Determination of the 1-MHz Fam value for Moscow for June, July and August;1600–2000 UT. (from Spaulding and Washburn, 1985).

Fig

ure

3.1

5.

In (

a) a

nd (

b) a

re s

how

nex

ampl

es o

f Sp

auld

ing

and

Was

hbur

n’s

corr

ecti

ons

to t

he C

CIR

Rep

ort

322

(fro

m S

paul

ding

and

Was

hbur

n, 1

985,

p. 1

8).

3.4 Ionospheric propagation

3.4.1 Magnetoionic theory

The Appleton equation

For an ionized medium the refractive index is expressed by the Appleton equa-tion. In its complete form this is a complicated expression using the dimension-less quantities X, Y, and Z, each of which is defined as a ratio between the wavefrequency and a frequency characteristic of the medium. The latter are the plasmafrequency,

N[Ne2/ 0me]1/2, (3.42)

the gyrofrequency,

B Be/me, (3.43)

and the collision frequency, , where N is the electron concentration (usually calledthe electron density), e is the charge on the electron (taken to be positive), me is themass of the electron, 0 is the permittivity of free space, and B is the magnetic fluxdensity in the medium. The plasma frequency is the natural frequency of oscilla-tion for electrostatic perturbations within the plasma, the gyrofrequency is the fre-quency of gyration of an electron in magnetic flux density B, and is the rate ofcollision between a given electron and other particles. Then the dimensionlessquantities are

X2N /2, (3.44)

YB/, (3.45)

and

Z/. (3.46)

In these terms the Appleton equation for the refractive index (n) of an ionizedmedium with N electrons cm3, permeated by a magnetic flux density B (W m1)and in which the electron-collision frequency is (s1) is given by

n21 (3.47)

where denotes the ordinary and the extraordinary wave. In (3.47), Y has beendivided into longitudinal and transverse components;

X

1 jZ Y 2

T

2(1 X jZ) Y 4

T

4(1 X jZ)2 Y 2L

1/2 ,


YLY cos (3.48a)

and

YTYsin, (3.48b)

being the angle between the direction of propagation and the magnetic field.Note that the refractive index is complex, with real and imaginary parts:n j .

Polarization

In order to calculate the effects of this anisotropic medium on the polarization ofa radio wave traversing the region, it is convenient to define the polarization ratioR as

RHy /HxEx /Ey, (3.49)

where Hy and Ey are the y-components of E and H, and Hx and Ex are the x-components of E and H, respectively. Then we can obtain the magnetoionic pola-rization equation (see Kelso, 1964; and Ratcliffe, 1959)

R " Y 2L . (3.50)

The polarization equation gives values of R that are complex. In general, thismeans an elliptical polarization. If R is purely real, the polarization is linear; if Ris purely imaginary, the polarization is circular. See Figure 3.16.

1/2

Y 4T

4(1 X jZ)2

jYL

Y 2T

2(1 X jZ)


Figure 3.16. The elec-tric-field polarization inthe plane of the wave-front. Ox and Oy are theprincipal directions andthe projection of theimposed magnetic field isalong Oy. The positivewave-normal is directedinto the paper, alongpositive Oz. The ordi-nary-wave ellipse isshown as a continuousline and the extraordi-nary-wave ellipse isshown as a dashed line(from Ratcliffe, 1959).

It is virtually impossible for an ordinary mortal to make much sense ofEquations (3.47) and (3.50) in their full glory – see Ratcliffe (1959) or Budden(1985) for a full discussion – but when special cases are taken the picture beginsto clarify. Luckily, many applications can be treated using these special cases.

Special case 1: Neglecting collisions and magnetic field

If there are no collisions and the magnetic field is neglected, the refractive index,n, is real:

n21X12N /2

n21Ne2/( 0me2). (3.51)

Then the phase velocity is

vpc/nc . (3.52)

The group velocity, using Equation 3.21, is

uc/ngc , (3.53)

where ng is the group refractive index. (Note that ng1/n in this case.)

Special case 2:The effect of a magnetic field

If the magnetic field is now included and the propagation is almost directly alongthe magnetic vector so that YT may be neglected, then

n21X/(1YL )

n212N /[(L ) (3.54)

and R j. There are now two waves, circularly polarized in opposite directions,having different velocities. These are characteristic waves, termed ordinary andextraordinary (for the upper and lower signs, respectively) by analogy with bire-fringence in crystals. In general, where YT#0, the characteristic waves are ellipti-cally polarized.

Special case 3:The effect of collisions

If collisions are significant (but in the absence of a magnetic field), then

n21X/(1jZ )

n212N/[(j)]. (3.55)

1 Ne2

0me2

1/2

/ 1 Ne2

0me2

1/2


Taking the imaginary part ( ) and applying Equation (3.25) gives the absorptioncoefficient

!

! . (3.56)

The refractive index (n) is modified by collisions between the electrons and heavyparticles, and the wave undergoes absorption – which physically is due to the con-version of ordered momentum into random motion of the particles after collision.At each collision, some energy is transferred from the wave to the neutral mole-cules and appears as thermal energy. Details of the microscopic processes involvedin ionospheric absorption are discussed by Ratcliffe (1959, Ch. 5) and derivationsof the equations describing macroscopic features of absorption are given by Davies(1969, Ch. 6).

We can conveniently divide absorption into two limiting types, commonlycalled non-deviative absorption and deviative absorption. Non-deviative absorp-tion occurs in regions where the product N is large and 1, and is character-ized by the absorption of LF, MF, and HF waves in the D region. Deviativeabsorption, on the other hand, occurs near the top of the ray trajectory or any-where else on the ray path where significant bending takes place (for small N and).

When the refractive index is 1, we can write

!4.6102 (dB km1). (3.57)

We can further simplify Equation (3.57) for the VHF case, since 22 , as

!4.6102N/2 (dB km1). (3.58)

In deviative absorption, 1, and

! . (3.59)

Near a reflection level, 21, and then the preceding equation reduces to

! , (3.60)

where is the group refractive index.One important case is for non-deviative absorption and the quasi-longitudinal

(QL) approximation, when

2c

2c (1 2)

N

2 2

e2

2 0mec 1

N

2 2

c

12

XZ

1 Z2


! . (3.61)

The absorption coefficient is therefore smaller for the ordinary than it is for theextraordinary wave. For a given value of the electron density, the absorptioncoefficient is a maximum at the level where

L. (3.62)

The absorption of the extraordinary wave becomes very strong at the higher levels( small) when the wave frequency is close to the gyrofrequency.

3.4.2 Reflection of radio waves from an ionospheric layer

Reflection at vertical incidence

If a pulse of radio waves of frequency f/(2) enters an ionospheric layer at ver-tical incidence from below, it will travel at the group velocity (u). Neglecting themagnetic field, u is given by Equation (3.53) and u decreases as the electron densityincreases with altitude. Provided that the layer is sufficiently intense, a level wherethe group velocity is zero (and the phase velocity infinite) will eventually bereached, and here the energy is reflected. At this level the plasma frequency( fNN /(2)) equals the wave frequency ( f ) and

N42 0me fN2 /e2. (3.63)

Numerically,

N (m3)1.241010[ f (MHz )]2. (3.64)

Above this level the wave is evanescent (Equation (3.32)).The time required for the journey to the reflection point and back is

t (3.65)

and the virtual height is

h . (3.66)

The virtual height is the height calculated on the assumption that the signal trav-eled at the speed of light (in vacuo). In fact, since the pulse always travels moreslowly in the layer, the virtual height is always greater than the true height.

If the electron density at the layer maximum is Nmax, the greatest radio fre-

ct2

h

0

dz

[1 ( fn / f )2]1/2

2c

h

0

dzn

e2

2 0mc

N

( L)2 2


quency that may be reflected at vertical incidence is the critical frequency of thelayer, fc, which is related to the maximum electron density by

Nmax1.241010f c2. (3.67)

A good discussion of the solution of Abel’s equation (3.66) (Appleton, 1930) isgiven by Kelso (1964). In the general case (including the geomagnetic field),several numerical techniques have been employed successfully to invert the iono-gram trace of the ordinary wave to give an equivalent monotonic electron-densityprofile (see the special issue of Radio Science, 1967). One of the most comprehen-sive of the numerical true height programs is the POLAN program developed byTitheridge (1985) and a discussion of this program is given by Davies (1990).

In the real ionosphere, where the geomagnetic field has to be taken intoaccount, there are two reflection conditions. The extraordinary wave is reflectedwhere

fN2f ( ffB) (3.68)

and the ordinary wave where

fNf. (3.69)

The first reflection occurs according to the QL approximation, whereas the secondrelates to the quasi-transverse (QT) approximation. If fBfN, the differencebetween the two critical frequencies is fB /2, that for the extraordinary wave beingthe greater.

3.4.3 Relations between oblique and vertical incidence

When the signal is incident obliquely on the layer, the process by which it isreturned to the ground can be appreciated as follows. Consider the ionosphericlayer to be composed of a large number of thin, uniform slabs, whose electrondensity increases with altitude. If successive slabs have refractive indices n1 and n2,Snell’s law relates the angles of incidence (1) and refraction (2) by

n1sin1n2sin2. (3.70)

Applying this law to each boundary in turn readily shows that, if a ray enters theionosphere at incidence 0, its angle to the normal in a slab with refractive indexnr is simply

sinrsin0/nr (3.71)


(the refractive index below the layer being unity). The ray therefore travels hori-zontally when

nrsin0 (3.72)

and this is the reflection condition (magnetic field neglected) for an obliquely inci-dent signal. The ray then returns to the ground by a similar path. The process isnow one of bending rather than reflection at a boundary.

Combining the two equations for fN in this section yields the secant law relat-ing vertical and oblique propagation:

fobfvsec0 (3.73)

where fob and fv are the frequencies of signals reflected from the same true heightwhen fob is incident at angle 0 and fv is incident vertically.

In order to determine values of sec0 and fob from vertical-incidence soundings(which measure the virtual height, h), we need the results of two more theorems.Breit and Tuve’s theorem states that the time taken to traverse the actual curvedpath TABCR in Figure 3.17 at the group velocity u equals the time necessary totravel over the straight-line path TER at the free-space velocity c. Referring to thegeometry shown in Figure 3.17, we can write the expression

tTER

(3.74)

t(TEER)/c. (3.75)

Dc sin a0

dxsin 0

1c


Figure 3.17. The geome-try describing verticaland oblique ionosphericpropagation (fromHunsucker, 1992).

Martyn’s theorem states that, if fv and fob are the vertical and oblique frequenciesrespectively, reflected from the same true height (h), then the virtual height atwhich the frequency fob is reflected equals the height of the equivalent triangularpath for the frequency fv. Referring to Figure 3.17 and defining the equivalent pathat oblique incidence for frequency fob as

Pob2TE, (3.76)

we obtain

Pv cos0Pob2DE (3.77)

Martyn’s theorem may be written more concisely as

hobhv. (3.78)

Newbern Smith (1939) devised the set of logarithmic transmission curves para-metric in range for curved Earth shown in Figure 3.18, which are sufficiently accu-rate for the distances shown. Details concerning the use of these curves to relatethe parameters given in Equations (3.76)–(3.78) may be found in Davies (1969)and in the URSI Handbook of Ionogram Analysis (1972).

3.4.4 Trans-ionospheric propagation

If the radio frequency exceeds the critical frequency of the ionosphere, the signalis not reflected but continues out into space. Similarly, signals from beyond the


Figure 3.18. Logarithmic ionospheric-transmission curves for a curved-Earth ionosphere(after Smith, 1939).

ionosphere may be received at the ground if their frequencies are sufficiently high.However, these signals are not necessarily unaffected by the ionosphere: there canbe significant and measurable effects on their phase, their polarization and theirintensity. In each case the effect becomes weaker with increasing frequency, andin practice they are significant from the upper part of the HF band, through theVHF band, and into the lower part of the UHF band. Another common featureis that the effects are cumulative and the total depends on an integral along thepropagation path.

Phase effects

In the Appleton equation for the refractive index, let X1 (radio frequency largerelative to the plasma frequency), YLYT0 (geomagnetic field neglected), andZ0 (collisions neglected). Then the second term of Equation (3.47) is much lessthan unity, and we can write

n1X /2

n1Ne2/(2 0me2). (3.79)

Inserting values for the constants, and using f instead of , gives

n140.30N (m3)/[ f (Hz)]2. (3.80)

The refractive index is smaller than unity by an amount proportional to the elec-tron density and inversely proportional to the square of the radio frequency.

If a radio wave travels a distance dl in an ionized medium, i.e. dl / wavelengths,its phase lags by 2dl /(2fndl /c) radians. Over a path l the advance of phaseis therefore

ndl Ndl. (3.81)

The first term is just the phase delay due to a wave of frequency f traveling a dis-tance l at the speed of light. The second is a phase advance that arises because therefractive index is less than unity and the phase speed greater than c. This term is

cumulative and simply proportional to the electron content, I Ndl, which is the

number of electrons in a column of unit cross-section along the propagation path.Numerically, the phase advance due to the medium is

(8.45107)I /f (radians). (3.82)

f is in hertz and I in m2 .

2 40.30cf

2f lc2f

c


Several significant applications follow.

(a) Since the phase advance depends upon the radio frequency, the electroncontent can be determined by comparing the effects on two frequenciestransmitted coherently from, for example, a satellite.

(b) Since the frequency is the rate of change of phase, another method is toobserve the Doppler shift in the frequency of a signal received from a sat-ellite passing overhead.

(c) If a carrier of frequency fc is modulated at frequency fm , the phase of themodulation is changed by

m 8.45107( fm/f c2)I. (3.83)

In this case the phase is retarded because the modulation travels at thegroup speed, which is less than the speed of light.

(d) Corresponding to this phase delay, the time delay of a pulse is

t8.45107I/(2f c2) (s). (3.84)

(e) If there is a gradient of electron content in a direction (x) perpendicular tothe propagation direction, the ray is deviated. This is wedge refraction. Thewave is deviated through an angle

[c/(2)](8.45107/f 2 )I /x. (3.85)

(In Equations (3.82)–(3.85) the constant 8.45107 is given to three significantfigures, therefore with an inaccuracy of 0.12%. To four figures, for more accuratework, the constant is 8.448107. In Equation (3.81) the constant 40.30 is accu-rate to within 0.025%. To five figures this constant is 40.302).

The Faraday effect

When the geomagnetic field is taken into account and propagation is almost alongthe field direction, there are two characteristic waves that travel at different speeds.These waves are circularly polarized in opposite directions, and their sum is alinear polarization. If the circularly polarized components make instantaneousangles O and E with respect to a reference direction, then the linear wave is at anangle

(OE)/2. (3.86)

See Figure 3.19.Let OE0 at the source. Then, after a distance l in the medium,

O2f [tnOl /c]


and

E2f [tnE l /c], (3.87)

giving

(f /c)(nOnE )l. (3.88)

At a sufficiently high frequency (e.g. 50 MHz) the gyrofrequency fBf, andthen the ordinary and extraordinary refractive indices differ by

nOnEXY( f 2N fB)/f 3, (3.89)

giving

l. (3.90)

Therefore the polarization angle changes progressively as the wave travels throughthe ionized medium. On substituting values and allowing for varying electrondensity and magnetic field strength, we obtain

fLNdl

BLNdl, (3.91)

since fL2.7991010 BL, BL being in webers m2 . We have now moved to the QLapproximation, to allow for propagation somewhat across the field. (In fact theQL approximation has wide application in the Faraday effect, being valid to a fewdegrees of normal to the field). As seen by an observer at the ground looking up,the polarization rotates anticlockwise in the northern hemisphere and clockwise

2.365 104

f 2

8.448 107

f 2

12c

f 2

N fB

f 2


Figure 3.19. Addition oftwo circularly polarizedwaves to give a linearwave, as seen by a sta-tionary observer lookingalong the geomagneticfield (from Hargreaves,1992).

in the southern hemisphere, irrespective of the direction in which the wave istraveling. A recent extensive discussion of the Faraday effect is given by Yeh et al.(1999).

Absorption

Equation (3.61) gives the absorption coefficient ! in the case of non-deviativeabsorption and the QL approximation. The signal amplitude falls by a factor of eover the distance 1/!. Provided that the radio frequency is considerably greaterthan the critical frequency of the layer, this formula applies to trans-ionosphericpropagation through the whole of the ionosphere. Whereas ! is in units of nepers,it is usual to express signal loss in decibels (dB), defined by the ratio between initial(P1 ) and final (P2) powers:

Absorption A (dB)10log10(P1/P2). (3.92)

The neper and the decibel are related by

1 neper8.686 dB. (3.93)

On putting in the appropriate values, Equation (3.92) gives

A (dB)4.611105 dl (3.94)

for the total absorption over the path.Since the collision frequency decreases sharply with altitude, most non-

deviative absorption occurs in the lower ionosphere, and it is maximized when theterms in the denominator of Equation (3.94) are equal. If is the larger term, theabsorption varies as 1/ and therefore decreases at the lower levels. However, overmost of the height range affected the second term dominates and then the totalabsorption is just proportional to the integral of N. Moreover, the gyrofrequencymay be neglected if it is much smaller than the radio frequency, which is certainlythe case at frequencies greater than about 30 MHz. Then Equation (3.94) sim-plifies to

A (dB) Ndl (3.95)

for the total absorption over the path.The limit to high-latitude communications is often set by the ionospheric

absorption, and measuring the absorption of the cosmic radio noise is a valuabletechnique in high-latitude studies.

1.168 1018

[ f (MHz)]2

N

2 (L)2


3.4.5 Principles of radio scintillation

Introduction

The phenomenon of scintillation, which appears principally in trans-ionosphericsignals, is caused by relative phase shifts in the propagating wavefront and by sub-sequent diffraction. The phase shifts are a direct result of spatial irregularity in themedium and specifically in the electron content, to which they are related byEquation (3.82). It should be noted that, other things being equal, the irregularcomponent of the electron content varies not linearly with the path length (slabthickness), but with its square root.

According to Huygens’ principle, each part of a wavefront may be regarded asa source of secondary wavelets, whose superposition builds up the wavefront at apoint further along. In diffraction theory this principle is applied to determinehow the amplitude and phase of a received signal are affected by passage througha region of irregularities. Diffraction theory applies to “small” irregularities, thecriterion for which is that there are at least several of them within the distance ofthe first Fresnel zone (see below).

Diffraction by a thin screen of weak irregularities and the concept ofthe angular spectrum

The simplest case to treat is that of a thin, shallow, phase-changing screen. In thismodel the irregularities are assumed to lie in an infinitely thin layer, and to intro-duce small (1 radian) phase perturbations along the wavefront of a wave passingthrough it, as in Figure 3.20

The incident wave is planar (the source being located at infinity), but the emerg-ing wavefront is irregular. To obtain the field at a point P in the observing planeOO, it is necessary to sum the contributions from each point of the emergingwavefront, EE. Since EE is irregular in phase, the field at OO will also be irreg-ular, and in general both the phase and the amplitude are affected.

Since the wavefield at the observing plane is made up from contributions frompoints all along the diffracting screen, it is clear that there is not necessarily a one-to-one relationship between the irregularities in the ionosphere and the wavefieldat the ground. There are, nevertheless, some relationships of a statistical nature.

The link between the properties of the screen and the variations observed at theground is the angular spectrum of the waves leaving the screen. Just as a wave mod-ulated in time may be expressed as a frequency spectrum that may be derived by aFourier transformation, so a wave modulated in distance may be expressed by aspectrum in angle. The spectrum of periodicities in the screen, F(d ), is related byFourier transformation to an angular spectrum of waves, f(sin), where d is thespatial wavelength of irregularities and is the angle of propagation measured fromthe normal. The same spectrum reaches the ground, though with the phase of eachsine wave modified by the distance traveled, where it may be transformed back to a


wavefield in the observing plane. Both F and f are complex, and therefore full infor-mation may be obtained only by observing both the amplitude and the phase.

The problem now is that, whereas it is easy to measure the amplitude of a receivedradio signal, it is much more difficult to measure the phase. However, it has beenshown that, if the observations are made sufficiently far from the screen (and pro-vided that the screen is shallow – i.e. the initial modulation is only small), the sta-tistical properties of the amplitude and phase irregularities at the ground are thesame as each other and the same as those at the screen. In that case, amplitudeobservations alone suffice to give the statistical properties of the ionospheric screen.

It is unlikely that irregularities will be sinusoidal or have any other analyticalform; they will more probably look like random noise. Such irregularities may behandled using the correlation function, .

If a(x) are the differences of a varying quantity A(x) from its mean A, and 2 isthe variance of a (the bars denoting averages over many values), thecorrelation function of A over the interval y is

(y) /2. (3.96)

The correlation function may sometimes be assumed to have a Gaussian form,

(d )exp[d 2/(2d 02) (3.97)

and in this case the angular power spectrum would also be Gaussian:

P(sin)exp[sin2/(2sin20)], (3.98)

[a(x)a(x y)]

[A(x) A]2


Figure 3.20. Diffraction of a plane wave incident on a thin phase-changing screen.

E E'a b

x

O O'P

Phase

Amplitude

Incident waveScreen

Emerging wave

Observingplane

where

sin0/(2d0). (3.99)

Figure 3.21 illustrates the relationship between and P in this case.

Fresnel-zone effects

The distance between the screen and the observer is significant because the size ofthe Fresnel zones depends upon the distance as well as the wavelength. Recall that,by definition, the first Fresnel zone extends to the point where the distance to theobserver exceeds the minimum distance by /2, the resulting phase differencebeing 180°. Referring to Figure 3.20, if the overhead point is a, we can pick a pointb such that PbPa/4. If the screen alters the phase only, the signal at EE maybe sketched as in Figure 3.22(a), where A is the unaffected signal and E is the per-turbation due to the screen.

At a point P on the observing plane, if the perturbation due to a alters the phaseof the signal, that due to b will affect its amplitude because of the extra /4 trav-eled. The resulting signal might now look like Figure 3.22(b), with both phase andamplitude fluctuations involved.

Since contributions may affect the amplitude only if they fall within the angularspectrum, it follows that

(D/2)1/2d0 (3.100)


Figure 3.21. (a) Correlation function and (b) angular power spectrum for a randomdiffraction screen.

(a) Correlation function

(b) Angular power spectrum

1

e –1/2

sin θ0

sin θ

d

d 0

P

ρ

1

e –1/2

for amplitude scintillation to appear at an observing plane at distance D from apure phase screen, the source being at infinity. This says that the signal receivedfrom a phase screen will contain both amplitude and phase perturbations if theobserver is sufficiently far from the screen for the first Fresnel zone to containseveral irregularities of typical size. At infinity, the fading power becomes equallydivided:

(A)/A()

s ()/√2, (3.101)

where (A) and () are the standard deviations of amplitude and phase.At a lesser distance there will be phase fluctuation, but the amplitude fluctua-

tion will not be fully developed, and this is often the situation in practice. If theradio wavelength, , is 6 m and the irregular screen is 400 km away, the radius ofthe first Fresnel zone is 1.5 km. Many of the irregularities will be largerthan that and therefore the amplitude fluctuations will not be fully developed.

The properties of a phase screen are important because the ionosphere behavesas a phase screen in most cases, and the bulk motion of the irregularities causesthe signal received at a fixed place to scintillate. If, by means of a specially devisedexperiment, it is possible to observe phase as well as amplitude scintillation, theFresnel-zone effect can be investigated directly by comparing the spectra of phaseand amplitude fluctuations. An example is shown in Figure 3.23

The irregularities in the ionosphere generally exhibit a power-law spectrum ofform !P, where ! is the wave number (2/d, in which d is the spatial wavelengthof the irregularities). We may generally suppose that the phase screen in the ion-osphere produces a pattern of amplitude and phase fluctuation over the groundthat is related to the spectrum of the irregularities themselves, and that scintilla-tions are observed because the pattern is moving across the observing point. It isby this means that the variation in distance is converted into a time variation.Since the conversion of phase to amplitude scintillation depends on the size of theirregularity, the low-frequency (arising from the large scale) end of the spectrum

D


Figure 3.22. Development of (a) phase and (b) amplitude perturbations from initial pertur-bations.

(a) Signal at EE

(b) Signal at OO

Total

Total

A

A

β0

αE

α0

is attenuated. The attenuation operates at frequencies less than u / , where uis the velocity (it being assumed that all the irregularities move together) and is the radius of the first Fresnel zone. Since is known and D may be assumed (tobe approximately 350 km), u can be determined by this means. This effect is seenin the spectra of Figure 3.23. When spectra can be determined, one can thereforeobtain further information about the irregularities and their motion, particularlyif the “Fresnel frequency” can be identified.

The above results are altered if the source is not at infinity (because the wave-front reaching the screen is then curved), and/or the phase screen introduces deepmodulation, s()1 radian (since that broadens the angular spectrum).

There is an extensive body of literature on the theory of scintillations.Hargreaves (1992) gives further details at an introductory level. The basic theoryand early work were reviewed by Ratcliffe (1956), and later developments by Yehand Liu (1982).

D2D


Figure 3.23. Spectra of the amplitude and phase recorded at 40 MHz from a geosynchro-nous satellite transmission. Power spectra are plotted on a log scale of relative values indecibels. The phase spectrum levels off due to detrending (at 3103 Hz), but the turn inthe amplitude spectrum marks the Fresnel frequency. (After W. J. Myers et al., J. Geophys.Res. 84, 2039 (1979), copyright by the American Geophysical Union.)

Indices and simple statistics of scintillation

The intensity of amplitude scintillation is usually expressed by using one of fourindices (Briggs and Parkin, 1963). If A is the amplitude, A is the mean amplitude,P is the power, PA2, P is the mean power, aAA, and pPP,

S1 |a |/A, (3.102)

S2( )1/2/A, (3.103)

S3 |p |/P, (3.104)

S4( )1/2/P. (3.105)

These are all dimensionless. S1 is the mean deviation of the amplitude normalizedby the mean amplitude, and S2 the root-mean-square deviation of the amplitudealso divided by the mean amplitude. S3 is the mean deviation of the power nor-malized by the mean power, and S4 the root-mean-square deviation of the power,similarly normalized. Note that S3 and S4 are similar to S1 and S2 but are writtenin terms of power instead of amplitude. From this selection of indices, S4 is themost commonly used.

It has been shown (Chytil, 1967) that the following approximate relations apply:

S10.42S4,

S20.52S4, (3.106)

S30.78S4.

An example of weak scintillation is shown in the top three panels of Figure 3.24.The S4 values are 0.016, 0.076, and 0.54 at 360, 140, and 40 MHz, respectively,

all of which are less than unity. The bottom panel of Figure 3.24 gives the ampli-tude spectra, normalized with respect to magnitude for easier comparison. Theturnover points indicate Fresnel frequencies of 0.07, 0.045, and 0.025 Hz, respec-tively, varying approximately as the square root of the radio frequency. The fadingspectrum varies as (fading frequency)3.5. For comparison, Figure 3.25 illustratesthe appearance of records with deep scintillation. Here the S4 values are respec-tively 0.13, 0.54, and 1.42. The character of the record changes dramatically whenthe modulation becomes deep.

In Figure 3.22b, the fading signal is represented as a steady component plusrandom in-phase and quadrature components. If the random components aresmall relative to the steady one, the amplitude of the total signal (A ) will fluctuateabout the mean with a Gaussian distribution. At the other extreme, if the steadycomponent is small relative to the random one, the amplitude distribution will bea “random walk” having the Rayleigh form. Between these extremes the family ofNakagami m-distributions (Nakagami, 1960) applies. Figure 3.26 illustrates

p2

a2



Figure 3.24. Examples of amplitude scintillation at three frequencies from a geosynchro-nous satellite, and their spectra. (R. Umeki et al., J. Geophys. Res., 82, 2752 (1997b).)

amplitude distributions for various S4 values covering the range between theGaussian (S40.1) and the Rayleigh.

Figure 3.27 gives a range of phase distributions, all of which are, of course, sym-metrical about zero, The m-distributions are characterized by a single parameterthat can be related to S4 and to the standard deviation of the phase.

3.4.6 Propagation involving reflection from a sharpboundary and full-wave solutions

Reflection at a boundary

The treatment of propagation outlined in the foregoing sections, which are basedon the concept of the refractive index, assumes that the medium is uniform. Ofcourse this is seldom the case, but in practice the assumption may be used pro-vided that any variations are not too large over a distance of several wavelengths.Such a medium is said to be slowly varying. There are, however, situations in which


Figure 3.25. Scintillations at 360, 140, and 40 MHz, showing the transition to deep fading:S40.13, 0.54, and 1.42. (R. Umeki et al., Radio Science, 12, 311 (1997a).)

this is plainly not so, and then a different sort of treatment is required.If the medium changes significantly within a wavelength then we may use the

physics of reflection at a sharp boundary, as at a partially reflecting mirror. If awave is normally incident at a sharp boundary, the coefficients of reflection andtransmission are determined by the condition that the tangential components ofthe E and H vectors must be continuous across the boundary

Referring to Figure 3.28, where the subscripts i, t, and r mean incident, trans-mitted, and reflected, the wave being incident from below,

EtEIEr (3.107)


Figure 3.26. Empirical amplitude distributions for a range of S4 values. (After R. K. Crane,Technical Note 1974–26, Lincoln Laboratory (1974).)

and

HtHiHr, (3.108)

the negative sign arising because the reflected wave propagates downward. In anon-magnetic medium,

H/En /( 00)1/2 (3.109)

and, by substitution, the reflection coefficient is given by

Er/EI(n2n1)/(n2n1). (3.110)

The fraction of power reflected is (Er /Ei)2 .


Figure 3.27. Empirical phase distributions for a range of S4 values. (After R. K. Crane,Technical Note 1974–26, Lincoln Laboratory (1974).)

When the wave is incident at an angle to the boundary a further condition mustbe applied, which is that the normal components of the electric and magnetic flux( E and H ) are also continuous across the interface. One familiar result thatfollows is Snell’s Law:

n1sinIn2sint, (3.111)

where i is the angle of incidence in the medium of refractive index n1, and t is theangle of the ray transmitted into medium n2 .

We now consider two special cases. First, let the plane of polarization (by con-vention the direction of the electric field) be perpendicular to the plane of inci-dence. Then application of the continuity conditions gives

sin(it) /sin(it), (3.112)

or,

. (3.113)

This is the first Fresnel equation for reflection.If the plane of polarization lies in the plane of incidence, the reflection coeffi-

cient is given by

‖tan(it)/tan(it) (3.114)

. (3.115)

This is the second Fresnel equation. When it90°, tan(it)$, and then‖0. This is the Brewster angle, given by tanBn2/n1, where the reflection coeffi-cient goes to zero if the E vector is in the plane of incidence – in practice, the waveis vertically polarized. The reflected wave is reversed in phase as the Brewster angleis crossed. There is no such effect if the wave is horizontally polarized.

(n2/n1)2cos i (n2/n1)2 sin2 i

(n2/n1)2cos i (n2/n1)2 sin2 i

(n2/n1)2 sin2 i cos i

(n2/n1)2 sin2 i cos i


Figure 3.28. The continuity of electric and magnetic vectors at a sharp boundary.

At normal incidence Equations (3.113) and (3.115) both revert to (3.110). Atgrazing incidence, as i→90°, ‖→1, but

→1, implying that there is a rever-

sal of phase on reflection.These remarks apply to reflection at the interface between dielectrics, n1 and n2

being both real. If the reflector is a partial conductor, the Fresnel equations stillapply but the refractive indices are now complex. In the general case reflectioninvolves a change of phase as well as of amplitude. Provided that the conditions ofa sharp boundary are satisfied and the appropriate refractive indices are used, theFresnel formulae are of wide application throughout the electromagnetic spectrum.

Full-wave solutions

There are (unfortunately) other cases in which the medium changes over a radiowavelength but the change is not sharp enough to count as a sharp boundary. Inthese cases the only approach is to develop a full-wave solution, which amounts tosolving Maxwell’s equations at each step through the layer by a numerical method.Conditions are imposed above and below the spatially varying medium to corre-spond to incident waves, and then the transmitted and reflected waves may bededuced. Though the method is applicable generally, preference would obviouslybe given to the simpler ones where they are valid. For more information about thistechnique the reader is referred to Budden (1985).

Sub-ionospheric propagation at ELF and VLF

At frequencies below about 30 kHz the base of the ionosphere is only a few wave-lengths above the ground, and across the boundary the ionosphere alters greatlywithin a wavelength. The propagation may now be considered in terms of reflec-tion at a sharp boundary. At oblique incidence the loss on reflection is relativelysmall, and in consequence these signals may propagate over great distances withan attenuation amounting to only 2–3 dB per 1000 km. They exhibit some inter-esting properties, one being that (except at high latitude) the diurnal variation ismore predictable than it is at higher frequencies, which makes them particularlysuitable for those applications, such as navigation and time transmission, whichrequire high stability.

In the lower ionosphere the collision frequency (2106 s1 at 70 km height) isgreater than the wave frequency at VLF. Neglecting the magnetic field, Equation(3.55) then gives the refractive index (n) as

n2 |1 jX /2 |12N /( j). (3.116)

The ionosphere now behaves as a metal rather than a dielectric, having conduc-tivity

( 02N)/Ne2/(me), (3.117)

where is the collision frequency.


Studies of the amplitude and phase of VLF signals received from transmittersat various distances indicate the effective reflection height (about 70 km by day) andthe ionospheric conductivity. Reflection coefficients are typically 0.2–0.5 There areactually four reflection coefficients because the presence of the geomagnetic fieldcauses changes of polarization on reflection as well as of amplitude and phase.

Putting typical values into the criterion of Equation (3.31) confirms that, atVLF and ELF, the lower ionosphere behaves as a conductor. Then, inserting thecondition [/( )]21 into Equation (3.29) leads to a skin depth (at which theamplitude falls by a factor of 1/e) of

1/ . (3.118)

The skin depth varies as the square root of the wavelength and inversely as thesquare root of the conductivity. The ground is also a partial conductor, and, evenin sea water, the most highly conducting part of the Earth’s surface, there is suffi-cient penetration to permit VLF and ELF communication with submerged sub-marines.

Over distances up to several hundred kilometers, VLF propagation can betreated by summing the ground wave and the first few hops (Figure 3.29). This isthe basis of geometrical-optical, or ray, theory.

For long-distance propagation, one must resort to waveguide theory as devel-oped by Budden (1961) and Wait (1970) and illustrated in Figures 3.30 and 3.31.This waveguide treatment is applicable because both the Earth and the ionosphereare partial conductors separated by a few wavelengths. In Figure 3.30 one assumesthat the signal at a point consists of component wavelets emanating from imagesof the source.

For long-distance VLF propagation the ionosphere behaves approximately likea conductor with a reflection coefficient of 1 and the ground has a reflectioncoefficient of 1. As in Figure 3.30, the images are located at z2h, 4h, . . . ,

0c / ()


Figure 3.29. Propagation in terms of a ground wave and two skywaves.

Ionosphere

GroundT R

ground wave1-hop

2-hop


Figure 3.30. Using the method of images to construct one of the pair of waves that willinterfere to produce the field patterns in the waveguide, such as those shown in Figure 3.31.The second wave (not shown) comes from the negative side. (After Davies, 1990.)

Figure 3.31. An idealiza-tion of the E field in theEarth–ionosphere wave-guide for waves polar-ized with their electricfields in the verticalplane and their magneticfields transverse to theplane of propagation(TM02 mode) (fromDavies, 1990).

but now they alternate in sign, which is equivalent to a change in phase of , andresonance occurs at

2hCn(n ), (3.119)

where Cn is n/(2h) and n1, 2, . . . .In Figures 3.30 and 3.31 there is no zeroth-order mode and the horizontal

wavelength g/Sn is given by

(1/g)2(1/)2 [n/(2h)]2, (3.120)

where Sn is the Fresnel coefficient showing that, for 2h/n, g is imaginary andhence the mode is evanescent. Thus there is a minimum cutoff frequency, fn, belowwhich waves will not propagate, where fnnc/(2h).

The cutoff frequency for the first-order mode during daytime, when the heightof the ionospheric D region is low, 2 kHz. For the case of a conducting iono-sphere, the cutoff frequency is given by

fn(n )c/(2h). (3.121)

So we see that the change of reflection coefficient R from 1 to 1 changes thecutoff frequency (for n1) from about 2 Hz to 1 kHz. When they are beingcompared with waveguide modes with perfectly conducting walls, the idealEarth–ionosphere modes should be denoted by n , rather than by n. A morecomplete analysis of the VLF waveguide mode must include Earth–ionosphereirregularities, changes in the height of the ionosphere, the effect of the geomag-netic field and collisions of electrons. There is a voluminous literature on VLFpropagation (see Budden, 1985, references; and Davies, 1990, pp. 371–379).

The main natural sources at ELF are lightning discharges, and the actual useof ELF for propagating signals is quite limited because of the practical constraintson constructing antennas several thousand meters long. Another importantfeature of ELF propagation is that the distance between source and receiver maybe comparable to the wavelength (for example, a 300-Hz ELF signal has a wave-length of 1000 km. At these extremely low frequencies, the ionosphere behavesmore like a conductor than a conducting dielectric and the displacement currentis small. Because of this large skin depth (see Ramo et al. 1965, pp. 249–299) in theD region for ELF, the reflection height is 90 km.

As an actual example, the U. S. Navy’s Wisconsin Test Facility (WTF) radiatesfrequencies in the 40–50 and 70–80 Hz ranges. At the WTF the antennas are two22.5-km quasi-orthogonal antennas. At middle latitudes the attenuation rate at 75Hz is about 1.2 dB Mm1 during the day and 0.8 dB Mm1 at night. ELF propa-gation is discussed in considerable detail in the June 1974 Proceedings of theIEEE. Anomalously strong ELF signals have also been received at antipodalregions (Fraser-Smith and Bannister, 1997).

12

12

12


Partial reflections at MF and HF

Turbulence in the lower ionosphere, at heights up to about 100 km, producesspatial irregularities on a scale sufficiently fine that partial reflections may bedetected from them in the band 2–6 MHz (wavelengths 1.5 km to 500 m). In thiscase the reflections are very weak, a mere 103 to 105 of the amplitude of a totalreflection, but they may be observed using a transmitter of high power and a largeantenna array for transmission and reception. Although they are not useful forcommunications, these partial reflections may be exploited in a technique formeasuring the electron-density profile of the lower ionosphere.

3.4.7 Whistlers

Whistlers are bursts of electromagnetic radiation in the VLF range that are pro-duced by lightning discharges. These bursts travel through the ionosphere andmagnetosphere in ducts approximately parallel to lines of force in the geomagneticfield and can be detected using low noise amplifiers with short antennas. Sinceabout 1951 these signals have been studied scientifically for the information theyreveal about the ionospheric and magnetospheric plasma. Other natural VLFemissions (called dawn chorus, risers, hiss, etc.) which are thought to originate inthe ionosphere can also be heard on whistler detection equipment. Some of thefascination with the whistler phenomenon is due to the fact that it is a remarkablesound in the audio range, resembling a human whistle, that can be heard on sen-sitive audio equipment and on telephone lines under certain circumstances. Thehistory of the scientific study of whistlers is covered by Eckersley (1925, 1928,1929, 1931, and 1932), Helliwell (1965, 1988), Davies (1990), Hunsucker (1991),and in reviews by Park and Carpenter (1978) and Carpenter (1988).

The starting points for whistler theory are Appleton’s equations for dispersionand polarization and the QL approximation. Figure 3.32 is a simplified presenta-tion of basic whistler signatures obtained near the source and near the conjugatearea of the source (i.e. the other end of the field line).

Another basic feature (not always present on a signature) is the “nose”(Helliwell, 1965) illustrated in Figure 3.33.

Helliwell (1965) showed that energy flow in the whistler will be guided alongducts in the geophysical magnetoplasma according to the following relation:

tan()(0.5tan) /(1 tan2), (3.122)

where is the angle between the ray path of the whistler and the wave normal, is the propagation angle limited by 0max, and fHcosmaxf, where fH is theelectron gyrofrequency.

Another important characteristic of a whistler wave packet is that the groupvelocity, vg, is

vg2c[ f 1/2(| fL|f )]/| fL| fN (3.123)

12



Figure 3.32. A sketch of the basic manifestation of a whistler and its initiating distur-bances: (a) illustrating the dispersion; (b) the frequency–time curve of a typical whistler; (c)the curve of √f with time, and (d) the initiating disturbance and multiple hops when thesource and receiver are at the same end of a geomagnetic field-line (from Helliwell, 1965).

where fL is the longitudinal component of fB and fLf, Equation (3.123) simplifiesto

vg2c( f 1/2 fL1/2 )/fN. (3.124)

The dispersion law for whistlers is

T fN fLds/[ f 1/2 ( fLf )3/2], (3.125)

which can be applied to determine Ndl along the field line.

3.5 Ionospheric scatter

One can qualitatively describe ionospheric scattering as either strong or weak interms of the received signal strength of the scattered wave at the receiving radarantenna. An example of the former is VHF/UHF backscatter echoes receivedfrom electron density gradients in the auroral or equatorial ionosphere, and anexample of the latter is incoherent backscatter by a VHF/UHF radar from theundisturbed E or F layer.

Another way of classifying scattered echoes is in terms of their backscattercross-section (, in m2 ) using pulsed radar systems, and their temporal stability. Acoherent echo exhibits a statistical correlation of the amplitude and phase fromone pulse to another, and emanates from quasi-deterministic gradients in electrondensity that have correlation times greater than 1 ms, which corresponds to a spec-tral width of the radar echo of less than 1000 Hz (sometimes less than 100 Hz). Italso has a backscatter cross-section 104–109 times greater than that from an inco-herent-scatter radar echo.

3.5.1 Coherent scatter

Other important considerations in the case of coherent backscatter are the relationbetween the size of the scattering irregularity relative to the free-space wavelength

12c

s


Figure 3.33. An idealized sketch of the frequency-versus-time characteristics of a “nosewhistler” (from Helliwell, 1965).

of the backscatter sounder, the mean fractional deviation in electron density of thescatterer, and the aspect angle between the radar LOS and the major axis of theirregularity. Figure 3.34 shows the approximate height–frequency domains oftypical ionospheric sounding systems.

The first quantitative description of coherent scatter from ionosphericirregularities was published by Booker (1956) (an extension of the Booker–Gordon (1950) troposcatter theory), when he developed a theory that describedbackscatter from field-aligned irregularities in the auroral E region. The resultsare also applicable to backscatter from F-region irregularities. The geometry ofscatter from an ionospheric irregularity is shown in Figure 3.35.

From the geometry in Figure 3.35, we can obtain one form of the Booker


Figure 3.34. Height–frequency regimes of various ionospheric radar probes (from Schlegel,1984).

Figure 3.35. The geometry for scatter from ionospheric irregularities.

ionospheric-irregularity scatter equation expressed in terms related to ionosphericparameters as

(, )(N/N )2(2L/N)2sin2 /N [1(4L/)2sin2(/2)], (3.126)

where (, ) is the backscatter cross-section of the irregularity, (N/N)2 is themean square fractional deviation in electron density, N is the wavelength ofplasma oscillation, and L is the scale size of the irregularity along B.

Relations for the backscatter cross-section in the cases of large and smallirregularities are derived in Hunsucker (1991, p. 56). Walker et al. (1987) startedwith the Booker scattering equation and derived a more general expression forthe backscattered power at the receiver (see also Hunsucker, 1991, pp. 56-58).

3.5.2 Forward scatter

Irregularities due to turbulence in the 75–90-km regions of the ionosphere permitone to design one-hop communication circuits at VHF (Bailey et al., 1955; Nortonand Wiesner, 1955). The ionoscatter mode typically uses frequencies from 30 to 60MHz, over distances of 1000–2000 km with system losses of 140–210 dB and ausable bandwidth of 10 kHz. Because of the high system loss, very-high-powertransmitters, large high-gain antennas and sensitive receiver front ends arerequired. Ionoscatter systems are also characterized by very high reliability andsecurity, but use of this bandwidth probably involves the highest cost per system ofall radio systems. In the late 1950s and early 1960s considerable use of the ionoscat-ter mode was made because of its 99.9% reliability and security, but, with the adventof satellite–Earth radio systems, use of the ionoscatter mode decreased drastically.

3.5.3 Incoherent scatter

The development of the incoherent-scatter radar technique has provided a verypowerful method for investigating the ionosphere. Evans (1969 and 1972) sum-marised the essentials of incoherent-scatter theory and practice and rigorous derivations of the salient equations are given by Krall and Trivelpiece (1973).

The basic theory of scattering of electromagnetic waves from free electrons wasdeveloped by the discoverer of the electron, J. J. Thomson, who in 1906 showedthat the energy scattered by a single electron is

W(resin)2, (3.127)

where W is the energy scattered by a single electron into unit solid angle per unitof incident electromagnetic flux (1 W m2 ); re is the classical electron radius,ree2/( 0mec

2 )2.821015 m; and is the angle between the direction of theincident electric field and the direction of the observer.


The radar cross-section of an individual electron would then be

e4(resin)21028sin2 (m2 )

and, for backscatter (/2),

e4re2. (3.128)

Fejer (1960) showed that the radar cross-section per unit volume is simply

Ne, (3.129)

where N is the electron density, and Buneman (1962) showed that the incoherentscatter effective radar cross-section (eff

) can be written as

eff1/[(12 )(1Te/TI2)] (3.130)

for Te/TI3.0, and Te is the electron temperature, TI is the ion temperature,4D/, where D is the Debye length, D6.9(Te /Ne)

1/2, in centimeters, and is the free-space wavelength of the radar signal.

Since the electrons are in random thermal motion, they will scatter signalswhose phases are varying with time and are not related to one another. At theradar-receiving antenna the signal powers will add so that, on the average, the cross-section per unit volume is that given by Equation (3.129), giving use to the name“incoherent scatter”.

The interesting history of the development of incoherent-scatter theory andpractice starting shortly after the end of WWII has been described by Davies(1990, pp. 106–111) and by Hunsucker (1991, pp. 58–64). Dougherty and Farley(1960) explained the discrepancy between the predicted and measured Dopplerbroadening of the echo spectrum in terms of the radar wavelength, electron andion temperatures, and the Debye length,

D69(Te/Ne)1/2 (m), (3.131)

Where T is in kelvins and N in m3.Incoherent scattering occurs from fluctuations in electron density having a

scale of D. The backscatter, then, is actually due to local fluctuations in electrondensity, instead of purely scatter from free electrons, and more correctly should becalled something like quasi-incoherent scatter, but the term incoherent scatter haspersisted. Some authors continue to refer to the incoherent-scatter phenomena asThomson scatter for a variety of reasons; however, the term Thomson scatter isnormally reserved for situations of scattering from free electrons without influencefrom ions.

In practice, this incoherent scatter is detected from the ionosphere principally


when D, although experiments at Arecibo have detected incoherent scatterwhen D (Hagen and Behnke, 1976). The spatial scale of the irregularities p isgiven by the Bragg formula,

p/[2sin(/2)] (3.132)

with the geometry as shown in Figure 3.35.The spectrum of an incoherent-scatter echo is very rich in information about

the magnetoplasma which it is probing. A few of these plasma properties are easyto obtain, most requiring only straightforward data-analysis techniques, but somerequire complex processing using specific models of ionospheric regions.

Figure 3.36 is an idealized sketch of the spectrum of an incoherent-scatter echofrom the ionosphere, showing the ion line on the left and the plasma line on theright. The ion line is centered on the operating frequency, f, and the energy back-scattered by irregularities of scale characterized by the Debye length that are inrandom motion, whereas the plasma line is centered on the plasma frequency fN

and is due to the thermal motions of electrons not under the influence of ions. Theplasma line is a weak line, except when it is enhanced by “hot” photoelectrons;and when the line is enhanced, both the electron density and the characteristics ofthe photoelectron flux may be measured.

In the lower ionosphere the motions of the ions (which in turn control those ofthe electrons) are increasingly affected by collisions with the neutral air. The spec-trum now becomes single-peaked, with width proportional to T/(mII

2 ), whereT is the temperature, mi and i are the mass and collision frequency of the ions,and is the wavelength of the radar. If 70 cm and T230 K, the line is 1000Hz wide at the height of 100 km, but, due to the increase of collision frequency,only a few hertz wide at 75 km.


Figure 3.36. An idealized sketch of the ISR spectrum.

In this, the collision-dominated region, the returned spectrum has the Lorentzform,

S( f )A/[1f 2 /(f )2 ]. (3.133)

(A is just a constant.) It is obviously much simpler than the F-region spectrum ofFigure 3.44, and is fully described by its half-width:

f16kT/(mii2). (3.134)

A Doppler shift is superimposed if the scattering volume is moving towards oraway from the radar. The spectrum is somewhat broadened if negative ions arepresent. Equation (3.134) also assumes that the ion and electron temperatures areequal.

3.6 HF-propagation-prediction programs

In the last two decades, over a dozen HF-propagation programs have been devel-oped for use on personal computers. Some representative examples are listed inTable 3.7. It should be emphasized that all these programs input median-valuedata and produce median values of MUF, LUF, signal strength, etc. as output andare basically intended for HF-circuit planning, not real-time prediction.

Most of the programs above take transmitter and receiver locations, time,month, year, and usually the number of sunspots as input, and provide MUF,LUF mode structure, antenna headings, great-circle distance and root-mean-square median-field-strength values for mid-latitude HF paths. The calculation ofsignal strength is especially difficult, because the exact mode structure on a par-ticular path is not accurately known and all the path losses (in the D region, in thetransmission line of the antenna, and from mismatch, ground reflection, etc.) aredifficult to accurately characterize (see Sailors and Rose, 1991; andAGARDograph No. 326, 1990). Also, HF-propagation mode structure and lossesat high latitudes are almost impossible to describe, so predictions of paths thatinclude ionospheric reflections and points of D-region penetration in the auroraland polar ionosphere are almost useless (see Hunsucker, 1992; and discussions inChapters 8 and 9 in this book).

There are several books covering the essentials of antennas, radio propagationat all frequencies, and related topics, such as those by Jordan and Balmain (1968),Sanders and Reed (1986), Rao (1977), Stutzman and Thiele (1981), Kraus (1988),Collin (1985), Hall and Barclay (1989), Freeman (1997), Balmain (1997), Hansen(1998), and Kildal (2000). There are also several recent books covering all aspectsof ionospheric radio propagation and magnetoionic theory, such as those byMaslin (1987), Davies (1990), McNamara (1991), and Goodman (1992).


3.7 Summary

It is, of course, impossible to cover the entire topic of radio propagation in onechapter, but we have attempted to list the essential elements of pertinent terrestrialpropagation modes and of antenna systems. It is fortunate that there are recentbooks available, which describe in considerable detail the particulars of thesemodes (Budden, 1985; Hall and Barclay, 1989; Davies, 1990; Goodman, 1992;Freeman, 1997). A very significant new development is the availability of PC orworkstation-based software to analyze antennas, terrain and propagation predic-tion, as listed in the tables of this chapter. Another new development is the avail-ability on the internet/www of URLs, which give near-real-time data for

3.7 Summary 175

Table 3.7. Representative PC-based HF-propagation-prediction programs

Name of program Description Source References andremarks

AMBCOM Includes some Hatfield (1980)effects of the high-latitude ionosphere

ASAPS 2 IPS (1991)FTZMUF2 foF2 and M3000 Dambolt and

MUF–LUF (?) Sussman (1988a, b)FTZ4 Improved

calculation ofHFBC84 several parametersHFMUFES4 Barghausen et al.

(1969)ICEPAC Includes some Stewart (1990),

effects of the high- private communicationlatitude W1FM (Lexington, ionosphere MA)

IONOSONDMINIFTZ4 Field strength Dambolt and

Sussman (1988a, b)MINIMUF MUF, LUF Rose (1982)

PROPHET Rose (1982)PROPMAN MUF/LUF, signal Roesler (1990)

strength, VOACAP User-friendly shell Lane (1993)

for IONCAP

Note:MUF, maximum usable frequency; LUF, lowest usable frequency.

radio-prediction purposes. One excellent example is the “space–weather”, mag-netospheric, and ionospheric data bases available from the U. S. NOAA SpaceEnvironment Center (http://www.sec.noaa.gov).


Section 3.2ARRL (1999) The ARRL Antenna Book. The American Radio Relay League,Newington, Connecticut.

ARRL (2000) The ARRL Handbook, 77th edition. The American Radio RelayLeague, Newington, Connecticut.

Balanis, C. A. (1997) Antenna Theory, Analysis and Design. Wiley, New York.

Hunsucker, R. D. (1991) Radio Techniques for Probing The Terrestrial Ionosphere.Springer-Verlag, Heidelberg.

Skolnik, M. F. (1980) Introduction to Radar Systems, 2nd edition. McGraw-Hill, NewYork.

Wolf, E. A. (1988) Antenna Analysis. Artech House, Norwood, MA.

Section 3.3AGARDograph No. 326 (1990) Radio Wave Propagation Modeling, Prediction andAssessment, pp. 69–72. AGARD/NATO.

Andersen, J. B., Hvid, J. T., and Toftgard, J. (1993) Comparison between different pathloss prediction models, COST 231-TD(93)-06, January, Barcelona.

Brent, R. I. and Ormsby, J. F. A. (1994) Electromagnetic propagation modeling in 3Denvironments using the Gaussian beam method. Joint Electronic Warfare CenterTechnical Report JDR 3-94.

CCIR Report 322-3c (1988) Characteristics and applications of atmospheric noisedata. XVth Plenary Assembly, Dubrovnik. International Telecommunications Union,Geneva.

Chamberlain, K. and Luebbers. R. (1992) GELTI Propagation Model: Theory ofOperation and Users’ Manual. Available through the authors.

Collin, R. E. (1985) Antennas and Radiowave Propagation. McGraw-Hill Book Co.,New York.

Eppink, D. and Kuebler, W. (1994) TIREM/SEM Handbook. DoD ECAC, Annapolis,Maryland.

Freeman, R. L. (1997) Radio System Design for Telecommunications. Wiley, New York.

Grosskopf, R. (1994) Propagation of urban propagation loss. IEEE Trans. AntennasPropagation 42, 1–7.

Hansen, R. C. (1998) Phased Array Antennas. Wiley, New York.

Hey, H. S. (1983) The Radio Universe, 3rd Edition. Pergamon Press, Oxford.

Hunsucker, R. D. (1992) Auroral and polar cap ionospheric effects on radio propaga-tion. IEEE Trans. Antennas Propagation 40, 818–828.


Jordan, E. C. and Balmain, K. G. (1968) Electromagnetic Waves and RadiatingSystems, 2nd Edition. Prentice-Hall, Inc., Englewood Cliffs, New Jersey.

Kraus, J. D. (1988) Antennas, 2nd Edition. Cygnus-Quasar Books, Powell, Ohio.

Marcus, S. (1994) Duct propagation over a wedge-shaped hill, BLOS Proc. AppliedResearch Laboratory, University of Texas, Austin, Texas.

Patterson, W. (1994) EM propagation program at NCCOSC, BLOS Proc. AppliedResearch Laboratory, University of Texas, Austin, Texas.

Rao, N. N. (1977) Elements of Engineering Electromagnetics. Prentice-Hall, Inc.,Englewood Cliffs, New Jersey.

Ryan, F. J. (1991) Analysis of Electromagnetic Propagation Over Variable TerrainUsing the Parabolic Wave Equation. Naval Ocean Systems Center, San Diego,California.

Sailors, D. B. (1993) A Discrepancy in the CCIR Report #22-3 Radio Noise Model.NCCOSC/NRaD, San Diego, California.

Sanders, K. F. and Reed, G. A. L. (1986) Transmission and Propagation ofElectromagnetic Waves. Cambridge University Press, Cambridge.

Spaulding, A. D. and Washburn, J. S. (1985) Atmospheric Radio Noise: WorldwideLevels and Other Characteristics. ITS, Boulder, Colorado.

Vincent, W. R. and Munsch, G. F. (1996) Power-line Noise Mitigation Handbook forNaval Receiving Sites, 3rd Edition. COMMNAVSECGRU, Meade, Maryland.

Section 3.4Appleton, E. V. (1930) Some notes on wireless methods of investigating the electricalstructure of the upper atmosphere. Proc. Phys. Soc. 42, 321.

Budden, K. G. (1961) Radio Waves in the Ionosphere. Cambridge University Press,Cambridge.

Budden, K. G. (1985) The Propagation Of Radio Waves: The Theory of Radio Wavesof Low Power in the Ionosphere and Magnetosphere. Cambridge University Press,Cambridge.

Carpenter, D. L. (1988) Remote sensing of the magnetospheric plasma by means ofwhistler mode signals. Rev. Geophys. 26, 535–549.

Crane. R. K. (1974) Morphology of ionospheric scintillation. Technical Note 1974–26,Lincoln Laboratory, MIT.

Davies, K. (1969) Ionospheric Radio Waves. Blaisdell Publishing Co., Waltham,Massachusetts.

Eckersley, T. L. (1925) Note on musical atmospheric disturbances. Phil. Mag. 49: (5),1250–1259.

Eckersley, T. L. (1928) Letter to the editor. Nature 122,768–769.

Eckersley, T. L. (1929) An investigation of short waves. J. Inst. Electr. Engineers 67,992–1032.

Eckersley, T. L. (1931) 1929–1930 developments in the study of radio wave propaga-tion. Marconi Rev. 5: 1–8.

Eckersley, T. L. (1932) Studies in radio transmission. J. Inst. Electr. Engineers. 71,434–443.


Fraser-Smith, A. C. and Bannister, P. R. (1997) Reception of ELF signals at antipodaldistances. Radio Sci. 32.

Hargreaves, J. K. (1992) The Solar–Terrestrial Environment. Cambridge UniversityPress, Cambridge.

Helliwell, R. A. (1965) Whistlers and Related Ionospheric Phenomena. StanfordUniversity Press, Stanford, California.

Helliwell, R. A. (1988) VLF wave stimulation experiments in the magnetosphere forSiple Station, Antarctica. Rev. Geophys. 26, 551–578.

Hunsucker, R. D. (1999) Electromagnetic Waves in the Ionosphere. In WileyEncyclopedia of Electrical and Electronics Engineering (ed. J. Webster), pp. 494–506.Wiley, New York.

Kelso, J. M. (1964) Radio Ray Propagation in the Ionosphere. McGraw-Hill, NewYork.

Park, D. and Carpenter, D. (1978) Very low frequency radio waves in the magneto-sphere. In Upper Atmospheric Research in Antarctica (ed. L. J. Lanzerotti and C. G.Parr). American Geophysical Union, Washington, DC.

Radio Science (1967). Special issue on analysis of ionograms for electron density pro-files. Radio Sci., 2, 1119–1282.

Ratcliffe, J. A. (1956) Some aspects of diffraction theory and their application to theionosphere. Rep. Prog. Phys., 19, 188.

Ratcliffe, J. A. (1959) The Magneto-ionic Theory and its Application to the Ionosphere.A Monograph. Cambridge University Press, Cambridge.

Smith, N. (1939) The relation of radio sky-wave transmission to ionosphere measure-ments. Proc. IRE 27, 332–347.

Titheridge, J. E. (1985) Ionogram Analysis with the Generalized Program POLAN.World Data Center-A, NOAA, Boulder, Colorado.

Umeki, R., Liu, C. H. and Yeh, K. C. (1997a) Multifrequency studies of ionosphericscintillations. Radio Science 12, 311.

Umeki, R., Liu, C. H. and Yeh, K. C. (1997b) Multifrequency spectra of ionosphericamplitude scintillations. J. Geophys. Res 82, 2752.

URSI (1972) URSI handbook on ionogram interpretation and reduction, 2nd Ed.,NOAA WDC-A, Rep. UAG-23, Boulder, Colorado.

Wait, J. R. (1970) Electromagnetic Waves in Stratified Media, 2nd Edition. PergamonPress, New York.

Yeh, K.-C., Chao, H. Y. and Lin, K. H. (1999) A study of the generalized Faradayeffect in several media. Radio Sci. 34, 139.

Yeh, K.-C. and Liu, C.-H. (1982) Radio wave scintillation in the ionosphere. Proc.IEEE 70, 324–360.

Section 3.5Bailey, D. K., Bateman, R. and Kirby, R. C. (1955) Radio transmission at VHF byscattering and other processes in the lower in the lower ionosphere. Proc. IRE 43,1181.


Booker, H. G. (1956) A theory of scattering by nonisotropic irregularities with applica-tion to radar reflection from the aurora. J. Atmos. Terr. Phys. 8, 204–221.

Booker, H. G. and Gordon, W. E. (1950) A theory of radio scattering in the tropo-sphere. Proc. Inst. Radio Engineers 38, 401–402.

Buneman, O. (1962) Scattering of radiation by the fluctuations in a non-equilibriumplasma. J. Geophys. Res. 67, 2050–2053.

Dougherty, J. P. and Farley, D. T. (1960) A theory of incoherent scatter of radio wavesby a plasma. Proc. R. Soc. A 259, 79.

Evans, J. V. (1969) Theory and practice of ionospheric study by Thomson scatterradar. Proc. IEEE 57, 496.

Evans, J. V. (1972) Ionospheric movements measured by incoherent scatter: A review.J. Atmos. Terr. Phys. 34, 175.

Fejer, J. A. (1960) Scattering of radiowaves by an ionized gas in thermal equilibrium. J.Geophys. Res. 65, 2635.

Hagen, J. B. and Behnke, R. A. (1976) Detection of the electron component of thespectrum in incoherent scatter of radio waves by the ionosphere. J. Geophys. Res. 81,

3441–3443.

Krall, N. A and Trivelpiece, A. W. (1973) Principles of Plasma Physics. McGraw-Hill,New York.

Nakajima, M. (1960) The m-distribution – A general formulation of intensity distribu-tion of rapid fading. In Statistical Methods in Radio Propagation (ed. W. C. Hoffman).Oxford, Pergamon.

Norton, K. A. and Wiesner, J. B. (1955) The scatter propagation issue. Proc. IRE 43,1174.

Schlegel, K. (1984) HF and VHF Coherent Radars for Investigation of the High-latitudeIonosphere. Max Planck Institut für Aeronomie, Katlenburg-Lindau.

Walker, A. D. M, Greenwald, R. A., and Baker, K. D. (1987) Determination of thefluctuation level of ionospheric irregularities from radar backscatter measurements.Rad. Sci. 22: 689–705.

Section 3.6Barghausen, A. F., Finney, J. W., Proctor, L. L. and Schultz, L. D. (1969) PredictingLong-term Operational Parameters of High Frequency Skywave TelecommunicationsSystems. ESSA, Boulder, Colorado.

Damboldt, T. and Suessmann, P. (1988a) FTZ High Frequency Sky-wave FieldStrength Prediction Method for Use on Home Computers. Forschungsinstitut der DBPbeim FTZ.

Damboldt, T. and Suessmann P. (1988b) A Simple Method of Estimating foF2 andM3000 with the Aid of a Home Computer. Forschungsinstitut der DBP beim FTZ.

Davies, K. (1990) Ionospheric Radio. Peter Peregrinus, London.

Hatfield, V. E. (1980) HF communications predictions, 1978. (An economical up-to-date computer code, AMBCOM). In Solar–Terrestrial Predictions Proc. (ed. R. F.Donnelly), Vol. 4, D2 1–15. US Government Printing Office, Washington DC.


Jordan, E. C. and Balmain, K. G. (1968) Electromagnetic Waves and RadiatingSystems, 2nd Edition, Prentice-Hall, Englewood Cliffs, New Jersey.

Lane, G. (1993) Voice of America Coverage Analysis Program (VOACAP). USInformation Agency, Bureau of Broadcasting, Washington DC.

Maslin, N. M. (1987) HF Communications: A Systems Approach. Plenum Press, NewYork.

McNamara, L. F. (1991) The Ionosphere: Communications, Surveillance, and DirectionFinding. Krieger Publishing Co., Malabar, Florida.

Roesler, D. P. (1990) HF/VHF Propagation resource management using expertsystems. In The Effect of the Ionosphere on Radiowave Signals and SystemsPerformance (IES90) (ed. J. M. Goodman), pp. 313–321. USGPO, available throughNTIS, Springfield, Virginia.

Rose, R. (1982) An emerging propagation prediction technology. In Effects of theIonosphere on Radiowave Systems (IES81) (ed. J. Goodman). US GovernmentPrinting Office, Washington, DC.

Sailors, D. B. and Rose, R. B. (1991) HF Sky Wave Field Strength Predictions.NCCOSC/NRaD, San Diego, California.

Section 3.7Briggs, B. H. and Parkin, J. A. (1963) On the variation of radio star and satellite scin-tillation with zenith angle. J. Atmos. Terrestr. Phys. 25, 339.

Goodman, J. (1992) HF Communications – Science and Technology. Van NostrandReinhold, New York.

Hall, M. P. M. and Barclay, L. W. (eds.) (1989) Radiowave Propagation. PeterPeregrinus Press for the IEE, London.

Nakajima, M. (1960) The m-distribution – A general formulation of intensity distribu-tion of rapid fading. In Statistical Methods in Radio Propagation (ed. W. C. Hoffman).Oxford, Pergamon.

General readingKildal, P.-S. (2000) Foundations of Antennas – A Unified Approach. Studentlitteratur,Lund.

Ramo, S., Whinnery, S., and van Duzer, T. (1965) Fields and Waves in CommunicationElectronics. Wiley, New York.


Chapter 4

Radio techniques for probing the ionosphere

4.1 Introduction

The purpose of this chapter is to review the basic techniques (and the newer mod-ifications and adaptations of these techniques) for studying the terrestrial iono-sphere, with particular emphasis on the capabilities and limitations of thetechniques when they are used to probe the high-latitude ionosphere. We are for-tunate to have several books and reports written since 1989 that have addressedthe general topic of ionospheric investigations using radio techniques (Kelley,1989; Liu, 1989; Davies, 1990; Hunsucker, 1991; Hargreaves, 1992; Hunsucker,1993 and 1999; pp. 502–505), so in this chapter we will emphasize the limitationsand capabilities of these techniques and update the information on deployment ofionospheric instrumentation at high latitudes. Figure 3.34 of Chapter 3 shows thefrequency–height regimes which various selected radio techniques can probe.

4.2 Ground-based systems

4.2.1 Ionosondes

In its simplest form, an ionosonde consists of a transmitter and receiver withcoupled tuning circuits, which is swept in frequency (usually in the frequencyrange of approximately 0.5–25 MHz). It can be either a pulsed or a CW-FM(chirp) system, and the transmitter and receiver can either be co-located (mono-static) or separated (bistatic). After the RF signals have been reflected by the ion-osphere they are received and processed by the receiver to produce ionograms. The

181

basic information in the received signal is the transit time for passage betweenionospheric layers and the Earth, frequency, amplitude, phase, polarization,Doppler shift, and spectrum shape (see Section 3.2.4). From these quantities, wecan obtain an ionogram, which is a plot of the virtual height of reflection versusfrequency. We can also deduce the true height of ionospheric layers as a functionof frequency, the line-of-sight (LOS) velocity, some communication parameters,and the vector velocity of ionospheric irregularities (with an array of severalantennas). Historically, the ionosonde was the instrument used to confirm theexistence of the ionosphere by Appleton and Barnett (1926) and by Breit and Tuve(1926). A brief account of the development of the primitive and first-generationionosonde is given in Sections 3.1 and 3.2 of Hunsucker (1991) and by Bibl (1998).

The so-called “standard” ionosondes used vacuum tubes and electromechani-cal tuning mechanisms and were very bulky and heavy, as shown in Figure 4.1. Atypical ionogram from a “standard” ionosonde in Yamagawa, Japan is shown inFigure 4.2, whereas an idealized ionogram is shown in Figure 4.3.

These standard ionosondes were produced in relatively large numbers, andwere deployed globally from c. 1942 until 1975. The photographically recordeddata provided by these sounders have contributed greatly to our state of knowl-edge of the ionosphere. The data, however, must be manually analyzed by trained“scalers” and the data film archived in controlled-climate storage facilities. A map

182 Radio techniques

Figure 4.1. NBS Model C-3 ionosonde installation. The power supply is on the left and theactual ionosonde is on the right.

of the global distribution of ionosondes (mainly the standard models) as of 1982is shown in Figure 4.4.

With the advent of reasonably priced compact personal computers, digitalsignal processing, new modulation-coding techniques, and VLSI, a new genera-tion of ionosondes was developed, starting in the mid-1960s and continuing intothis century. Many of these ionosondes are portable and all have much smallervolume, weight, and power consumption than did the standard ionosondes, andthey produce much better ionograms. The modern sounders also permit the dele-tion of discrete frequencies that are contaminated by interference, and the dele-tion of frequencies that may interfere with other services. Advances inantenna-array theory have also made it possible to deploy arrays of receivingantennas in such a way as to permit direction-of-arrival (DOA) determinationfor echoes, permitting the production of “skymaps” for selected heights.


Figure 4.2. A “typical ionogram” from a “standard” ionosonde (frequency range 0.5–12 MHz ,height range 1000 km, power 10 kW, sweep time 20 s, linear frequency scale. Note the heavyvertical lines – caused by MF and HF interference.

Figure 4.3. An idealized ionogram.


Figure 4.4. A map of all ionosondes known to have existed as of 1982.

Representative examples of the new sounders available at the time of writing areshown in Table 4.1. An ionogram obtained from a typical modern ionosonde isillustrated in Figure 4.5.

Most of the ionosondes which produce ionograms such as that shown in Figure4.5 are of the “modern” type, since the “standard” ionosondes are obsolescentand extremely difficult to maintain. An up-to-date description of the modernsounders and their deployment is given by Wilkinson (1995). The modern iono-sondes permit the study of a wide range of ionospheric irregularities as illustratedschematically in Figure 3.34.

Capabilities and limitations

A limitation of all ionosondes is that they can yield information on the ionosphereonly up to the height of maximum ionization of the F2 layer (the “bottomside” ofthe ionosphere). Also, unless one extends the low-frequency end of the sweep (toat least 250 kHz) by increasing the height of the transmitting antenna tower andusing relatively high power, not much information can be obtained from the D


Table 4.1. Typical available ionosondes

Name of Sounder Specifications Source

Digisonde Portable Frequency range 1–32 MHz University of Sounder (DPS) Power 300 W pulse Massachusetts Center for

Height range 90–1000 km Atmospheric Research, Doppler sounding, etc. 600 Suffolk Street, 3rd

Realtime data transfer via Floor, Lowell, MA 01854, the internet USA www.uml.eduAutomatic scaling available

Canadian Advanced Frequency range 1–20 MHz Scientific Instruments, Digital Ionosonde (CADI) Power 600 W pulse (13-bit Ltd, 2233 Hanselman

Barker code) Avenue, Saskatoon, CA Height range 90–1024 km S7L6A7, USADoppler sounding, etc.Realtime data transfer viathe internet

Ionosonde: HF Diagnostics Frequency range 1–30 MHz Center for Remote Module, 01-2000 Power 1 kW CW and Sensing, Inc., 11350

peak Random Hills Road, Doppler sounding, etc. Suite 710, Fairfax, VA

22030, USA

Advanced Digital For specifications contact KEL Aerospace Pty Ltd, Ionosonde, IPS-71 www.kel.gov.au 231 High Street,

Ashburton, Victoria 3147,Australia

region. This is in contrast with the incoherent-scatter-radar (ISR) technique,which, however, is much more expensive and definitely not as portable. Anotherlimitation is that, during episodes of intense E-region ionization (“blanketing-E”),it is not possible to obtain much information on the F region. Approximate costsof new “modern” ionosondes currently vary from about $ 30000 to over $ 250000.

At auroral latitudes all ionosondes are subject to several rather severe limita-tions – namely that, during some of the most “interesting” times, auroral-E ion-ization or D-region absorption precludes the gathering of any ionosphericinformation on the layers above! These “interesting” times include magneticstorms and substorms and associated auroral and polar-cap absorption, intenseauroral events, and extreme spread-F conditions.

4.2.2 Coherent oblique-incidence radio-sounding systems

We shall refer to the systems which utilize coherent radars to obtain either directbackscatter or ground-reflected backscatter from ionospheric features as obliquebackscatter sounders (OBSs). The systems may be either bistatic or monostatic in


Figure 4.5. A typical modern digital ionogram (compare with Figure 4.2).

configuration. OBS systems are also referred to in the literature as ionosphericradars, coherent scatter radars (CSRs), backscatter sounders, and auroral radars.They are discussed in considerable detail in Greenwald et al. (1978), Liu (1989,Sections 11 and 12), Hunsucker (1991, pp. 94–109), and Hunsucker (1993, pp.441–450). Specifically, the WITS Handbook, edited by Liu (1989) devotes Sections11 and 12 (64 pages) to two types of OBS systems: auroral radars and HF ground-scatter radars in Appendix A1.2, as well as fundamentals of plasma dynamics andelectrodynamics of the equatorial, mid-latitude, and high-latitude ionosphere inChapters 2, 3, 5, and 6.

Basic principles

A coherent-scatter echo exhibits a statistical correlation of the amplitude andphase from one pulse to another, and emanates from quasi-deterministic gra-


Figure 4.6. An idealized sketch of the ground backscatter mode (a) and a sketch of directbackscatter from field-aligned irregularities (FAIs) in the auroral oval (b). In reality, the HFray path is usually refracted by the ionosphere, making it orthogonal to the FAI.

(a)

(b)

dients in electron density, which have correlation times usually greater than 1 ms.One can also describe backscatter as “strong” compared with incoherent-scatterechoes (the “scattering cross-section” for coherent backscatter is 104–109 timesgreater than that for incoherent scatter). In general, coherent backscatter isobtained when the ray path from the transmitting antenna intersects large elec-tron-density gradients or field-aligned irregularities, at near-perpendicular inci-dence. Thus, coherent backscatter is 40–90 dB stronger than incoherent scatter,and is qualitatively similar to specular reflection. However, for a full understand-ing of the ionospheric physics, considerable plasma theory must be employed. Theessence of the plasma-theory description is that, when plasma instabilities arepresent in the ionosphere, the amplitude of fluctuations in the medium can growto much higher levels than the thermal background. Coherent scatter occurs whenthe wave vector of the medium (km) equals twice the wave vector of the transmit-ted wave (kt).

Rather complete descriptions of the history of the development of the OBStechnique, and basics of the various systems, were given by Croft (1972), inChapter 11 of the WITS Handbook, and in Hunsucker (1991, Sections 4.1.1, 4.2.1,and 4.3.1). It is interesting to note that the first observation of coherent backscat-ter (from the ground) was made by Mogel in 1926, but not really understood until1951, when it was explained independently by Dieminger (1951) and Peterson(1951). There is another class of sounders known as oblique ionosondes or“synchronized-sounders,” which are used primarily for assessing propagationcharacteristics of the ionosphere for HF communication circuits (see Goodman,1992, Chapter 6). There is also an important “subset” of OBSs, most oftenreferred to as over-the-horizon (OTH) radars, which are used by military servicesand other government agencies primarily for the detection of airplanes, ships, andmissiles. The hardware and software are quite sophisticated, and the subject hadbeen highly classified until fairly recently, when some of the systems were madeavailable for ionospheric and oceanographic research. Descriptions of some of theOTH radar systems and results are given by Barnum (1986), Brookner (1987), ina special issue of the IEEE Journal on Oceanic Engineering (1986), and in a specialsection of Radio Science (1998).

Modern OBS systems typically operate in the HF and VHF bands and usecontinuous-wave (CW), pulse-coded, or FMCW modulation. They obtain iono-spheric information either from direct backscatter from field-aligned irregular-ities, or by backscatter from irregularities via a ground-reflected mode, asillustrated in the idealized sketch in Figure 4.6.

In the groundscatter mode (at the top of Figure 4.6), the echoes returned to thereceiver will be affected by irregularities near the ionospheric-reflection point, bythe Earth-surface characteristics, and by field-aligned irregularities (FAIs), wherethe second hop enters the ionosphere. It is necessary to analyze the Doppler veloc-ity, the phase characteristics, and the spectral shape of the echo to identify thescattered echo of interest. The bottom part of Figure 4.6 illustrates the mode


involving direct backscatter from FAIs, which may be significantly influenced byionospheric refraction (depending on the frequency of the sounder). Figure 4.7summarizes the type of information from an OBS which may be of interest toplasma physicists.

Types of oblique sounders currently in use

Having generically described the sounders in the previous section, we will proceedto classify and describe them by their operating frequency and describe several ofthe systems currently deployed globally. The lowest-frequency OBS systems arethe VLF sounders described by Kossey et al. (1983), sweeping between 5 and 30kHz using pulse widths 100 ms. Figure 4.8 illustrates the basic system configu-ration and Figure 4.9 shows data obtained during disturbed periods in the polarlower ionosphere.

To the best of the authors’ knowledge, no VLF sounders are at present in oper-ation. However, VLF sounding remains a practical technique for probing the Dand E regions of the ionosphere in some detail, especially at high latitudes.

In the HF region (3–30 MHz) of the radio spectrum, the OBS technique hasbeen employed since the mid-1920s. See Hunsucker (1991, Chapter 4) for adescription of the history and theory for OBS systems. Perhaps the best examplesof the HF OBS technique is the SuperDARN (Dual Auroral Radar Network)system (Greenwald et al., 1995) and the PACE (Polar and Conjugate Network)system (Baker et al., 1989). These HF radars operate in the frequency range of


Figure 4.7. A summary of coherent-scatter radar investigations from a plasma-physicspoint of view (after Schlegel, 1984).

NATURALPHENOMENA

ARTIFICIALLY CREATEDIRREGULARITIES

COHERENT RADAR

SPECTRUM

VELOCITY INTENSITY

NATURE OF IRREGULARITIES

COMPARISON WITHTHEORETICAL APPROACHES

COMPARISON WITH ROCKETS SATELLITES INCLUDING SCATTER

COMPARISON WITH CONDUCTIVITIES ELECTRON DENSITIES TEMPERATURES

8–20 MHz with an azimuth coverage of 52° and extend in range from a fewhundred kilometers to more than 3000 km. Backscatter from F-region iono-spheric irregularities is typically observed from 10% to 60% of this range inter-val. The first HF radar of this type is located in Goose Bay, Labrador (Greenwaldet al., 1985) and has been in continuous operation since 1983.

The present SuperDARN system covers over most of the northern polar iono-sphere and part of the south polar ionosphere. The fields of view of the existing,funded, and proposed northern-hemisphere SuperDARN radars are shown inFigure 4.10 (and listed in Table 4.2) and the southern-hemisphere HF radar cover-age is shown in Figure 4.11.

The SuperDARN radars utilize ionospheric refraction to achieve orthogonal-ity with the magnetic-field-aligned irregularities in the high-latitude F region, andtheir frequency range of 8–20 MHz permits achieving orthogonality over afactor of more than six in electron density. They are also frequency-agile, permit-ting observations at two or more different frequencies to be interwoven. Anexample of a SuperDARN-derived polar plasma-convection pattern is shown inFigure 4.12. The SuperDARN antenna array consists of 16 log-periodic antennas(LPAs) in the primary array and four LPAs to form a small-scale interferometerarray for elevation-angle determination, as shown in Figure 4.13.

RF signals from or to these antennas are phased with electronically controlledtime-delay phasing elements that allow the beam to be steered into 16 directionscovering the 52° azimuth sector. The azimuthal resolution of the measurements is


Figure 4.8. (a) The VLF pulsed-ionosonde technique. (b) An example of typical observedwaveforms. (c) The spectrum of a typical transmitted pulse. (After Kossey et al., 1983.)

(a)

(b)(c)

TRANSMITTER

NORMALSKYWAVEPULSE

CONVERTEDSKYWAVE PULSE

RECEIVERGROUNDWAVEPULSE

GROUNDWAVE NORMAL SKYWAVECOMPONENT

CONVERTED SKYWAVECOMPONENT

RE

LAT

IVE

AM

PLI

TU

DE

0 100 200 300 400 500

TIME (MICROSECONDS) FREQUENCY (kHZ)

0.0 20.0 40.0 60.0 80.0 100.0

0.40

0.30

0.20

0.10

0.0

AM

PLI

TU

DE

(×

10–1

)

276

0000

197

8

275

0000

197

8

274

0000

197

8

273

0000

197

8

272

0000

197

8

271

0000

197

8

270

0000

197

8

269

0000

197

8

268

0000

197

8

267

0000

197

8

266

0000

197

8

265

0000

197

8

264

0000

197

8

167

0000

197

9

166

0000

197

9

169

0000

197

9

168

0000

197

9

165

0000

197

9

164

0000

197

9

163

0000

197

9

162

0000

197

9

161

0000

197

9

160

0000

197

9

159

0000

197

9

158

0000

197

9

157

0000

197

9

156

0000

197

9

157

0000

197

9

356

0000

197

8

355

0000

197

8

358

000

0 1

978

357

0000

197

8

354

0000

197

8

353

0000

197

8

352

0000

197

8

351

0000

197

8

350

0000

197

8

349

0000

197

8

348

0000

197

8

347

0000

197

8

346

0000

197

8

345

0000

197

8

344

0000

19

78

TIME OF DAY – UT

50 70 90 110

130

06

1218

24T

IME

(G

MT

)

SOLAR ZENITH ANGLE (DEG)

50 70 90 110

130

06

1218

24T

IME

(G

MT

)


50 70 90 110

130

06

1218

24T

IME

(G

MT

)


SE

PT

EM

BE

RD

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EM

BE

RJU

NE

100

200

300

400

TIM

E –

S

EC

ON

DS

100

200

300

400

TIM

E –

S

EC

ON

DS

100

200

300

400

TIM

E –

S

EC

ON

DS

Fig

ure

4.9

.V

LF

pul

se-r

eflec

tion

dat

a fo

r a

dist

urbe

d po

lar

peri

od (

afte

r K

osse

y et

al.,

198

3).


Figure 4.10 Locations and fields of view of the eight operating northern-hemisphereSuperDARN HF radars, as well as the STARE radar in northern Scandinavia and theremaining SABRE radar in Wick, Scotland (after Greenwald et al., King Salmon (C),operated by the Communications Research Laboratory in Japan; Kodiak (A), operated bythe Geophysical Institute UAF in the USA; Prince George (B), operated by the Universityof Saskatchewan in Canada; Saskatoon (T), operated by the University of Saskatchewan inCanada; Kapuskasing (K), operated by the JHU/APL in the USA; Goose Bay (G), oper-ated by the JHU/APL in the USA; Stokkseyri (W), operated by the CNRS/LPCE inFrance; ykkvibær (E), operated by the Radio and Space Plasma Physics Group,University of Leicester in the UK (also known as Cutlass/Iceland); and Hankasalmi (F),operated by the Radio and Space Plasma Physics Group, University of Leicester in the UK(also known as Cutlass/Finland).

Tab

le 4

.2.

Sup

erD

AR

N r

adar

s op

erat

ing

in t

he n

orth

ern

hem

isph

ere

Rad

arID

Loc

atio

nA

ffilia

tion

Lat

itud

e (°

N)

Lon

gitu

de (

°E)

Ope

rati

onal

CU

TL

ASS

a /F

inla

ndF

Han

kasa

lmi,

Fin

land

Uni

vers

ity

of L

eice

ster

62.3

226

.61

Apr

il 19

95C

UT

LA

SSa /

Icel

and

EP

ykkv

ibæ

r, Ic

elan

dU

nive

rsit

y of

Lei

cest

er63

.77

20

.54

Dec

embe

r 19

95Ic

elan

d W

est

WSt

okks

eyri

, Ice

land

CN

RSb

63.8

6

20.0

2O

ctob

er 1

994

Goo

se B

ayG

Lab

rado

r, C

anad

aJH

U/A

PL

c53

.32

60

.46

June

198

3K

apus

kasi

ngK

Ont

ario

, Can

ada

JHU

/AP

Lc

49.3

9

83.3

2Se

ptem

ber

1993

Sask

atoo

nT

Sask

atch

ewan

, Can

ada

Uni

vers

ity

of S

aska

toon

52.1

6

106.

53Se

ptem

ber

1993

Pri

nce

Geo

rge

BB

riti

sh C

olum

bia,

Can

ada

Uni

vers

ity

of S

aska

toon

53.9

8

122.

59M

arch

200

0K

odia

kA

Kod

iak

Isla

nd, A

lsas

kaU

AF

d57

.62

15

2.19

Janu

ary

2000

Not

es:

aC

o-op

erat

ive

Uni

ted

Kin

gdom

Tw

in L

ocat

ed A

uror

al S

ound

ing

Syst

em.

bC

entr

e N

atio

nal d

e la

Rec

herc

he S

cien

tifi

que.

cJo

hns

Hop

kins

Uni

vers

ity

App

lied

Phy

sics

Lab

rato

ry.

dU

nive

rsit

y of

Ala

ska,

Fai

rban

ks.

dependent on radar operating frequency and ranges from 2.5° at 20 MHz to 6°at 8 MHz. Since most of the observations are made in the frequency range 12–14MHz, the nominal azimuthal resolution of the radar is 4°. At a range of 1500km, this corresponds to a transverse spatial dimension of 100 km.

A secondary parallel antenna array of four LPAs located 100 m in front of theprimary array is used to determine the vertical angle of arrival of the backscat-tered signal. This secondary array also uses a phasing matrix and functions as aninterferometer to determine the relative phases of the backscattered signals arriv-ing at the two arrays. The phase information is converted into an elevation angle,


Figure 4.11. Fields of view of southern-hemisphere SuperDARN HF radars (afterGreenwald et al., 1995). Halley (H), operated by the British Antarctic Survey in the UK(also known as the Southern Hemisphere Auroral Radar Experiment (SHARE)); SANAE(D), operated jointly by the University of Natal and the PUCHE in the Republic of SouthAfrica; Syowa South (J), operated by the National Institute of Polar Research in Japan;Syowa East (N), operated by the National Institute of Polar Research in Japan; Kerguelen(P), operated by the CNRS/LPCE in France; and TIGER (R), operated by the La TrobeUniversity in Australia.

which is used to determine the propagation modes of the backscattered signal asa function of range, as well as the approximate height of the scatterers. This sec-ondary antenna array is also visible in Figure 4.13. The range resolution of theSuperDARN measurements is determined by the transmitted pulse length(200–300 ms) and is equivalent to 30–45 km.

Electronic steering of the SuperDARN antenna array occurs on microsecondtime scales, which allows the radar to be scanned rapidly through a number ofbeams or to dwell for an extended time on a single beam. Typically a radar willscan in a sequential manner with a dwell time of 6 s in each beam and a full-scantime of 96 s.

Although very useful information has been obtained using single HF radars, itbecame apparent that bi-directional common-volume observations with radarseparations greater than 500 km were the best approach to advancing the study ofhigh-latitude convection with HF radars (Ruohoniemi et al., 1989). The commonfield of view of a pair of HF SuperDARN antennas covers 15–20° of invariant lat-itude and 3 h of magnetic local time. The fields of view of several pairs of HF


Figure 4.12. A typical polar plasma-convection pattern (courtesy of R. Greenwald).

radars extend the spatial coverage of the high-latitude auroral zone and the polar-cap boundary over many hours of magnetic local time. If ionospheric irregular-ities were to fill this common viewing area, it would be possible to monitor thedynamics of plasma convection over a significant part of a convection cell. Therates of occurrence of HF scattering during a solar-cycle maximum are given byRuohoniemi and Greenwald (1997).

Figure 4.14 is a sketch of the manner in which VHF and HF radars interceptfield-aligned irregularities in the high-latitude E and F regions and Figure 4.15shows a comparison between F-region Doppler velocities obtained simultane-ously with the Sondrestrom ISR and the Goose Bay HF radar. More details onthe SuperDARN system may be found in the review paper by Greenwald et al.,(1995) and on the SuperDARN homepage on the internet.

At VHF/UHF frequencies, OBS systems are primarily used as auroral radarsand sometimes, at near-equatorial latitudes, to investigate irregularity structuresassociated with the equatorial electrojet. See Kelley (1989) for the physics ofauroral and equatorial VHF/UHF echoes. Examples of VHF/UHF radars usedin research into auroral and equatorial ionospheric irregularities are the CornellUniversity Portable Interferometer (CUPRI) (Providakes, 1985), theSaskatchewan Auroral Polarimetric Phased Ionospheric Radar Experiment(SAPPHIRE), (Kustov et al., 1996 and (1997). Auroral radars are exemplified bythe Scandinavian Twin Auroral Radar Experiment (STARE), which was firstdescribed by Greenwald et al., (1978). The STARE system consists of two pulsed


Figure 4.13. The SuperDARN HF antenna array at Kapuskasing, Ontario (afterGreenwald et al., 1995).


Figure 4.14. Idealized ray paths for VHF and HF radars to E-region and F-region FAIs(after Greenwald et al., 1995).

Figure 4.15. A comparison of F-region Doppler velocities obtained with the Goose BayHF radar and velocities obtained by the Sondrestrom ISR (after Greenwald et al., 1995).

bistatic phased-array radars located at Malvik, Norway and Hankasalmi,Finland. Beams from the radars are directed over a large common-viewing area(approximately 16000 km2) centered on the auroral zone in northern Scandinavia– as illustrated in Figure 4.16.

The Doppler data from the two radars are combined to determine the vectorvelocity of the irregularities with 20-km20-km spatial and 20-s temporal reso-lution. An example of the data obtained with the STARE system and simultane-ous all-sky-camera data is shown in Figure 4.17 illustrating a westward-travelingauroral surge. (See Section 6.4.) A list of OBSs (coherent radars) deployed glo-bally as of 2000 is shown in Table 4.3.

Some of the advanced OBS systems employ arrays of interferometer antennas(Farley et al., 1981) similar to those used in radio astronomy. The Fourier trans-form of the digitized signals from the respective antennas is taken, and thecomplex cross-correlation spectrum for each pair is determined in the timedomain. Spaced-antenna analysis can also be carried out in the frequency domain


Figure 4.16. A map of the eight overlapping beams of the STARE radar over northernScandinavia (after Greenwald et al., 1978).

(Briggs and Vincent, 1992), offering some advantages over time-domain analysis.Two new novel approaches in the design of OBS systems are the Frequency-

Agile Radar (FAR) (Tsunoda et al., 1995) and the multi-use system described byGanguli et al. (1999), which may be used in modes other than as an OBS, and theManatash Ridge Radar (a passive bistatic radar for upper-atmospheric radioscience) (Sahr and Lind, 1997), which utilizes transmissions from standard FMbroadcast stations.


Figure 4.17.

Superimposed epochanalysis of the spatialdistribution of auroralluminosity (upper panel)and equivalent currents(lower panel) during thepassage of a westwardtraveling surge atapproximately 1911 UTon 27 March 1977 (fromInhester et al., 1981).


Table 4.3. Currently deployed OBS (HF/VHF/UHF) systems

Radar

Location Name Type Reference/description

Finland COSCAT/XMTRa Auroral/pulsed/bistatic McCrea et al. (1991);929.5 MHz

Finland and COSCAT/RCVRSa Bistatic/pulsed and CW 0.5 kW, 4° elevation, Sweden 2° azimuth

U. K. and SABRE Auroral/pulsed Jones et al. (1981); 150 Sweden MHz; twin radars

Scandinavia STARE Auroral/pulsed/bistatic Greenwald (1987)

Canadian SAPPHIREa Auroral and polar cap/ Kustov et al. (1996); Arctic CW/bistatic 50 kW

NE Canada SHERPA Auroral and polar/pulsed Hanuise et al. (1992)

Polar SUPERDARN Polar cap and auroral/ Greenwald et al.pulsed (1995); 6–16 MHz;

1 kW each into16 antennas,52º azimuth sector

Crete SESCAT Mid-latitude, E region/ Haldoupis andCW/bistatic Schlegel (1993); 50.52

MHz, 1 kW, four Yagi arrays

(Portable) CUPRI E region/monostatic Providakes et al.(1985); 49.92 MHz,25 kW, five antennas

(Portable) FAR D, E, and F regions/ Tsunoda (1992); pulsed 2–50 MHz, various

pulse widths

Halley Bay, PACE Polar cap F region/pulsed Baker et al. (1989); Antarctica 8–20 MHz, 1 kW each

into 16 antennas,52° azimuth sector

Antarctica SYOWA Auroral/pulsed 50 and 112 MHz,15 kW, 3–14-elementcoaxial antennas

Peru Jicamarca Equatorial/pulsed/ Kelley (1989, Chapter monostatic 4); 50 MHz (oblique

and verticalincidence), 49.9 MHz

Some advantages and disadvantages of auroral and HF radars

Auroral radarsThe radio ray path from these radars must intercept the Earth’s magnetic field atnear-normal incidence, so siting of the radars is of critical importance. Thisrequires that transmitters and receivers be located at high-latitude sites, which aresometimes rather inhospitable and distant from “civilization,” which, in turn,complicates the logistics. Also, in order to achieve the narrow azimuthal beam-widths required, rather large antenna arrays are required, affecting the cost.

HF radarsHF radar systems require larger areas for the antenna array than do VHF/UHFsystems. Siting of the radars, although it is not as critical as it is for auroral radars,is important. In order to cover the entire polar cap (as in the SuperDARNsystem), considerable international cooperation is required. Because of theirremote location, some of the sites are quite expensive to maintain. During severeauroral or polar-cap absorption the lower HF frequencies used may be unusable.


Table 4.3. (cont.)

Radar

Location Name Type Reference/description

Kwajalein Altair Equatorial/monostatic/ Tsunoda (1981); 155.5 pulsed MHz

Japan MU Radar mid-latitude, monostatic/ Kato et al. (1989); pulsed 46.5 MHz

Notes:Acronyms for radars: COSCAT: Coherent scatter, CUPRI: Cornell UniversityPortable Radar Interferometer, CW: Continuous Wave, PACE: Polar Anglo-AmericanConjugate Experiment, SABRE: Scandinavian and British Radar Experiment,SAPPHIRE: Saskatchewan AuroralPolarimetric Phased ArrayIonospheric Radio Experiment, SESCAT: Sporadic-E scatter, SHERPA: System HFd’Etude Radar Polaires Auroral, STARE: Scandinavian Twin Radar Experiment,DARN: originally was Dual Auroral RadarNetwork – now SUPERDARN refers to the network of HF backscatter sounders thatmainly probes the polar F region, FAR: Frequency Agile Radar.The SABRE radar in Sweden has been decommissioned, but the radar in Wick,Scotland, is still operational.

4.2.3 Incoherent-scatter radars

Incoherent-scatter radars (ISRs) are a relatively new development compared withcoherent backscatter techniques – they were first developed and deployed duringthe early 1960s. The fundamentals of the theory of incoherent scatter from the ion-osphere are covered by Evans (1969), in Section 4.7 of Davies (1990), in Section2.3.2 of Hunsucker (1991) and in Section 3.5.3 of this book. ISR technique hasmatured and proven to be one of the most powerful Earth-based radio techniquesfor probing the ionosphere and thermosphere and even for probing into the meso-sphere under certain conditions. At present there are nine functional ISRs (someoperating only sporadically), as shown in Figure 4.18.

Most of the ISRs in use today have been described in some detail in Chapter 7of Hunsucker (1991) and in Section 5 of Hunsucker (1993). The newest additionto the global array of ISRs is the Longyearbyen, Svalbard installation (Figure4.19) – which is part of the EISCAT system, whose parameters are listed in Table4.4. The design features of the Svalbard ISR are described in detail by Wannberget al., (1997). The other operational ISRs are shown in Figure 4.18 and currentfacility addresses and contact personnel are listed in the current version of theNCAR CEDAR Data Base.

4.2.4 D-region absorption measurements

The power density (or attenuation) of radio waves at a distance, d, from the trans-mitter is reduced by geometric effects, refraction, absorption in the atmosphere,


Figure 4.18. A map showing currently operational ISRs (courtesy of EISCAT Association).

and scattering and diffraction by objects in the ray path. For frequencies used inionospheric techniques (ELF/UHF), most of the absorption occurs in the Dregion and is characterized as either deviative or non-deviative absorption. Thetheory of ionospheric absorption is treated in Davies (1990, pp. 65–66 and215–217), Hunsucker (1991, pp. 50–53), Hargreaves (1992, pp. 65–66 and 71–72),and Section 3.4.1 of this book.

Current status and global deployment

Since there are several radio techniques for measuring ionospheric absorption, weemploy the URSI designations for the most-used methods. See Rawer (1976),Davies (1990, pp. 217–219), and Hunsucker (1991, Chapter 7, pp. 165–183) forextensive descriptions of these techniques. Certain of these techniques are cur-rently in use, whereas others have fallen into disuse for various reasons.

The URSI A1a and A1b methods

The URSI A1a method is usually employed at mid-latitudes, since the frequen-cies used (2–5 MHz) would be highly absorbed at auroral and polar latitudes.Basically, this method uses a stable, constant-output pulsed transmitter, anantenna with a uniform, vertically directed main lobe (and low sidelobes), plusa stable, sensitive receiver to analyze a signal that traverses the D region twice,being reflected by the E region. This technique requires very careful, frequentcalibration of the system, plus a measurement of the E-region reflection coeffi-cient. A variant of this method is the URSI A1b method, which uses the samebasic equipment and modified equations for oblique incidence at short distances.The URSI A1a and A1b techniques were used rather extensively from the


Figure 4.19. A photo of the new EISCAT ISR in Longyearbyen Svalbard (Spitzbergen)(courtesy of EISCAT Association).

Ta

ble

4.4

.P

aram

eter

s of

the

EIS

CA

T r

adar

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tem

(co

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sy o

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p.)

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n

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msø

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una

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nkyl

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tes

69°3

5N

67°5

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9N

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HF

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quen

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MH

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mid-1950s through the mid-1970s, but, to the best of the authors’ knowledge,only a few installations are still in operation. However, it remains a usefulmethod – especially in view of advances in VLSI, DSP, antenna theory, and com-puter techniques.

The URSI A2 method

Brief discussions of the URSI A2 cosmic-noise method of measuring absorptionmay be found in Davies (1990, pp. 218–219 and in Hargreaves (1992, pp. 71–72),and a rather extended discussion in Hunsucker (1991, pp. 169–178). The instru-ment used to make URSI A2 absorption measurements is called the riometer(Relative Ionospheric Opacity Meter, Extra-terrestrial ElectromagneticRadiation). It was designed and built at the Geophysical Institute of theUniversity of Alaska (Little and Leinbach, 1959), and was first globally deployedduring the International Geophysical Year (IGY), 1957–1959. It was based onwork done in the early 1950s by several investigators, and the riometer was foundto be ideally suited for measuring the strong D-region absorption at high latitudes.Indeed, both polar-cap and auroral-zone absorption were verified using thisinstrument. See also Hargreaves (1969).

In essence the riometer is just a stable radio receiver, and, in its usual form, thisstability is achieved by switching the receiver input rapidly between the signal anda stable local noise source, a principle first enunciated by Machin et al. (1952). Theriometer operates at some frequency above the penetration frequency of the ion-osphere so that it receives the signal coming from outer space – i.e. the cosmic-radio noise. Since the intensity of the cosmic noise source does not vary,reductions of the received intensity are interpreted to mean that the signal hasbeen absorbed somewhere in the ionosphere.

The cosmic-noise absorption in decibels can be calculated by using

A10log10(P0/P), (4.1)

where P0 is the power output in the absence of the ionosphere and P is the poweroutput of the riometer. A plot of typical riometer results is shown in Figure 4.20.

Although the cosmic noise may be assumed constant over time, it is not con-stant over the sky. The riometer antenna, which points in a fixed direction fromthe observing site – to the zenith, for example – is scanned around the radio skyas the Earth rotates, coming back to the same place every sidereal day (24 h,4 min). In order to measure the absorption, we must know what the intensitywould have been in the absence of the absorption. This is usually estimated bysuperimposing measurements over some period of time as a function of siderealtime, and taking a line along the top of the distribution to indicate the intensitywhen absorption is absent. The resulting curve is generally called the quiet-daycurve (QDC), and, although the idea is simple, the accurate derivation of theQDC can be the most difficult part of absorption measurement by the riometertechnique (Krishnaswamy et al., 1985).


Most riometers have operated with a small antenna that has a wide beam – e.g.60° between half-power points. This has been done for practical reasons, but itdoes bring a disadvantage in that the antenna pattern projects to a region about100 km across in the D region. Therefore a standard riometer installation does nothave good spatial resolution. In recent years, however, there has been an increasein narrow-beam work and in the use of imaging riometers.

The absorption depends on the radio frequency as the inverse square (seeSection 3.4.1), and this is one factor that influences the choice of a frequency forthe riometer. At higher VHF frequencies the antenna can be smaller (for a givenbeamwidth) but the instrument becomes less sensitive to weak absorption. At thelower VHF frequencies the antenna must be large and also there is more interfer-ence from ionospherically propagated signals. The compromise has generally ledto using the 30–50-MHz band. When data are obtained at several frequencies, itis usual to reduce the results to 30 MHz for comparison purposes,

A(30 MHz)A( f )(30)2/f 2 (4.2)

The first generation of riometers (from the IGY/IGC era) used vacuum tubes, andsolid-state circuits were introduced into this type of instrument in the 1960s, whichpermitted the riometer to be packaged as a small unit with low power comsump-tion. A problem with the solid-state riometer, however, was a lack of discrimina-tion against interference in the front end, but this has been remedied using ceramicfilters and integrated circuits (Chivers, 1999, personal communication).


Figure 4.20. Auroral radio absorption at 30 MHz over a 6-h period. On the riometer chart(lower panel) the “noise diode current” is proportional to the received cosmic noise power,and the straight line is the ‘quiet-day curve’ representing the power that would be receivedin the absence of absorption.

Imaging riometry

Technical developments have now made it possible to construct riometer systemsthat produce a large number of narrow beams simultaneously, sufficient to con-struct a picture of the absorbing region out to, say, 150 km (horizontal) from theinstallation. Several such systems are operating at the time of writing (2002), andseveral more are planned. These systems are called imaging riometers.

The first Imaging Riometer for Ionospheric Studies (IRIS) was installed at theSouth Pole in 1988–1989 (Detrick and Rosenberg, 1990). It forms 49 beams andthe best resolution is about 29 km at the 90-km level (Figure 4.21).

In principle, one could use 49 riometers to record the signals, but, to reduce thenumber, this system switches the signals sequentially among seven riometers; and,although this implies some loss of sensitivity, it is nevertheless adequate for obser-vations at a time resolution of 10 s. The operating frequency is 38.2 MHz.

The imaging riometers have demonstrated that the absorption contains fea-tures of finer scale, whose motions may be also be observed. This type of system


Figure 4.21. Projection of the IRIS beams at 90 km altitude (Derrick and Rosenberg,1990). The beam centers are marked as dots, and the 3-dB levels as solid lines. The dashedcircle is the projection of a typical wide-beam riometer antenna.

is expected to produce a lot of new information about the structure and dynam-ics of auroral radio absorption, and the occurrence of finer-scale absorption musthave implications for the effect of auroral absorption on HF radio propagationrelated to high-resolution systems. Some results are given in Sections 7.2.2 and7.2.4.

URSI A3a and A3b methods

The URSI A3 technique uses short, one-hop ionospheric modes at LF throughHF frequencies at mid-latitudes, and the basic geometry is shown in Figure 4.22.The A3a method consists of CW field-strength measurements at oblique incidenceover ground distances of 200–400 km, using frequencies from 2–3 and 6 MHz. Thevertical-plane antenna patterns must be very uniform, so that small changes inreflecting-layer height will not affect the system losses, and one dominant modemust be used. Transmitter outputs and receiver sensitivities must be stable and cal-ibrated, and no significant groundwave should be present to contaminate theresults. This method is probably most applicable for long-term measurement ofseasonal and sunspot variations of D-region absorption at mid-latitudes.

The main difference with the URSI A3b mode is that it uses frequencies in and


Figure 4.22. The geometry for the A3 absorption-measurement method. The dashed linefrom R to the Pole Star is an idealized ray path for the A2 (riometer) method (after Rawer,1976).

27 MHz

Pole starcosmic noise(method A2)

F layer

E layer

2.6 MHz 6 MHz

200 MHz

R T200−400 km

D layer(Absorber)

CW propagation(method A3)

below the MF band, where the groundwave is quite strong. Therefore, a vertical-loop antenna, with its plane perpendicular to the direction of the transmitter, isused to null out the groundwave, and another antenna is used to receive theskywave. The URSI A3a and A3b methods are described in considerable detail inthe URSI Handbook, by Rawer (1976).

Gardner and Pawsey (1953) and Belrose and Burke (1964) pioneered the devel-opment of the partial-reflection-experiment (PRE) technique. This involves ahigh-powered transmitter and a sensitive receiver, operating at frequencies notnear the plasma frequency. The receiving antenna array has vertically directedlobes, which can distinguish between the downcoming x and o polarizations. So,by measuring the amplitudes of both magnetoionic components, one may obtaininformation on the D-region electron density, collision frequency, and absorption.The PRE technique has been further enhanced by measuring both the amplitudeand the phase of the downcoming waves. This is a differential-phase measurement.Belrose (1970) and Meek and Manson (1987) have used MF radars in the inter-ferometric mode to obtain more information on the middle atmosphere and thelower D region. PRE theory and experimental results were outlined in Hargreaves(1992, pp. 28–29 and 76–77) and in Hunsucker (1991, pp. 180–182). Othertechniques that have been used to measure D-region absorption are described inHunsucker (1991, pp. 182–183) and in Hunsucker (1993, pp. 459–464). Table 4.5summarizes most of the absorption-measurement techniques.

4.2.5 Ionospheric modification by HF transmitters

During the early years of radio broadcasting Butt (1933) and Tellegin (1933) pub-lished papers describing observations of the transfer of modulation from onetransmitted signal to another signal, and Tellegen correctly described the phe-nomenon as radio-wave interaction in the ionosphere. This was labeled in follow-ing publications as the “Luxembourg effect” (or the “Luxembourg–Gorkiieffect”). Bailey (1937) was apparently the first to suggest that the ionosphere couldbe “heated” by a powerful HF transmitter and that this heating could produce newinformation about the ionosphere. “Ionospheric heating” was not experimentallyconfirmed until the 1960s, and results were not published until 1970, by Utlaut.

Experimental and theoretical studies of “ionospheric cross-modulation,”however, were pursued from the 1940s until the 1970s, when funding for thisresearch decreased, due to the high operating and maintenance costs of these facil-ities and the advent of other less expensive facilities.

Davies (1990) devoted an entire chapter (pp. 506–537) to ionospheric modifi-cation, as did Hunsucker (1991, pp. 142–164). The former stressed results of mod-ification experiments, whereas the latter stressed the technique. Anotherdescription (mainly theoretical) of ionospheric modification was Chapter 10 (pp.267–284) by Erukhimov and Mityakov in the WITS Handbook (Liu, 1989).Radio-wave interaction and ionospheric heating were also discussed byHargreaves (1992, pp. 93–94).


Basic principles

It is possible to modify the ionosphere by heating it with a high-powered HFtransmitter, releasing chemicals, using plasma-beam injection, explosions, andtropospheric (severe weather – Davies (1990, pp. 507–511)) and VLF wave injec-tion. We will restrict our discussion to HF waves interacting with the ionosphere.

A generic wave-interaction experiment is described in Figure 4.23 and theaccompanying caption. Similarly, a generic HF heating experiment is describedschematically in Figure 4.24 and the stages of the heating process are shown inFigure 4.25.

An outline of cross-modulation theory was given by Hunsucker (1991, pp.146–152); HF heating theory was given on pp. 152–155; and also by Erukhimovand Mityakov in the WITS Handbook (Liu, 1989). Some special theoretical con-siderations, which apply to HF heating of the high-latitude ionosphere, were


Table 4.5. Capabilities and limitations of absorption-measurement techniques

URSIdesignationor othername Capabilities Limitations Remarks

A1 method Quite sensitive Interference, cannot For mid- or low measure high values latitudes

A2 method Large dynamic range Not as sensitive as Passive, low cost, some other methods can be used to

measure polar-capand auroralabsorption

A3a method Quite sensitive Interference, more Mid- or low latitudecomplex than A1

A3b method Sensitive Interference Mid- or low latitude

PRE method Can obtain electron Interference, MF radar can be and collision-frequency complex system, used to probe the profiles more expensive than middle atmosphere

others

fmin method Can give a qualitative Very sensitive to Really not too indication of variation variations in usefulof absorption equipment

parameters

LOF Gives a value that can Interference, difficult Not used very muchbe applied fairly to interpretdirectly to HF circuits

Satellite HF Global coverage Too many variables Not too usefulbeacon


Figure 4.23. The geometry and nomenclature describing a generic ionospheric cross-modulation experiment (from Hunsucker, 1991). WT, “wanted” transmitter; DT, “disturb-ing” transmitter; R, receiver; A, WT keying sequence; B, DT keying sequence; C, detectedecho amplitude of the wanted wave (for 50% cross modulation) at the receiver. The bottompanel shows the technique for measuring the height of attenuation. The upper trace is thereceived wanted echo; the lower trace is the DT pulse.


Figure 4.24. Some of the effects produced by high-power HF heating facilities (afterCarlson and Duncan, 1977).

Figure 4.25. A schematic representation of the four stages of ionospheric heating (fromJones et al. 1986).

ENHANCEDIONLINE

ENHANCEDPLASMA

LINE

HEATERANOMOLOUSABSORPTION

ENHANCEDELECTRON

TEMPERATURE

ELECTRONDIFFUSION

ALONG FIELD

PARAMETRICDECAY

INSTABILITY

PONDEROMOTIVEFORCE

PLASMASTRIATIONS

10 m

THERMALPARAMETRICINSTABILITY

NON-LINEARTHERMALEFFECTS

HEATERON

1—10 ms 1—10 s 10 s 1 m

TIME

presented by Stubbe et al. (1985), in a special edition of Radio Science, edited byWong et al. (Wong, 1990). Table 4.6 lists the ionospheric modification facilities inoperation from c. 1970 to the present time.

Capabilities and limitations of ionospheric-modification techniques

The HF stimulation of the ionospheric plasma produces both linear and non-linear effects, and a wide spectrum of scale sizes and lifetimes of irregularities, aswell as modulating ionospheric-current systems to produce VLF and ELF prop-agation. This has proven to be an extremely important technique, stimulatingmany experimental and theoretical advances (see Carlson and Duncan, 1977;Hunsucker, 1991, pp. 162–163, and the Proceedings of the AGARD Conference onIonospheric Modification, 1991.) It has also been demonstrated that the auroralelectrojet can be modified by HF-modulated stimulation, to produce both VLF


Table 4.6. Ionospheric-modification facilities in operation since 1970

HF heaters and theirlocations Parameters Remarks and references

NAIC; Arecibo, 18° N/67° W; Operational in 1971; Puerto Rico, USA 300 MW/3–15 MHz Gordon et al. (1971)

EISCAT; Tromsø, Norway 69.6° N; 1200 MW

HIPAS UCLA, USA 64.9° N/146.9° W; Wong et al. (1983)50 MW/2.8–4.9 MHz

Established by the USSRGissar (Dushanbe) 38° N; 6–8 MW/4–6 MHz Operational in 1981

Erukhimov et al. (1985)

Khar’kov 50° N; 6–12 MHz Bogdan et al. (1980)

Moscow 56° N; 1000 MW/1.35 MHz Schluyger (1974)

Sura Radiophysics 56° N; 4.5–9 MHz Belov et al. (1981)Research InstituteNizhni Novgorod

Zimenki 56° N; 20 MW/ Getmatsey et al. (1973)4.6–5.7 MHz

Monchegorsk 68° N; 10 MW/3.3 MHz Kaputsin et al. (1977)

HAARP, Alaska, USA 63° N; 145.1° W; www.haarp.alaska.edu2.8–10 MHz

Notes:1. Several diagnostic techniques are usually employed at the HF heater sites to detectionospheric changes caused by the heater. Some typical diagnostics include ISRs,ionosondes, coherent radars, and spectro-photometers.2. The description of the facilities in this table incorporates the latest informationavailable to the author at the time of writing.

and ELF radiation. This technique is quite expensive, both in terms of initial costsand in terms of operating and maintenance costs, which means that most opera-tions are in the campaign mode. Also, because of the high levels of effective radi-ated power and the large area needed for high-gain antenna arrays,environmental-impact studies can drive up the capital costs, and require specialmeasures to reduce possibly harmful radiation effects.

4.3 Space-based systems

4.3.1 A history of Earth-satellite and radio-rocket probing

Hey et al. (1946) were probably the first scientists to realize that extraterrestrialsources could be utilized to study the ionosphere. Subsequently, Smith et al. (1950),Little (1952), and Hewish (1952) showed that the radio-star emanations could beused to study the irregular nature of the ionosphere. Radar echoes from the moonresulted in the discovery of the ionospheric Faraday-rotation effect (Murray andHargreaves, 1954; Browne et al., 1956; Evans, 1956). With the advent of theartificial-Earth-satellite era (Sputnik, October 1957), satellite radio beacons wereutilized to study the ionosphere. As electronics technology and rocket-boostercapabilities advanced, it became possible to actually place miniaturized ionosondesinto orbit, starting with the Canadian–US Alouette I topside sounder in 1962.

Actual in situ measurements of the ionospheric plasma from rockets and satel-lites have been made since the late 1940s, and a variety of radio-frequency (RF)probes has been utilized. The Langmuir probe, retarding-potential analyzers,plasma-drift meters, etc. are not really RF devices; they have been described byKelley (1989, pp. 437–454), but will not be discussed in this book.

During the last decade, several books that discuss Earth-satellite and rocket-radio techniques for probing the Earth’s ionosphere have been published (Liu,1989, pp. 44–147; Davies, 1990, pp. 260–296; Hunsucker, 1991, pp. 187–207;Hargreaves, 1992, pp. 64–65 and 67–71).

4.3.2 Basic principles of operation and current deploymentof radio-beacon experiments

The first class of satellite experiments carries an onboard transmitter (a radiobeacon) and utilizes a network (sometimes global in coverage) to receive thetransmissions. The daily, seasonal, geographic, and magnetic-storm-timevariations of the total electron content (TEC) of the ionosphere have beenobtained for the global ionosphere from various radio-beacon-experiment(RBE) satellites since the early 1960s. These TEC studies have yielded infor-mation on the large-scale changes in the ionosphere, such as orders-of-magnitude changes in TEC and medium-scale variations such as those caused

4.3 Spaced-based systems 215

by atmospheric gravity waves (AGWs). Another class of experiments meas-ures the scintillations in phase and amplitude of a stable (usually multi-frequency) beacon transmitter, thus providing information on the finestructure of ionospheric irregularities.

The TEC can be determined from RBE satellites by measuring the differentialDoppler effect between two signals (Bowhill, 1958), the Faraday rotation of theelectric vector, the modulation phase (or group delay) between two different fre-quencies, or the carrier-phase difference between two widely spaced frequencies.Most of the TEC studies, from the early 1960s through the mid-1970s, simplymonitored the transmissions of radio beacons aboard the satellite whose primarypurpose was to track the satellite, and both near-polar-orbiting and geostation-ary satellites were used as “targets of opportunity.”

The first results obtained using geostationary satellite RBEs were reported by(Garriott et al., 1965). Hargreaves developed the first proposal for a geosynchron-ous RBE specifically designed for ionospheric studies, which was described byDavies et al.,1975. More recent RBE studies involve the geostationary ETS-1 andETS-2, the US Navy NNSS (TRANSIT) satellites, and the GPS constellation.Other RBE satellites, used for studies both of TEC and of scintillation, wereWIDEBAND and POLAR BEAR.

More recently, the constellation of GPS satellites has provided much new infor-mation on ionospheric morphology and the structure of irregularities from TECand tomographic methods (Davies, 1990; Crain et al., (1993). The geometry andequations describing Faraday rotation, scintillation, and other TEC methodolo-gies are described by Fremouw et al. (1978), Basu et al. (1988), Ho et al. (1996),and Pi et al. (1997), and in Sections 3.4.4 and 3.4.5 of this book.

4.3.3 Topside sounders

As mentioned in the introductory paragraph to this section, it became possible toplace miniaturized sounders in satellites in the early 1960s, thus initiating the eraof continuous global monitoring of the ionosphere using topside ionosondes.Several topside sounders have been launched and have performed beyond expec-tations in the last three decades: Alouette I (1962), Explorer (1964), Alouette II(1965), ISIS-I (1969), Cosmos-381 (1970), ISIS-B (1964), ISS-B (1978), EXOS-B(1978), Intercosmos-19 (1979), EXOS-C (1984), Cosmos 1809 (1984) and ISEE-1and 2 (1979). Strictly speaking, EXOS-B, EXOS-C, and ISEE-1 are not topsidesounders in the ionosonde sense, but they are “relaxation sounders” used to exciteplasma waves in situ. Again, we are fortunate to have extended descriptions in theliterature: the WITS Handbook, edited by Liu (1989), Davies (1990, pp. 261–273),Hunsucker (1991, pp. 200–203), and Hargreaves (1992, pp. 64–65). Vast quantitiesof data have been obtained using topside sounders, some of which have not beenanalyzed. As an example, the Alouette/ISIS series of sounders provided 50 satel-lite years of measurements, and has led to the publication of over 1000 papers (seeJackson, 1986, and Benson, 1997).


4.3.4 In situ techniques for satellites and rockets

In situ RF probes used aboard rockets and satellites were described in detail in theWITS Handbook, by Hunsucker (1991, pp. 205–207), and by Hargreaves (1992,pp. 52–53). These methods of trans-ionospheric propagation can be adapted toinvestigate the lower ionosphere. Since the signal need not penetrate the denserpart of the ionosphere, its frequency can be reduced to make the observationsmore sensitive. The electron density and collision frequency can then be deter-mined as functions of height as the rocket ascends and descends.

One basic type of instrument is the RF impedance probe, which was first sug-gested by Jackson and Kane (1959)]. Its basic principle of operation is that theinput impedance of an electrically short antenna is given by a capacitive reactance(1/(C0)) in free space, but the behavior departs from C0 when it is immersed in aplasma.

Another basic in situ probe is the resonance probe, which is identical to therelaxation sounders mentioned in the previous subsection. It consists of a trans-mitter and a receiver immersed in the plasma, which excites the plasma in such away as to make it oscillate at the various magnetoionic frequencies, as describedby Benson and Vinas (1988). Other sensors include the Langmuir probe and itsderivatives, mass spectrometers, particle detectors, and magnetic and electric-fieldinstruments (see pp. 49–58 of Hargreaves, 1992).

4.3.5 Capabilities and limitations

Each of the three techniques (involving RBEs, topside sounders and in situprobes) discussed in this section does some things very well and other things notso well. However, when these three techniques are employed together in cam-paigns, they provide considerable information about the ionospheric plasma.Table 4.7 attempts to summarize the salient capabilities of these techniques.

4.4 Other techniques

The techniques discussed in this section are no less important than those discussedin previous sections. However, some of them are variants of certain basic methods,whereas others are quite new and in the process of being implemented.

4.4.1 HF spaced-receiver and Doppler systems

Unfortunately, there is some confusion between the spaced-receiver technique(SRT) and Doppler techniques for measuring the motion of ionospheric irregu-larities. This may be due in part to the fact that both techniques use multiplereceiving antennas, although the antenna spacing for the Doppler method isusually much less than that in the SRT. The concept of the SRT was conceived by


Ratcliffe and Pawsey (1933) and by Pawsey (1935), and was first applied experi-mentally by Mitra (1949). Discussions of these techniques were given by Kelley(1989, pp. 431–434), Davies (1990, p. 243–245), Hunsucker (1992, pp. 207–211),and Hargreaves (1992, pp. 300–302), and in some recent papers.

The SRT usually involves one transmitter and several receivers, with the loca-tion of the receiving antennas optimally spaced in regard to the horizontal scale-size of the particular ionospheric irregularity to be investigated. Figure 4.26illustrates the wide range of irregularities in the terrestrial ionosphere.

An extended discussion of the SRT is given by Hargreaves (1992, pp. 300–302)and by Hunsucker (1993, pp. 470–473).


Table 4.7. Advantages and limitations of radio beacons and topside sounders

Technique Advantages Limitations

Radio beacons Global coverage of the (1) Relatively expensive; (2) for TEC studies ionosphere from polar-orbiting rather complex calibration

satellites; constant beacon problems; yields a vast quantityparameters; not-too-complex of data (sometimesreceiving system; continuous overwhelming!); also, rather coverage for a large area from painstaking data analysis is geostationary satellites; ability required. At present, there are to study TIDS. few RBEs suitable for

ionospheric studies; (3) thepolar-orbiting satellites are, ofcourse, always moving inreference to the Earth station,thus convoluting spatial andtemporal effects.

Radio beacons The averages of global and Interpretation of these data in for scintillation temporal coverage listed above the context of extant theories isstudies also apply to the polar-orbiting a non-trivial task. Remarks 1,

and geostationary satellites, 2, and 3 above also apply.respectively, used in scintillationstudies. Many earth stationscan use the same beacon forTEC and scintillation studies.

Topside sounders Since all topside sounders use More-complex instrumentationrelatively high-inclination than most beacons.orbits, they have good globalcoverage. They are also freefrom D-layer absorption effects,and provide much informationon the ionospheric above theF2 peak.

4.4.2 The HF Doppler technique

This technique is quite useful for monitoring small, transient changes in the ion-osphere. It has been incorporated into several of the modern digital ionosondesand coherent radars, as well as being used as a “stand-alone” technique. Basically,in its first implementation, this technique used a very stable transmitter and oneor more stable receivers and local oscillators. These heterodyned the receivedskywave signal and then the beat frequency was usually recorded on tape at slowspeed. The data tapes were then speeded up by a factor of several thousand andthe amplitude and phase of the Doppler variation with time were spectrum ana-lyzed. This version of the stand-alone HF/CW Doppler sounder was pioneered inBoulder, Colorado in the early 1960s (Watts and Davies, 1960; Davies, 1962;Davies and Baker, 1966). Modern Doppler techniques utilize digital signal pro-cessing and computers instead of tape recorders. A thorough treatment of iono-spheric phase and frequency variations and of the HFD technique was given byDavies (1969), and other descriptions may be found in Jones (1989, Chapter 4, pp.


Figure 4.26. A composite spectrum summarizing the intensity of ionospheric irregularitiesas a function of wavenumber, over a large spatial scale (after Booker, 1979).

1

0.1

1e –2

1e –3

1e –4

1e –51e-2 0.1 1 1e+1 1e+2 1e+3

Irregularity Scales, km

Irre

gula

rity

Leve

l ∆N

/N

λ LF

MultipleScattering

(AnomalousAttenuation

Effect,Lacuna Effect)

WeakDiffraction

(PhaseStructure Function

Method)

MultibeamReflections

(Spread F;∆f / f Method;PolarizationDistortions;

s2 , s3 Indices)

Tilts, AGWs

(Echolocation,Slow PhaseVariations)

383–398); the WITS Handbook, edited by Liu; Hunsucker (1991, pp. 211–213);Hargreaves (1992, pp. 66–67); and Haldoupis and Schlegel (1993).

4.4.3 Ionospheric imaging

For over four decades now, ionospheric physicists and engineers have discussedand used radio methods to image the ionosphere. Rogers (1956) was probably thefirst to suggest using the wavelength-reconstruction method for this purpose.Schmidt (1972) proposed using VHF signals from a satellite to localize iono-spheric irregularities, and a description of a two-dimensional technique was givenby Parthasarathy (1975) and Schmidt and Taurianen (1975). Stone (1976) devel-oped a more sophisticated holographic radio camera, using a 32-element antennaarray oriented perpendicular to the path of the beacon satellite, with which he pro-duced three-dimensional reconstructions from measured data. Additional detailsconcerning the development of radio-imaging techniques (including computer-ized ionospheric tomography) from c. 1975 to the present may be found in Nygrenet al. (1997), Pryse and Kersley (1992), and in reviews by Hunsucker (1993 and1999) and Kunitsyn and Tereschenko (1992).

4.5 Summary

As we move into the twenty-first century, we see an extensive deployment of state-of-the-art, sophisticated ground- and space-based radio installations for probingthe terrestrial ionosphere – probably surpassing the deployment during theIGY/IGC. There has also been a sea change in the availability of near-realtime andarchived data from these radio installations on the internet. Ionospheric scientiststhus have rapid access to an unprecedented assemblage of data as well as usingemail to rapidly communicate with the principal investigators of the variousobservatories.

There is now a global distribution of modern ground-based instruments suchas digital ionosondes, coherent VHF/UHF radars (CUPRI, COSCAT, STARE,SABRE CANOPUS, . . .), incoherent-scatter radars (EISCAT, Millstone Hill,Jicamarca, Arecibo, MU Radar, and Russian installations), imaging riometers(IRIS), and ionospheric HF heaters (HIPAS, HAARP, Arecibo, EISCAT, . . .).For the first time, we now have near-realtime access to solar, interplanetary, andmagnetospheric data from a new generation of scientific satellites such as ACE,WIND, POLAR, and FAST.

When one is analyzing data from these instruments located at high latitude, onemust remember that there are some limitations – especially for those using HF.Under especially disturbed conditions (magnetic storms, etc.) ionosondes may bestrongly affected by D-region absorption and intense E-region ionization, and HFradars may also be affected by these phenomena.


An invaluable compilation of details of and data from most of the radiotechniques listed in this chapter is contained in the CEDAR (Coupling, Energeticsand Dynamics of Atmospheric Regions) Data Base published by the NationalCenter for Atmospheric Research (NCAR) in Boulder, Colorado. CEDAR is aprogram sponsored by the US National Research Foundation (NSF) which spon-sors many research programs at institutions in the USA, holds an annual meetingin June in Boulder, Colorado, and updates its data catalog.


Section 4.1Hargreaves, J. K. (1992) The Solar–Terrestrial Environment. Ch. 3, Techniques forobserving geospace. Cambridge University Press, Cambridge.

Hunsucker, R. D. (1991) Radio Techniques for Probing the Terrestrial Ionosphere –Physics and Chemistry in Space, Vol. 22 – Planetology. Springer-Verlag, Berlin.

Hunsucker, R. D. (1993) A review of ionospheric radio techniques: present status andrecent innovations, Ch. 22. In The Review of Radio Science, 1990–1992 (ed. W. R.Stone). Published for the International Union of Radio Science (URSI) by OxfordUniversity Press, Oxford.

Hunsucker, R.D. (1999) Electromagnetic waves in the ionospheric. In WileyEncyclopedia of Electrical and Electronic Engineering (ed. J. Webster), Vol. 6, pp.494–506. Wiley, New York.

Kelley, M. C. (1989) Appendix A – ionspheric measurement techniques. In The Earth’sIonosphere – Plasma Physics and Electrodynamics. Academic Press, New York.

Liu, C.-H. (1989) World Ionosphere/Thermosphere Study. WITS Handbook, Vol. 2,Instrumentation. SCOSTEP, University of Illinois Champaign-Urbana, Illinois

Section 4.2Appleton, E. V. and Barnett, M. A. F. (1925) On some direct evidence for the down-ward atmospheric reflection of radio waves. Proc R. Soc. A 109, 621–641.

Bailey, V. A. (1937) Interaction by resonance of radio waves. Nature 139, 68–69.

Baker, K. B., Greenwald, R. A., and Ruohoniemi, J. M. (1989) PACE: Polar Anglo-American conjugate experiment. Eos 22, 785–799.

Barnum, J. R. (1986) Ship detection with a high-resolution HF skywave radar. IEEE J.Oceanic Eng. 11, 196.

Belrose, J. S. and Burke, M. J. (1964) Study of the lower ionosphere using partialreflection. J. Geophys. Res. 69, 2779–2818.

Belrose, J. S. (1970) Radio wave probing of the ionosphere by the partial reflection ofradio waves (from heights below 100 km). J. Atmos. Terr. Phys. 32, 567–596.

Booker, H. G. (1979) The role of acoustic gravity waves in the generation of spread-Fand ionospheric scintillations. J. Atmos. Terr. Phys. 41, 501–515.

4.6 References and bilbliography 221

Breit, G. and Tuve, M. A. (1926) A test of the existence of the conducting layer. Phys.Rev. 28, 554.

Briggs, B. H. and Vincent, R. A. (1992) Spaced antenna analysis in the frequencydomain. Radio Sci., 27, 117–129.

Brookner, (1987) Array radars: an update. Microwave J., February, 117–137.

Butt, A. G. (1933) World Radio, April, 28.

Carlson, H. C. and Duncan, L. M. (1977) HF excited instabilities in space plasmas.Radio Sci., 12, 1001.

Croft, T. A. (1972) Skywave backscatter: a means for observing our environment atgreat distance. Rev. Geophys. Space Phys. 10, 73–155.

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Chapter 5

The high-latitude F region and the trough

5.1 Circulation of the high-latitude F region

5.1.1 Introduction

The high-latitude ionosphere is greatly influenced by the outer magnetosphereand the solar wind, the essential connection being via the geomagnetic field.Through this connection the high-latitude F region is exposed to the interplane-tary medium and thence to disturbances originating in the Sun. The circulationof the magnetosphere (Section 2.4.1) establishes a corresponding circulationpattern in the high-latitude F region. Although production by solar EUV is stillimportant, these added features lead to a more complex ionosphere, which exhib-its some striking differences both from the middle- and from the low-latitudezones. In describing the F region at high latitude, therefore, we shall be particu-larly concerned with two underlying factors:

(a) the dynamic nature of the high-latitude ionosphere, the pattern of circula-tion of the F region being mainly controlled by the solar wind and its vari-ations, and

(b) the influence of energetic particles from the magnetosphere and the solarwind, to which the region is generally more accessible than is the iono-sphere at lower latitudes.

The auroral zones, which occur within the high-latitude region, are particularlycomplex, and the trough of depleted ionization on its equatorward side has its ownpattern of behavior. The present chapter deals with the behavior of the high-latitude F region, its patterns of circulation, and their consequences. The auroralphenomena are discussed in Chapter 6.

227

5.1.2 Circulation patterns

In the F region the ion–neutral-species collision frequency is small relative to thegyrofrequency, and therefore the plasma moves with the magnetic-field linesrather than with the neutral wind. The motion may also be considered as themotor effect of the cross-polar electric field mapped down from the magneto-sphere. The plasma speed v, the electric field E and the magnetic flux density Bare related by vE/B (Equation (2.12)). Since the polar magnetic field is almostvertical, with a value of about 5105 Wb m2, typical plasma speeds of200–1000 m s1 correspond to electric fields of 10–50 mV m1.

The integral of the electric field across the polar cap, which may be determinedfrom satellite measurements, provides an estimate of the total electric potentialdifference across the magnetosphere between its dusk and dawn sides. Various for-mulae have been derived to express the polar-cap potential () in terms of thesolar-wind speed (vsw), the total flux density of the interplanetary magnetic field(IMF) (B) and the “clock angle” of the IMF () as seen from the Earth. If Bz andBy are the northward and westward components of the IMF, then

tan1|By/Bz | if Bz0 (northward),or

180°tan1|By/Bz | if Bz0 (southward).

A recent analysis (Boyle et al., 1997) gives

104v2sw11.7Bsin3(/2), (5.1)

where vsw is in km s1, B is in nanoteslas, and is in kilovolts. If vsw400 km s1

and B5 nT, the first term gives 16 kV and the second one between 0 and 58.5kV, depending on the orientation of the IMF. In terms of the magnetic activityindex Kp (Section 2.5.4), which is derived entirely from ground-based data,

16.515.5Kp. (5.2)

The basic flow pattern caused by the polar-cap electric field is simple enough.The plasma flows from the noon sector to the midnight sector directly over thepole, and there is a return flow around the low-latitude edge of the polar cap, inthe vicinity of the auroral oval, and so back to noon. See Figure 5.1(a).

The speed of flow is typically several hundred m s1. The flow over the polarcap corresponds to the motion of open field-lines from the cusp to the tail (Section2.4.1), and the return flow corresponds to the sunward flow of closed field-linesdown the flanks of the magnetosphere. However the co-rotation effect, conven-iently represented by the co-rotation electric field (Section 2.4.4), must also beincluded, and then the flow pattern becomes distorted as in Figure 5.1(b). The two

228 The high-latitude F region

circulation cells are now different, and the evening cell is particularly affectedbecause here the return flow and the co-rotation act in opposite directions. Thereare some field-lines that follow long, complicated paths, while others may circu-late endlessly in small vortices. All these features have ionospheric consequences.In addition the whole pattern is constantly changing in response to variations ofthe solar wind.

The IMF exerts a major influence on the circulation pattern of the polar iono-sphere. Because of the stronger coupling at the magnetopause, the magnetospherecirculates most strongly when the IMF has a southward component (Figure 2.19).This is the situation in Figure 5.1. The control exercised over the drift by the IMFhas been proved by measurements of the drift at high latitude at times when sat-ellites were situated in the solar wind just outside the bow shock (Willis et al.,1986; Todd et al., 1986).

There is a more complicated pattern of circulation when the IMF is northward,but its nature has been more controversial. Various two-, three-, and four-cell pat-terns have been proposed. Several agreed features distinguish it from the patternfor southward IMF.

(1) It is more structured, but the speeds are lower.

(2) The region of moving plasma is restricted to higher latitudes.

(3) There is a region of sunward convection at the highest latitudes.

5.1 Circulation 229

Figure 5.1. Plasma convection at high latitude. (a) Polar convection pattern without co-rotation. (R. W. Spiro et al., J. Geophys. Res. 83, 4255, 1978.) (b) Examples of convectionpaths of plasma at 300 km altitude in the northern hemisphere under the combined electricfields due to the magnetosphere and co-rotation. The large dots indicate the starting pointsused in the calculations. The time between successive dots is 1 h, except for the return to thestarting point. Each path is an equipotential, whose value is indicated. The boundary of thepolar cap is a circle (not marked) of radius 15°, centered 5° towards midnight from the geo-magnetic pole. (After S. Quegan et al., J. Atmos. Terr. Phys. 44, 619, Copyright 1982, withpermission from Elsevier Science.)

The east–west component of the IMF (usually called By) also affects the circu-lation, presumably because of shifting connection regions at the magnetopause(Figure 5.2).

Figure 5.3 shows versions of the circulation patterns of the northern polarregion according to the directions of the north–south and the east–west compo-nents of the IMF. Versions (a) and (b) agree that the circulation is generally weakerwhen Bz is positive (i.e. northward), though the details differ. The influence of theeast–west component over the form and size of the cells is clear in version (a) butless so in version (b). The latter shows two-cell patterns throughout, whereas in(a) three- or four-cell patterns appear when the IMF is northward.

Rich and Hairston (1994) have made a comprehensive compilation of poten-


Figure 5.2. Geometry of the IMF and the geomagnetic field viewed from the Sun. Regionsof preferred merging for various orientations of the IMF are indicated by shaded boxes.The principal merging region changes its location according to the “Sun–Earth” (Bx) and“east–west” (By) components of the IMF. (R. A. Heelis, J. Geophys. Res. 89, 2873, 1984,copyright by the American Geophysical Union.)

MERGING REGIONS FORBy > 0

MERGING REGIONS FORBy < 0

Bx > 0Dominant Merging

Region

Bx < 0Dominant Merging

Region

AWAY SECTORBx < 0

Dominant MergingRegion

AWAY SECTORBx > 0

Dominant MergingRegion

tial distributions (equivalent to flow diagrams) from satellite measurementsrecorded between 1988 and 1990, divided according to season, and to magnitudeand orientation of the IMF. Using advanced ionosondes at polar-cap stations innorthern Canada, Jayachandran and MacDonald (1999) find a marked seasonalvariation in the flow pattern. When the IMF is southward, the central region ofthe flow, that which passes over the pole, is observed to be towards magnetic mid-night in winter but towards 2000 local magnetic time in the summer, with agradual transition between. For northward IMF, those times are about 2 h earlier(i.e. 2200–1800 LT). There is (of course!) a considerable spread about these trendson individual days.

The effect of the east–west IMF component should be in opposite directions innorthern and southern hemispheres. Cases studied by Dudeney et al. (1991) usingHF coherent radar (Section 4.2.2) appear to verify this. Lu et al. (1994), using

5.1 Circulation 231

Figure 5.3. Patterns of the circulation of the high-latitude F region in the northern hemi-sphere for various orientations of the IMF. The viewpoint is that of an observer lookingdown on the polar region. In each diagram noon is at the top and the geomagnetic pole isin the center. (a) A conceptual picture based on various studies including European inco-herent-scatter radar data. (After S. W. H. Cowley and M. Lockwood, Ann. Geophysicae 10,103, 1992, copyright notice of Springer-Verlag.) The two-cell pattern for southward IMF(top row) gives way to three- or four-cell patterns when it is northward (bottom row). Thecolumns are respectively for when the east–west component is directed towards the west, iszero, and is directed towards the east. (b) Results from a HF radar in North America, for amoderate level of disturbance (Kp from 2 to 3 inclusive). In these patterns the IMF isnorthward at the top, southward at the bottom, westward on the left and eastward on theright. (J. M. Ruohoniemi and R. A. Greenwald, J. Geophys. Res. 101, 21743, 1996, copy-right by the American Geophysical Union.) The two-cell pattern dominates throughout,though with differing magnitude.

18

12

06

LT = 0hrs

By < 0 By = 0 By > 0

Bz % 1nT

Bz & 1nT

50° Auroral oval

%

&

(a)

magnetogram interpretation combined with incoherent-scatter radar, distinguishthree cases.

(1) The northern and southern patterns of circulation are mirror images whenthe IMF is southward.

(2) When the IMF has a northward component that is smaller in magnitudethan the east–west component, the patterns are similar to each other butof different intensities.

(3) If the IMF is strongly northward, the patterns in the summer and winterpolar caps are very different.

Figure 5.4 illustrates these patterns.Cowley and Lockwood (1992) point out that the polar circulation is driven by

two components, one the dayside coupling between the solar wind and the geo-magnetic field, and the other the night-time ionosphere’s reaction to changes in


Figure 5.3. (cont.)

5.1 Circulation 233

Figure 5.4. Convection in northern and southern hemispheres under various conditions ofthe IMF: (a) southward component but east–west component larger, (b) northward compo-nent but east–west component larger, and (c) northward component with smaller east–westcomponent. The plots show the equipotentials, which are also the streamlines of the polarflow. (G. Lu et al., J. Geophys. Res. 99, 6491, 1994, copyright by the American GeophysicalUnion.)

the magnetotail. The latter are expected to be delayed 30–60 min behind a changein the IMF. It is not surprising, then, that it takes some time for the circulation tosettle into a new pattern following a change in the IMF. According to Hairstonand Heelis (1995), analyzing a limited number of cases, a new convection patternappeared 17–25 min after the IMF turned from northward to southward. For anorthward turning the lag was 28–44 min. Since the IMF is always changing tosome extent, there will obviously be some times when the polar circulation is in astate of transition and will not conform to any particular model.

A discussion of observations of high-latitude convection is given by Kelley(1989).

There can be an abrupt change of plasma speed, or even a reversal of direction,across the boundaries of circulation cells, particularly when the IMF is north-ward. The plasma drift is equivalent to an electric field (as measured by a station-ary observer), which is communicated along the field-lines to the E region.Therefore the Pedersen current (Section 1.5) in the E region also alters abruptly.To maintain continuity, current then flows up the field lines as a Birkeland current.See Figure 5.5. The corresponding downward flow of electrons is probably thecause of the sun-aligned arcs observed in the polar cap when the IMF is north-ward (Section 6.3.2).

5.2 The behavior of the F region at high latitude

5.2.1 The F region in the polar cap

The tongue

Figure 5.6 shows F-region critical frequencies measured with ionosondes withinthe northern polar cap. As would be expected, the values are generally much largerin the sunlit region than they are in the dark. On this occasion there is also atongue of ionization, drawn out from the day side, over the pole, and into the nightsector. This disrupts the pattern that we might have expected, which ought toexhibit a sharp gradient between the sunlit and dark regions. The tongue is mostpronounced near the equinoxes, when the terminator crosses the polar cap. Thetongue may be broken into dynamic patches, which will be discussed in moredetail later in this chapter.

The polar F region is at its most variable when it is at its darkest – during winter,and when the magnetic pole is anti-sunward of the geographic pole. Critical fre-quencies can be very low: values of f0F of approximately 2–3 MHz (electron den-sities from several times 104 to 105) are common, and f0F1 MHz (implying apeak electron density as low as 1.4104 cm3) has been reported from ionosondedata (Whitteker et al., 1978). The lowest values occur in the dark, anti-sunward,part of the polar cap and generally near local midnight. (Also see Section 5.5.)


The UT effect

A remarkable observation is that the daily variations of the F region depend onuniversal time as well as on local time. There is, for example, a daily variation atthe South Pole, even though the solar zenith angle is virtually constant there over24 h. The electron density there, as elsewhere in the Antarctic, peaks about 0600UT, which happens to be near magnetic midnight.

5.2 The F region at high latitude 235

Figure 5.5. (a) Field-aligned current due to velocity shear in a magnetoplasma. B, magneticfield; v, velocity; E, electric field; IH, IP, and I‖: Hall, Pedersen, and field-aligned currents.(b) The field-aligned current associated with the polar-cap aurora at the boundary betweencirculation cells. (Reprinted from H. C. Carlson et al., Adv. Space Res. 8, 49, copyright1988, with permission from Elsevier Science.)


Figure 5.6. Maps of the F-region critical frequency (f0F2) showing the development of a“sporadic-F” event on 12 October 1957. (G. E. Hill, J. Atmos. Sci., 20, 492, 1963.) Theplots are successively for 1700, 1800 and 1900 UT and the sunlit hemisphere is at thebottom of each plot. The contours range between 4 and 13 MHz.

The explanation of the UT variation depends on the separation of the geo-graphic and magnetic poles. The neutral-air wind in the thermosphere blows overthe polar regions generally away from the Sun. At 0600 UT in the Antarctic and1800 UT in the Arctic the geographic pole is on the midnight side of the magneticpole, and the drag of the neutral particles against the ions therefore acts to lift theionosphere (as described in Section 1.3.4). The rate of recombination of ions isthereby reduced and the net ion density is increased. This is also the time of daywhen the largest amount of the geomagnetic polar cap is sunlit, and it is thereforewhen the circulation pattern will be most effective at bringing solar-produced ion-ization over the pole. It is significant that, in the northern hemisphere, where theseparation between the poles is smaller, the UT effect is less pronounced than it isin the south.

In the polar cap the F1 layer can be almost as strong as the F2 layer, and onoccasions it may be even stronger. This produces the so-called “G condition” onionograms.

5.2.2 The effect of the polar cusps

On the day side of the Earth are two regions, one in each hemisphere, where thegeomagnetic field-lines provide a direct connection between the ionosphere andthe magnetosheath (Section 2.2.5). In the simplest models of the magnetosphere,in which there is no circulation, they correspond to the neutral points on thesurface of the magnetosphere. Field-lines at lower latitude are closed, whereasthose at higher latitude are “open”, connecting with the solar wind and the IMFor sweeping back into the magnetotail. In more realistic, dynamic, models(Sections 2.4.1–2) the cusps are where the dayside field-lines open before beingswept over the poles (Figure 5.7(a)). The cusps are significant regions of the mag-netosphere and also of the ionosphere.

In the ionosphere the cusp regions have several signatures.

(1) Charged particles with energies similar to those in the magnetosheath maybe detected. Whereas the cusps are typically located near 78° geomag-netic latitude, and are about 5° wide, the particle observations show thecusps extending over all daylight hours and merging into the auroral oval(Section 6.2.1). There is also a second, smaller, region extending only a fewhours from local noon. The particle flux from the magnetosheath is highlyvariable over short times (or over small distances, since these observationscome from orbiting satellites).

(2) Luminous emissions at 630 nm are enhanced, indicating the occurrence oflow-energy excitation of the upper atmosphere. Emissions typical of theaurora (Section 6.3.3) are actually reduced – a feature sometimes called thenoon gap. These photometric observations reveal a considerable variationin the latitude of the cusp, from 84° under very quiet geomagnetic condi-tions to 61° under very disturbed conditions.



Figure 5.7. Aspects of the polar cusp and its F-region effects. (a) Details of the polar cusp:MS, magnetosheath; LLBL, low-latitude boundary layer; EL, entry layer; and PM, Plasmamantle. (G. Haerendel et al., J. Geophys. Res. 83, 3295, 1978, copyright by the AmericanGeophysical Union.) (b) A tomographic image of the F region on 14 December 1996 at10:46 UT showing signatures arising from magnetic reconnection. The dashed line marksthe boundary between closed and open field-lines, and other features are described in thetext. (I. K. Walker et al., Geophys. Res. Lett. 25, 293, 1998, copyright by the AmericanGeophysical Union.)

(a)

(b)

(3) Owing to the influx of particles from the magnetosheath the density andtemperature of the ionosphere is increased and there is a greater degree ofirregularity. Owing to the opening of the field-lines, ionospheric plasmamay flow out into the magnetosphere, where its ionospheric origin hasbeen recognized from its temperature and composition.

(4) Magnetic pulsations of type Pi2 (of period approximately 30 s) areenhanced. (See Section 2.5.6.)

The image of Figure 5.7(b), which was obtained by the tomography technique(Section 4.4.3), shows features of the F region due to magnetic reconnection at thecusp. The boundary between closed and open field-lines is marked, and, fromscanning-photometer observations, Walker et al. (1998) were able to identify iono-spheric effects due to (1) precipitation of electrons from the ring current on thelast closed field-lines, (2) a downward field-aligned current on the first open field-lines, and (3) dispersion of precipitating soft ions on the flux tubes convectingpoleward. The last effect shows up as the increasing height of the layer maximum.

5.2.3 The polar wind

The circulation of the magnetosphere carries field-lines from the closed region,through the cusp, and into the polar region where they are open to the solar windor go deep into the tail of the magnetosphere. These tubes of force lack an effec-tive outer boundary. Furthermore, the scale height is large for light ions at hightemperature (Equations (1.3) and (1.46)). Therefore the ionospheric plasma mayreadily flow upward, and, in the absence of a boundary, the flow may continue aslong as the tube remains open.

A steady outward flow is one of the solutions of Equation (1.43), describing themotion of a minority gas under the forces due to a pressure gradient and gravity.As was pointed out in Section 1.3.4, the separation between the heavy ions(oxygen) and the electrons produces an electric field directed upward. When lightions (hydrogen and helium) are also present, they are accelerated by this electricfield, which tends to drive them upward. Detailed consideration shows that grav-itational attraction is able to bring about a state of hydrostatic equilibrium(Equation (1.47)) in the oxygen ions, but that H is light enough for the electricfield to cause the dynamic equilibrium state having a steady outflow above somealtitude. He may also flow out, though to a lesser extent.

This continuous outflow of light-ion plasma is the polar wind. In theory theflow can even reach supersonic speeds, but the details depend on what is assumedabout the flow speed at a great distance. The term “polar wind” is sometimesrestricted to the supersonic regime, in which case subsonic flow would be a “polarbreeze.” The flow is limited by collisions with stationary ions, and by the rate ofproduction of H by the charge exchange between oxygen ions and neutral atomichydrogen (Equation (1.69)) in the topside ionosphere. Since the concentration of


O is far from uniform over the polar cap, the polar wind must be similarly vari-able.

The lighter ions are the most affected by the outflow, and it is commonlyobserved in satellite measurements that the concentration of H is greatly reducedrelative to the O in the topside ionosphere over the polar caps. The upward speedcan be several km s1. The flux of H is heated by collision with the heavy ions,and its temperature is significantly raised. The theory of the polar wind has beenreviewed by Raitt and Shunk (1983). Figure 5.8, from that paper, shows compu-tations illustrating the reduction of topside ion density and the upward drift speedof H for various assumptions about the outer boundary.

One important point established by satellite observations is that the polar windis a significant source of the plasma in the magnetosphere. That material then con-vects with the magnetospheric circulation and eventually reaches the plasma sheetat a distance from the Earth that depends on the nature of the ion but is estimatedgenerally to be within 50RE. Figure 5.9 illustrates some aspects of the interchangeof plasma between ionosphere and magnetosphere.

Plainly, the polar wind is a mechanism that removes ionization from the polarionosphere from above. Typical loss rates are 3108 cm2 s1 for H ions and3107 cm2 s1 for He. It is secondary to the loss by recombination acting mosteffectively in the lower F region, and for which electron-content observations leadto estimates in the range 109–1010 cm2 s1 at middle latitudes.

5.2.4 The F layer in and near the auroral oval

On a long-term view the F region in the auroral zone exhibits properties similarto those typical of middle latitudes. Figure 5.10 shows how the average electrondensity near the peak of the layer varies diurnally during summer and winter atsunspot maximum and minimum. These measurements are by incoherent-scatterradar at Tromsø, Norway (geographic latitude 69.6° N, invariant latitude 67°,L6.5). The winter anomaly (Section 1.4.5) is seen at sunspot maximum but notat sunspot minimum, which is also the case at mid-latitude. The electron densityis larger in the months either side of the winter solstice, indicating the presence ofa semi-annual anomaly as well (Farmer et al., 1990).

In addition, there are additional factors that make the ionosphere more irreg-ular in both time and distance. In the poleward part of the auroral oval andextending several degrees into the polar cap, the electron density may be enhancedby the precipitation of low-energy electrons (maintaining the F-region penetrationfrequency at at least 3 MHz). There may be large variations over short distances,probably due to irregularity in the intensity of the particle precipitation.

The precipitation (of particles with energy 300 eV) is particularly strong inthe cusp region (75°–80° magnetic), where the penetration frequency may beincreased by several megahertz. This precipitation creates irregularities tens ofkilometres across, which then break down into smaller structures (tens of metres



Figure 5.8. Theoretical properties of the polar wind, showing the density of H and thefield-aligned drift speed. The various curves are for a range of H escape speeds between0.06 and 20 km s1 at 3000 km. The range of O is given by the shaded region. (W. J. Raittand R. W. Schunk, Energetic Ion Composition in the Earth’s Magnetosphere, Terra ScientificPublishing, Tokyo, 1983, p. 99.)

4000

3000

2000

1000

0

4000

3000

2000

1000

0

ALT

ITU

DE

(km

)A

LTIT

UD

E (

km)

2 2 6 10 14 18

101 102 103 104 105 106

DENSITY (cm3)

VELOCITY (KM.SEC1)

HO

across or less) as they drift in the general convection (Muldrew and Vickrey,1982).

No doubt the transport of plasma over the pole also contributes significantlyto the ionization observed in the vicinity of the auroral oval near midnight.Structures moving over the pole, provided that they continue to drift in the con-vection pattern (Figure 5.1), are expected to become distorted on reaching theHarang discontinuity and be diverted eastward or westward along the oval(Robinson et al., 1985). As will be discussed in Section 5.3.2, it is clear from theirproperties that at least some of the structures in the oval are not of local origin.

On the equatorward side of the auroral oval the F region tends to be depletedof ionization. This is the “main trough”, sometimes known by its older name of“mid-latitude trough.” The depletion in the trough can be as much as by a factorof ten, though it is often not so great. It is a complex feature, created by the com-bination of loss processes and the circulation pattern in the region where the high-and mid-latitude ionospheres meet. The trough is considered in detail in Section5.4.

5.3 Irregularities of the F region at high latitude

5.3.1 Introduction

Spatial irregularities are a common feature of the atmosphere and ionosphere, andtheir scales of variation cover a wide range in both time and distance. The exis-tence of F-region irregularites has been known for at least 40 years from their


Figure 5.9. Ionospheric sources of plasma for the magnetosphere. Ions leaving the high lat-itudes tend to separate according to mass. They may subsequently be trapped in the plasmasheet and drift towards the Earth, being energized by betatron acceleration. Computationsindicate that the ionosphere is a significant source of magnetospheric plasma. (After C. R.Chappell, Rev. Geophys. 26, 229, 1988, copyright by the American Geophysical Union.)

5.3 Irregularities 243

Figure 5.10. Yearly, summer, and winter diurnal variations of electron density near thepeak of the F layer at Tromsø. (a) sunspot maximum (August 1981–August 1983), (b)sunspot minimum (April 1986–March 1987). (Reprinted from A. D. Farmer et al., J.Atmos. Terr. Phys. 52, 561, copyright 1990, with permission from Elsevier Science.)

600

500

400

300

200

100

0

4 8 12 16 20 24

Ele

ctro

n de

nsity

(10

3 cm

–3)

Yearly Average250–300 km

Summer250–300 km

Winter250–300 km

Local time

Local time

Yearly Average250–300 km

Summer250–300 km

Winter250–300 km

600

500

400

300

200

100

0

Ele

ctro

n de

nsity

(10

3 cm

–3)

4 8 12 16 20 24 4 8 12 16 20 24

4 8 12 16 20 244 8 12 16 20 244 8 12 16 20 24

(a)

(b)

effects on trans-ionospheric radio propagation, originally in observations of radiostars, though our knowledge of them is still incomplete. We are here concernedwith the two principal kinds affecting the F region: enhancements extending overtens and even hundreds of kilometers, which can be observed by incoherent-scatter radar and other ionospheric techniques; and irregularities smaller thanabout 10 km, which, by a diffraction mechanism, produce in propagating radiowaves the phenomenon of scintillation.

5.3.2 Enhancements: patches and blobs

We first consider enhancements of large size occurring in the polar cap and theauroral zone. They may be 50–1000 km across and are remarkable for their highplasma density. Even when they are observed during the polar winter night, theirdensity can be more typical of that of the daylit mid-latitude ionosphere. Thereare several techniques by which they may be observed. Some of the first reportscame from polar-cap ionosonde data, when they were described as “sporadic-F.”Figure 5.6 showed a good example in which the evolution of a patch may be seen.Speeds of 2000–5000 km h1 (500–1400 m s1) were reported. The cause of themotion was correctly interpreted as being due to an electric field, but it was (incor-rectly) supposed to arise in the E region rather than the magnetosphere.

While much of the information about patches has come from ionosondes, theycan also be detected by virtue of the 630-nm airglow which they emit. Other tech-niques, such as incoherent-scatter radar and tomography, have been significant inthe more recent studies of enhancements.

Patches

Enhancements within the polar cap are generally called patches. They are seenduring the winter night under disturbed conditions, and the F-region electrondensity may be increased by as much as a factor of ten above the background,which would typically be about 105 cm3. They tend to be stronger at times of highsunspot number.

It seems clear that this type of enhancement is not produced locally, but wasformed some distance away and has then drifted in the polar convection to thepoint of observation. Because the F region decays only slowly by recombination,the lifetime of the patches should be quite long enough for them to cross the polarcap at a speed of several times 100 m s1 (up to 1 km s1) from a source on the dayside. This possibility has been verified by computations that have also demon-strated how a change of polar circulation, for instance due to an increase in theflow of the solar wind or a sudden change in the IMF (Anderson et al., 1988; Sojkaet al., 1993), can detach plasma from the dayside cusp region and carry it over thepole into the midnight sector along a path such those shown in Figure 5.1(b).Lockwood and Carlson (1992) have attributed patch creation to the enhancedplasma flow during a flux-transfer event (Section 2.4.2). What happens when the


enhancement reaches the night sector is less clear, but it probably becomesstretched along the auroral zone in the return flow or merges into the mid-latitudeionosphere (Robinson et al., 1985).

Computer modeling of the high-latitude F region (Sojka et al., 1994) suggeststhat, at the winter solstice, patch formation should be absent between 0800 and1200 UT and at a maximum from 2000 to 2400 UT. From then until the equinoxthere should be strong patches all day. In the summer they should be considerablyweaker.

While much is still not understood about these larger structures of the polarionosphere, a number of observational facts have been established about them.

(a) They are roughly circular, and between 200 km and 1000 km in size.

(b) The patches are smaller than the gaps between them, suggesting that weshould consider them as enhancements of ionization above a low back-ground rather than as depletions within a higher background.

(c) The degree of enhancement in a typical patch is 2–10 times the ion densityof the background.

(d) The gradients at the edges of patches are fairly sharp, on a scale of a fewkm to about 100 kilometers, and these gradients are the same in all hori-zontal directions.

(e) The patches appear when the IMF is southward.

(f) They move with the general plasma drift in the polar cap and at the samespeed, neither overtaking the general flow nor lagging behind.

(g) They occur during all seasons of the year but more frequently during thewinter.

A different pattern is seen in the weaker circulation which occurs when the IMFhas a northward component and conditions are less disturbed. At such times theairglow emissions form thin strips with noon–midnight alignment, and these driftslowly across the polar cap in a dusk-to-dawn direction. In these elongated struc-tures the electron density is enhanced by a factor of 5–8 at times of high sunspotnumber, but by a smaller amount (a factor of two) near solar minimum (Buchauet al., 1983). Figure 5.11 compares the structures typically associated with north-ward and southward IMF.

Blobs

In the auroral zone the enhancements are generally known as blobs. They aresmaller than the patches in the horizontal, extending for tens of kilometers ratherthan hundreds. Some of them peak low in the ionosphere, in the E region or thelower F region. Figure 5.12 illustrates the structures of the ionosphere as seen bytwo different techniques: (a) was derived by the tomography technique (Section4.4.3) from electron-content data in the Scandinavian sector, and (b) was obtainedby a scanning incoherent-scatter radar (Section 4.2.3) in Alaska. The upper panel


gives an overall view showing the mid-latitude ionosphere on the left, the morestructured auroral region on the right, and the main trough (Section 5.4) inbetween, while the two lower panels show similar features as contour diagramsemphasizing the irregularities.

There is some uncertainty about the cause of blobs in the auroral zone. Theyseem to vary greatly in size. There is some evidence, though it is perhaps not yetdefinitive, that they move with the plasma drift of the auroral F region as a whole.It seems clear that more than one source is involved, since, as Figure 5.13 illus-trates, they may occur over different altitude ranges. Moreover, those at the higherlevels are generally cooler than their surroundings by about 10%, whereas thosepeaking in the lower F region tend to be hotter by about 20%. (According to theresults of Burns and Hargreaves (1996), typical electron temperatures are about1280 and 1540 K, repectively, for the two types, compared with about 1410 K forthe plasma outside the blob – all these values being medians over a number of sep-arate determinations.) It is generally assumed that the higher structures arrive aspatches drifting from the polar cap (since they are also cooler than their surround-ings), but the exact connection and the mechanism which breaks them up areunknown. Those blobs which are hotter and appear at lower altitudes are morelikely to have been produced by particle precipitation nearer to the point of obser-vation. Figure 5.12 shows one of these lower blobs, and also examples of the boun-dary blob which is situated just poleward of the main trough. The boundary blobis a long-lived feature that may continue for several hours.


Figure 5.11. Typical irregular structures of the polar F region. (a) arcs with noon–midnightalignment and dusk–dawn drift during northward IMF (Bz 0), and (b) patches driftingtowards midnight during southward IMF (Bz 0). The coordinates are corrected geomag-netic latitude (CGL) and local time (CGLT), and the heavy lines mark the auroral oval(Section 6.2). (After H. C. Carlson, private communication.)


Figure 5.12. (a) A tomographic image of the ionosphere in the Scandinavian sector, 15October 1993, pre-midnight, showing the mid-latitude ionosphere, the main trough(Section 5.4), and the structured auroral ionosphere. (L. Kersley, private communication,1998.) (b) Blobs and other features observed with the Chatanika incoherent-scatter radaron 11 November 1981. The time of each scan is marked, and the main trough, a boundaryblob, an auroral blob, and the auroral E layer (Section 6.5.4) may be seen from south tonorth. A distance of 100 km is about 0.9° of latitude. Since Alaskan time is UT – 10 h,these are in the early evening. (C. L. Rino et al., Radio Sci. 18, 1167, 1983, copyright by theAmerican Geophysical Union.)

(b)

(a)


Figure 5.13. Three kinds of blob observed with the EISCAT incoherent-scatter radar. (a)F-region enhancement peaking at 250–400 km (cooler than the surroundings). (b) an inter-mediate type having an F-region peak and related E-region structure, and (c) a low-altitudeblob peaking below 200 km (hotter than the surroundings). (Reprinted from C. J. Burnsand J. K. Hargreaves, J. Atmos. Terr. Phys. 58, 217, copyright 1996, with permission fromElsevier Science.)

(a)

(b)

Table 5.1 compares the main properties of the various types of enhancement.A comprehensive review of high-latitude enhancements of the F region was

made by Tsunoda (1988). A collection of relevant papers was published as aspecial section of Radio Science (1994).

5.3.3 Scintillation-producing irregularities

The irregularities of smaller scale produce scintillation phenomena in trans-ionos-pheric radio signals. The theory of scintillation was outlined in Section 3.4.5,where it was seen that the radius of the first Fresnel zone is an important param-eter. For a radio frequency of 100 MHz the first Fresnel zone has a radius of about1 km if the effective diffraction screen is at a height of 300 km; therefore irregular-ites smaller than about 1 km produce both amplitude and phase scintillation.Irregularities larger than that produce phase scintillation only.

Distribution and occurrence

Scintillation occurs at all latitudes, including the polar region, but it tends to beparticularly severe at and around the auroral zone (Aarons, 1982; Yeh and Liu,1982). See Figure 5.14. (The other region of heavy scintillation is at the equator.)The auroral scintillation zone is offset from the magnetic pole and exhibits ageneral correspondence to the auroral oval (Sections 6.2.1 and 6.3.5), being nearerto the equator in the night sector. Both in the auroral and in the polar regions the


Figure 5.13. (cont.)

(c)

Table

5.1

.A

sum

mar

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larg

e-sc

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irre

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at

high

lati

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Typ

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Mag

nitu

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Dur

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n

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p pa

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lar

cap

100s

to

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s

106

cm

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F r

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d B

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al.

whe

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of

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out

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(198

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the

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lar

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25

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0 m

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50

0 km

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d eq

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.(19

80)

the

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to

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th.

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nced

by

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of

the

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1984

)th

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, Po

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l.(1

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–100

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200

abou

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l 25

0 m

s

1

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al.

(198

4)ni

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dary

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; 198

5b)

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rate of occurrence and the intensity maximize at night, and there is also a daytimemaximum in the auroral region only. The seasonal variation depends on the lon-gitude. Figure 5.15 shows the seasonal and daily occurrence patterns for anauroral station in the European sector (Kiruna). The occurrence and the intensityof scintillation increase strongly with the sunspot number; the occurrence alsoincreases with magnetic activity (Kp), but this effect is only slight in the polar cap.

The period and depth of fading

The period of fading varies considerably, but is generally in the range of secondsto a few minutes. It depends on the apparent motion of the irregularities as wellas on the depth of the fading. Figure 5.16 shows an example of amplitude scintil-lation.

The intensity of amplitude fading is commonly measured using the index S4

(defined in Section 3.4.5). In these terms it depends on the radio frequency as f1.5

if the fading is not too severe, but less steeply for strong scintillation.The observed depth of scintillation also depends on the direction of propaga-

tion between the sender and the receiver (for instance from a satellite to a groundstation). Increasing obliquity tends to make the fading more severe because theray traverses a longer path through the ionosphere, thereby encountering moreirregularity in total. Details depend on the form of the individual structures. Itmay be assumed that there will be considerable elongation along the geomagneticfield, and, according to Rino (1978), auroral irregularities are extended east–west,giving a sheet-like form. Since individual irregularities are strongly field-aligned,there is another maximum for rays traveling directly along the direction of themagnetic field (because rays traveling almost directly along the magnetic field tendto remain within a single irregularity). These effects may be seen in Figure 5.15.


Figure 5.14. The principal regions of scintillation at L band (1.6 GHz). (S. Basu et al.,Radio Sci. 23, 363, 1988, copyright by the American Geophysical Union.)


Figure 5.15. The occurrence of scintillation over magnetic latitudes 55°–80° observed fromKiruna (64.3° N, 102.8° E CGM), September 1984–September 1986. The contours showthe percentage of time for which the scintillation at 150 MHz exceeded S40.2. The con-tours 1–5 represent 25%, 35%, 45%, 55% and 65% respectively. (a) Variation with month.Note the summer maxima. (b) Variation with local time, low magnetic activity (Kp%1). (c)Variation with local time, moderately high magnetic activity (Kp&4). (L. Kersley et al.,Radio Sci. 23, 320, 1988, copyright by the American Geophysical Union.)


Figure 5.16. Examples of scintillation fading observed in Alaska in transmissions at (a) 140MHz and (b) 360 MHz from a geosynchronous satellite (ATS-6), on 30 March 1979. Thesatellite was at low elevation to the south, and the raypath crossed the F region at about 60°geomagnetic latitude. The fading is considerably greater at the lower frequency, with a ratioof almost four between the scintillation indices (c).

Average values of S4 at 137 MHz over a range of latitude and at various timesof day (geomagnetic local time) are shown in Figure 5.17. All these measurementswere made between autumn and early summer (October to May) at Hornsund,Svalbard (invariant latitude 73.4°), whose position is marked on the plots with anarrow. Since the magnetic field is nearly vertical over the polar cap, the propaga-tion path is closest to the magnetic-field direction when the satellite is at the samelatitude as the receiving station. The maximum in S4 at the latitude of Hornsundis clearly present at night.

A value of S4 equal to 0.25 corresponds to fading with about 1 dB standard


Figure 5.17. The S4 scintillation index at Hornsund, Svalbard (invariant latitude 73.4° N)at various local magnetic times. The bars indicate the standard deviation. The latitude ofthe receiving station is marked with an arrow, and horizontal lines indicate the typical lati-tude of the auroral oval at that time of day. (A. W. Wernick et al., Radio Sci. 25, 883, 1990,copyright by the American Geophysical Union.)

deviation. However, the fading may be much more severe on occasion, and espe-cially so in the auroral zone. Table 5.2 shows the incidence of intense scintillationat the auroral station Narssarssuaq (Greenland) at two radio frequencies.

Note that severe fading is considerably more common under magnetically dis-turbed conditions and at night. At the highest latitudes (82° magnetic latitude)the scintillation is associated with polar arcs (Section 6.3.2), and fading of morethan 28 db (peak–peak) has been observed at 250 MHz.

Spectrum

The irregularities causing scintillation may be considered as an irregular, spatialdistribution that is drifting but also evolving in time. The temporal variationobserved at a single site includes the intrinsic time variation, but the main part ofthe variation is likely to be due to the relative motion between the irregularitiesand the probing signal. A satellite in low orbit converts the spatial spectrum alongthe orbit to a temporal spectrum according to the orbital speed. In the case of ageostationary satellite, the temporal change arises from the drift of the irregular-ities through the satellite-to-ground ray.

Examples of the intensity spectrum of 137-MHz scintillations recorded atHornsund are shown in Figure 5.18. Since the transmitting satellite, HiLat, wasin orbit at an altitude of 800 km, we expect that the time variation will be duemainly to the motion of the satellite across the spatial irregularities – though exactconversion would require knowledge of the irregularity motion as well. The largemaxima in Figure 5.18 are due to the effect of diffraction (Section 3.4.5), whichprevents large-scale phase irregularities generating amplitude scintillation at theground. The peak marks the Fresnel frequency. The falling part of the spectrumrepresents a range of spatial size from about 700 to 130 m (when the satellite isoverhead). These are power-law spectra, as is commonly the case, and, in theHornsund data set, the average spectral index, q, is generally between 2 and 3.That is, for a factor of ten in fading frequency the intensity declined by a factorbetween 100 and 1000 (20–30 dB). Amplitude fading tends to be dominated by theFresnel frequency.


Table 5.2. Depth of scintillation atNarssarssuaq

Percentage occurrence

&12 dB &10 dBat 137 MHz at 254 HMz

Kp Day Night Day Night

0–3 2.9 18 0.1 2.63 19 45 0.9 8.4

Direct measurements

Spatial fluctuations of electron density can be measured in situ using satellite-borne probes, though the high velocity of an orbiting satellite limits the structu-ral detail that can be resolved in this manner. In Figure 5.19, which showsmeasurements of ion (and therefore electron) density made on an orbiting satel-lite, the fluctuations are as much as 20% of the mean.

In some cases it has been observed that the small-scale irregularities whichproduce scintillation are located at the edges of large-scale enhancements, andFigure 5.19 is such an example. There are mechanisms (such as the gradient-driftand Kelvin–Helmholtz instabilities) that can cause a large patch to break up at theedges, thereby generating smaller ones, which may break up in turn. By this meansthe larger structures can progressively break down to give smaller ones in a


Figure 5.18. Typical spectra of amplitude scintillation in 137-MHz signals received fromthe HiLat satellite at Hornsund: (a) 24 April 1986 and (b) 28 October 1985. q is the spectralindex. (A. W. Wernick et al., Radio Sci. 25, 883, 1990, copyright by the AmericanGeophysical Union.)

S4 = 0.854 q = 2.359 ± 0.140

S4 = 0.226 q = 2.792 ± 0.200

20

30

40

50

60

70

30

40

50

60

700.1

0.1

1 10 100

1 10 100

Inte

nsi

ty S

pec

tru

m (

dB

)In

ten

sity

Sp

ectr

um

(d

B)

Frequency (Hz)

Frequency (Hz)

(a)

(b)

cascade process. The relationship between large structures in the polar cap and theincidence of radio scintillation has been discussed by Buchau et al. (1985).

Modeling

For forecasting purposes, empirical models have been developed to represent theintensity of scintillation at high (and other) latitudes. Scintillation depends on thespatial variation of the electron content, rather than on its actual value (seeSection 3.4.5), and varies with parameters such as magnetic latitude and longi-tude, time of day, season, magnetic activity, and sunspot number. The high-lati-tude model proposed by Secan et al. (1997) is derived from scintillations observedin the 137.67-MHz transmissions from several orbiting satellites (Wideband,HiLat and Polar BEAR) received at stations in Greenland, Norway, Canada, andthe USA (Washington State) between 1976 and 1988. Figure 5.20 gives anexample showing a quantity called the irregularity strength parameter (defined asthe power-spectral density of the variation in electron density at the wave numberfor 1 km, multiplied by the thickness of the irregular region), which is propor-


Figure 5.19. (a) Relative irregularity and (b) ion density measured on a satellite crossingthe polar cap. In (a) the variation N was taken with respect to a linear least-squares fit tothe electron density measured for 3 s; the plotted N/N therefore refers to irregularitiessmaller than about 25 km. (After S. Basu et al., The Effect of the Ionosphere onCommunication, Navigation, and Surveillance Systems (ed. Goodman), p. 599. IES’87,National Technical Information Service, US Government Printing Office, Springfield,Virginia, 1987.)

tional to the variance of the vertical electron content. The calculation of the depthof scintillation is then based on the theory of a phase screen (Section 3.4.5) withan assumed power-law spectrum of intensity. The scintillation index deriveddepends also on the propagation direction, the radio frequency of the signal, thespeed at which the propagation path crosses the plasma irregularities, andassumptions about the height of the effective phase screen and the form of theirregularities.

The data compilation underlying this model revealed several significant fea-tures.

(1) The high-latitude scintillation region has a well defined boundary, acrosswhich the irregularity strength increases by more than a factor of ten.


Figure 5.20. Contours of the irregularity parameter CkL for 2300 UT on 21 July at solarmaximum (sunspot number 175) and high geomagnetic activity (Kp6), from modelversion 13.04. CkL is the height integrated power spectrum of irregularities for a periodicityof 1 km, here shown as the logarithm, and is proportional to the variance of the electroncontent. (J. A. Secan et al., Radio Sci. 32, 1567, 1997, copyright by the AmericanGeophysical Union.)

(2) The peak of the enhancement associated with the auroral zone lies 2° pole-ward of the boundary of particle precipitation at midnight, but 14° pole-ward of it at noon. (The aforsaid precipitation boundary is theequatorward edge of the region where auroral electrons of energies 50 eVto 20 keV are precipitated, as determined by Gussenhoven et al. (1983).See also Figure 6.6.)

(3) Equatorward of the scintillation boundary there is a transition region,most evident from 0800 to 1600 magnetic local time, before the irregularitystrength assumes the lower values typical of middle latitudes.

(4) The auroral enhancement has maxima near midnight and noon, both ofwhich become more intense with increasing Kp. The night maximumoccurs later as Kp increases, and the day maximum occurs later withincreasing sunspot number.

(5) The polar cap contains a strong enhancement after noon, and a minimumafter midnight. The overall level of irregularity in the polar cap increaseswith the sunspot number and decreases with increasing Kp.

These features do not show up clearly in Figure 5.20, but are illustrated inFigure 1 of Secan et al. (1997), to which the reader is referred for further detailsof the model and its use.

5.4 The main trough

5.4.1 Introduction

An ionospheric trough is a region of depleted ionization, limited in width butextended in the east–west direction, with more intense regions to the north andsouth. We deal here only with depletions that are observed regularly and appearto be permanent or semi-permanent features of the F region, accepting that theyvary in intensity and location. A depletion that is not elongated would bedescribed as a hole.

The reader should be warned that the terminology of troughs and holes hasbeen subject to some ambiguity in the literature, as may well happen when phe-nomena have not yet been fully defined. Most investigations relate to an F-regiontrough that seems to mark the boundary between the mid-latitude and high-latitude ionospheres. Originally this was called the “mid-latitude” trough, a termthat continues to be used. It has also been called the “main” trough, and that termwill be prefered here, first to emphasize its importance as the principal trough-likefeature of the F region, and second because its occurrence is by no meansrestricted to middle latitudes. Under a blanket definition of middle and high lati-tudes, the trough would appear sometimes in one and sometimes in the other, andit is probably more helpful to consider it as the variable boundary between the


high- and middle-latitude regions of the ionosphere, at least on the night side ofthe Earth. Various other troughs and holes that are wholly within the high-latitude region are observed, and these will be called simply high-latitude troughsor holes, as the case may be.

Ionospheric troughs are depletions of the heavy ions, principally O. They arerelated to, but not identical to, depletions of light ions (H and He) in the topsideionosphere and the protonosphere as far as the equatorial plane. (The inner edgeof the main depletion in the plasmasphere is, of course, the plasmapause – Section2.3.2.)

5.4.2 Observed properties and behavior of the main trough

Observations

The main trough was first observed in the early 1960s by the topside sounderAlouette 1 (Muldrew, 1965; Thomas et al., 1966) as a local depletion of electrondensity when the satellite crossed the frontier between Canada and the USA. Inthose early days it was sometimes known as the Canadian-border effect. Since thenit has been studied from the ground by a variety of techniques, particularly electron-content measurement, incoherent-scatter radar, and by using ionosondes.

An example from Dynamics Explorer 2, showing the variations of electrondensity and temperature across the northern high-latitude region at the height ofthe satellite (733–371 km), is shown in Figure 5.21. The main (mid-latitude)trough appears just after 0931 UT near 60° invariant latitude, and two othertroughs are seen at higher latitudes. The electron temperature was enhanced in themain trough, and this is typical. The main trough is wider in the example of Figure5.22, which was derived from ISIS-2 topside ionograms. Here the trough is morethan 15° wide, and the complexity of detail in the trough region is indicated. Thenumbers 1–8 pick out a number of features, namely:

(1) a latitudinal variation in the mid-latitude ionosphere;

(2) the equatorward wall of the trough;

(3) the trough minimum;

(4) the poleward wall of the trough (which is often sharp, as it is here);

(5) an auroral enhancement;

(6) a decline on the poleward side of the auroral oval; and

(7, 8) structure within the polar cap.

Troughs are also observed in the electron content but generally they do notexhibit the sharp gradients or as much detail as those observed by satellite-borneprobes or topside sounders. Some examples are given in Figure 5.23. The reasonfor the different appearance is probably that the electron content is an integral ofthe electron density rather than the value at one height. Figure 5.23 shows selected


Fig

ure

5.2

1.

Lat

itud

e pr

ofile

s of

ele

ctro

n de

nsit

y (l

eft-

hand

sca

le)

and

elec

tron

tem

pera

ture

(ri

ght-

hand

scal

e) m

easu

red

on t

he s

atel

lite

DE

-2, 2

2 N

ovem

ber

1981

, sho

win

g m

id-l

atit

ude

(mai

n) a

nd h

igh-

lati

tude

trou

ghs.

(R

epri

nted

fro

m A

. S. R

odge

r et

al.,

J.A

tmos

.Ter

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hys.

54

, 1, c

opyr

ight

199

2, w

ith

perm

issi

onfr

om E

lsev

ier

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nce.

)

examples in which the trough is clearly defined. Some troughs are more structuredthan these, and some have a second minimum.

Figure 5.24 indicates by means of a schematic diagram the structure of thetrough as it affects the electron isopleths on the bottom side of the ionosphere near60° geomagnetic, and in Figure 5.25 we see them both on the topside and on thebottom side obtained by tomographic analysis of electron-content data.

A summary of principal properties (northern hemisphere)

Following Moffett and Quegan (1983), the location and occurrence of the trough(in the northern hemisphere) may be summarized as follows.

The trough is primarily a night-time phenomenon, extending from dusk todawn. It has on occasion been observed at all local times.


Figure 5.22. Features of the main trough, recorded by the topside sounder ISIS-2 on 18December 1971. The local time is near midnight. (M. Mendillo and C. C. Chacko, J.Geophys. Res. 82, 5129, 1977, copyright by the American Geophysical Union.)

N(HMAX)

N(450)

N(550)N(650)N(750)N(850)N(950)

N(HSAT)

CORRECTED GEOMAGNETIC LATITUDE40 50 60 70 80

106

105

104

103

102

86

4

2

86

4

2

86

4

2

86

4

2

500

400

300

200

500

400

300

200

H(MAX)(km)

EL

EC

TR

ON

CO

NC

EN

TR

AT

ION

(E

L/C

M

3 )

1

1

2

2

3

3

4

4

5

5

6

67

7

8

8


Figure 5.23. Troughs in electron content on four separate occasions when the trough wasnarrow and well defined. The observations were made in Scandinavia and time is marked inUT. (Reprinted from L. Liszka, J. Atmos. Terr. Phys. 29, 1243, copyright 1967, with permis-sion from Elsevier Science.)

Figure 5.24. A sketch of the trough as it often appears near Halley, Antarctica. (J. R.Dudeney et al., Radio Sci. 18, 927, 1983, copyright by the American Geophysical Union.)

It is observed most regularly in the winter and equinoctial seasons. Itoccurs more rarely in summer, and then only near local midnight.

The poleward edge of the trough, which is usually sharp, is close to theequatorward edge of the region of diffuse aurora.

The trough moves to lower latitudes as the night proceeds. Under geomag-netically quiet conditions it can turn back to higher latitudes during theearly morning.

It also moves to lower latitude with increasing geomagnetic activity; solaractivity as such appears to have no effect.

There is no general agreement regarding the depth of the trough or itswidth, or on how these properties vary with the time of day.

Formulae for variations with time and magnetic activity

Knowledge of the locations both of the trough minimum and of the poleward edgeis important for radio communication at high latitudes and for trans-polar paths.The position of the trough minimum as a function of local time and Kp has beenexpressed by the linear relationship

'T'0aKpbt, (5.3)

where 'T is the invariant latitude of the trough minimum, '0 is its invariant lati-tude at midnight (t0) if Kp0, t is the local time in hours reckoned from mid-night (negative before, positive after), and a and b are coefficients. The values of'0, a, and b given in Table 5.3 were derived from independent sets of observa-tions.

These formulae have the merit of simplicity, but they cannot give the wholestory because there is no provision for poleward motion in the morning. Halcrowand Nisbet (1977) and Spiro (1978) have derived equations of non-linear form.

Equation (5.3) implies that, at a given latitude, the trough minimum appears


Figure 5.25. The troughas seen by tomography.Results are from theScandinavian sector,early afternoon, 17November 1995. Notethe narrow upwardextension on the pole-ward side. (L. Kersley,private communication,1998.)

earlier if Kp is higher. The dependence (in h per unit of Kp) is just a/b, or 2.0, 4.2,3.8, and 1.2, respectively, for the coefficients of Table 5.3.

The increase in the latitude of the trough after about 0700 LT is seen in theelectron-content observations (Liszka, 1967) reproduced in Figure 5.26. However,these data are fitted quite well by the formula of Kohnlein and Raitt (shown super-imposed) during the hours around midnight. (Liszka’s observations were mainlyfor times of low Kp.) The same data give a Kp dependence of about 2° of latitudefor one unit of Kp within the range 0–3 (Figure 5.27), which again agrees with theformula of Kohnlein and Raitt. Note, however, that individual values are spread2°–3° of latitude about the trend. Rodger et al. (1986) have commented that Kp isa poor predictor of the position of the poleward edge of the trough, and this isprobably true for all its features except in the statistical sense.

The incoherent-scatter results of Collis and Häggström (1988) were obtainedfrom a review of observations made during a year at sunspot minimum. Thetroughs were observed during the afternoon and evening hours but none wasrecorded during the summer period between early April and late August. Theirformula gives the strongest variation of latitude with time of day, and significantlyhigher latitudes during the afternoon than does that of Kohnlein and Raitt. Notethat the trough in Figure 5.25 occurred at 72°–74° during the afternoon.

In addition to their formula for the latitude of the minimum, Best et al. (1984)also produced expressions in terms of L and for the electron temperature:

L(trough minimum)5.40.5Kp0.13t, (5.4)

L(Te maximum)5.20.4Kp0.12t, (5.5)

Te(maximum)32508.06 /Dst K. (5.6)

In Equation (5.6), Dst is the magnetic-storm index (Section 2.5.2) in nanoteslas.


Table 5.3. Coefficients for Equation (5.3)

a (degrees LT for per unit of b (degrees which

Reference Data source '0 KP) h1) applicable

Rycroft and Satellite 62.7 1.4 0.7 1900–0500Burnell (1970) Alouette-1

Kohnlein and Satellite 65.2 2.1 0.5 2000–0700Raitt (1977) ESRO-4

Best et al. (1984) Satellite 64.0 0.5 0.13 Not statedIntercosmos18

Collis and EISCAT 62.2 1.6 1.35 1300–0100Häggström (1988)

75 70 65 60

1618

2022

002

0406

0810

LMT

Win

ter

1964

–65

Spr

ing

1965

Sum

mer

1965

Aut

umn

1965

Win

ter

1965

–66

Geographic latitude, °N

Kp

= 0

Kp

= 2

Fig

ure

5.2

6.

Lat

itud

e of

the

tro

ugh

agai

nst

loca

l tim

e, f

rom

a y

ear’s

ele

ctro

n-co

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t ob

serv

atio

ns a

t K

irun

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wed

en. T

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cal.

(Rep

rint

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zka,

J.A

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29

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The

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from

the

Koh

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n an

d R

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form

ula

have

bee

n ad

ded.

75 70 65 60

01

23

45

Kp

WIN

TE

R 6

4–65

SP

RIN

G 6

5

SU

MM

ER

65

AU

TU

MN

65

WIN

TE

R 6

5–66

°N

GE

OG

RA

PH

ICL

AT

ITU

DE

Fig

ure

5.2

7.

Lat

itud

e va

riat

ion

of t

he t

roug

h in

ele

ctro

n co

nten

t w

ith

the

mag

neti

c in

dex

Kp:

(a)

190

0–20

00 L

MT,

and

(b)

030

0–04

00 L

MT.

(R

epri

nted

from

L. L

iszk

a, J

.Atm

os.T

err.

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75 70 65 60

01

23

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Kp

WIN

TE

R 6

4–65

SP

RIN

G 6

5

SU

MM

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65

AU

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MN

65

WIN

TE

R 6

5–66

°N

GE

OG

RA

PH

ICL

AT

ITU

DE

(a)

(b)

The southern hemisphere

The known synoptics of the main trough have been derived mainly from observa-tions in the northern hemisphere. Mallis and Essex (1993) studied the trough inthe southern hemisphere as observed in the electron content, and conclude thatthere are some marked differences between the hemispheres. In the south, troughsare observable in all seasons and at all times of day. They occur less frequently inwinter than they do at the equinoxes or in summer, with a relatively high incidenceby day. Compared with the north, the southern hemisphere has more troughs byday but fewer by night. It is assumed that these differences are due to hemisphericdifferences in polar circulation.

5.4.3 The poleward edge of the trough

Introduction

The results in the previous section refer mainly to the minimum of electron densityin the trough, but the poleward edge is also a feature of special interest. Valid ques-tions are why the electron density increases again to the poleward side, and whythat increase is so sharp. The sharpness of the poleward edge may also be put touse, since it is often the trough feature which is the most easily detected and themost precisely located.

Orientation

The orientation of the trough has been studied using the poleward edge. Theequatorward drift of the trough during the night suggests that, at a given time, thetrough should not lie exactly along a contour of constant invariant latitude butshould be oriented at a small angle to it. This property was investigated in theAntarctic using the Advanced Ionospheric Sounder (AIS) at Halley (76° S, 27° W,L4.2) by Rodger et al. (1986). The AIS can measure the direction of arrival ofan ionospheric echo as well as its range. Assuming that the reflection is specular,the position of the perpendicular from the sounder to the edge of the trough canbe plotted, and thus the orientation observed.

The results are illustrated in Figure 5.28, which plots the positions of the echoesfrom troughs observed on 16 occasions. From Halley the perpendicular to thecontours of constant invariant latitude is east of south, and close to the directiondetermined for the period 0000–0159 LT (i.e. line number 4 in panel (c)). Beforethis time, therefore, the poleward edge is tilted towards lower latitude at later localtime (i.e. to the east), and the reverse is true after 0200 LT. The sense of these tiltsis consistent with a general equatorward motion during the earlier part of thenight and a poleward motion later.

In panel (d), the orientations are mapped into the equatorial plane at L4.2and compared with the “teardrop” model of Kavanagh et al. (1968) representing


the equipotentials resulting from a simple magnetospheric electric field. It appearsthat the trough is aligned with the equipotential and, thus, with the direction ofplasma drift.

Electron precipitation and the poleward edge

Following the first observations by Bates et al. (1973) it has often been noted thatthe minimum of the trough lies some few degrees equatorward of the edge of theregion of auroral precipitation, and it is therefore natural to postulate that auroral


Figure 5.28. On the orientation of the main trough. (a) The location of the poleward edgewith respect to Halley on 20–21 June 1982. The time is local. (b) Collection of the polewardedge locations for all observations during 2200–2359 LT, with the best-fitting straight line.(c) The best-fitting straight lines for six 2-h periods. (d) Perpendiculars to the lines in (c)projected to L4.2 in the equatorial plane and compared with the Kavanagh model ofmagnetospheric equipotentials. (Reprinted from A. S. Rodger et al., J. Atmos. Terr. Phys.48, 715, copyright 1986, with permission from Elsevier Science.)

(a) (b)

(c)(d)

ionization is what causes the electron density to increase on the poleward side ofthe trough. Supporting evidence comes from particle measurements (Rodger etal., 1986) and from incoherent-scatter radar (Jones et al., 1997). Electron precip-itation (at 1 keV) was present on nearly every trough overpass of DynamicsExplorer-2 (DE-2) before 2230 magnetic local time. The radar evidence is of anenhanced electron temperature on the poleward side of the trough. These obser-vations confirm earlier results published by Pike et al. (1977).

Later in the night, however, after the passage of the Harang discontinuity(Section 2.5.3), the association was less clear. Two other classes of event were seen,one in which the poleward edge was accompanied by softer electrons (50 eV), andone in which the level of electron precipitation did not alter over the trough. In theDE-2 study, all three types occurred with about the same frequency after mid-night. The radar study, also, was unable to establish a clear association with elec-tron precipitation during the second half of the night. The source of the ionizationforming the poleward edge is therefore less clear in the post-midnight sector. It issupposed that transport of ionization in the polar circulation is important.

5.4.4 Motions of individual troughs

Most of the studies which produced formulae for the latitude of the trough asfunctions of the time of day and Kp (Equation (5.3)) were based on observationsfrom (or on signals transmitted from) orbiting satellites. As such, the data consistof a sequence of snapshots taken on different occasions; there is no opportunityto observe any one trough continuously. Therefore these formulae do not necces-sarily describe the instantanous motion of the trough. The trough shown in Figure5.28(a), for example, moved equatorward at 1.3° h1, a faster drift than would beindicated by any of the formulae except the last of Table 5.3, which, indeed, wasbased on the tracking of individual examples (by incoherent-scatter radar).

Results from the AIS, tracking the poleward edge from Halley station (L4),also tend to show relatively high speeds. In the examples in Figure 5.29(a),showing the change of invariant latitude with time, many of the slopes exceed1°h1. If the higher speeds were maintained for several hours, these troughs wouldcover a greater range of latitude than is actually observed. However, it is also sig-nificant that, in some cases, the slope flattens out, indicating that the drift is notuniform. The drift speed also varies greatly from one example to another. Theexamples in Figure 5.29(b), also from Halley, cover the hours 2130–0800 LToverall, though every example extends into the period 0000–0400 LT. The driftspeed varies by a factor of ten (from 60 to 600 km h1), with half the speedsbetween 100 and 300 km h1 and the median at 200 km h1 (1.8° h1).

The examples in Figure 5.29(c) are from electron-content measurements froma site in the auroral zone. (The local time is UT 1 h.) At this higher latitude thetrough is seen during the afternoon, but note that the locations and the speedsagain agree with the formula of Collis and Häggström (CH) rather than with



Figure 5.29. The latitudinal drift of the main trough. (a) The poleward edge observed bythe Halley Advanced Ionospheric Sounder for five nights of 1982. In each case Kp2.(Reprinted from A. S. Rodger et al., J. Atmos. Terr. Phys. 48, 715, copyright 1986, with per-mission from Elsevier Science.) (b) A collection of Halley troughs from 1982–1983, showingthe variability of the speed of equatorward drift. (Time is counted from the appearance of aweak precipitation event associated with the poleward edge.) (W. G. Howarth and J. K.Hargreaves, private communication.) (c) Trough minima from electron-content measure-ments in the auroral zone in Scandinavia. Values of Kp are marked and the formulae ofKohnlein and Raitt, and of Collis and Haggestrom have been superimposed. (Reprintedfrom J. K. Hargreaves and C. J. Burns, J. Atmos. Terr. Phys. 58, 1449, Copyright 1996, withpermission from Elsevier Science.)

(b)

–5 –4 –3 –2

200 km N

200

300

400 km S

1 2 3 4

Time (hr) after passing over Halley

100

that of Kohnlein and Raitt (KR). These troughs would not link up with thoseshown in Figure 5.29(a) if they continued to move equatorward at the same speed.Thus, the evidence indicates that, although formulae based on satellite data mayexpress the latitude at which the trough is likely to be seen, individual troughsmove considerably faster than those formulae would indicate.

One explanation (Rodger et al., 1986) is based on the effect of substorms, whichon some occasions are seen to be related to a partial filling of the trough from thepoleward side. Figure 5.30 illustrates the point, showing how the polar edge steep-ened between two successive orbits of the satellite DE-2, a substorm having com-menced in the interim. This filling was most likely due to enhanced particleprecipitation due to the substorm. This is a new factor, not included in theassumptions of Equation (5.3), but it is not clear whether this is the whole expla-nation.

5.4.5 Mechanisms and models

The main trough caused by plasma decay

Since the main trough lies between the mid- and high-latitude ionospheres, onemay reasonably expect that its cause has some connection with the different cir-culation patterns in those two regions. Various attempts to predict the position ofthe trough have been made by modeling the ionosphere mathematically (Moffettand Quegan, 1983). These models represent the high-latitude convection in a


Figure 5.29. (cont.)

steady state and, although they might not include all the physical processes thatcould be relevant, they do predict a main trough in about the right place. The basiccause is that there are some convection paths (e.g. path 5 in Figure 5.1(b)) whichdo not encounter a production region for several hours, a time long enough forthe plasma density to decay to a low value. Measurements by incoherent-scatterradar (Collis and Haggstrom, 1988) support this theory, showing that the troughminimum generally lies in a zone where the plasma flow (with respect to the Earth)is strongly westward. Such a flow tends to offest the Earth’s rotation and henceprolong the time for which the plasma remains in a dark region.

One possible complication is that a steady-state pattern of convection isunlikely to continue for very long, due to the constant variations in the solar windwhich drives the polar convection. While ionization decay is now accepted as theessential cause of the main trough, we are some way from being able to predictdetails of the trough for any given day.

Other mechanisms

Rodger et al. (1992) reviewed all the mechanisms that could create, or help tocreate, ionization troughs, and concluded that the difference in velocity between


Figure 5.30. (a) Two consecutive passes of DE-2 near Halley on 14 August 1982, showing asteepening of the poleward edge. (b) The Halley magnetometer indicated that a substormoccurred between those two orbits. (Reprinted from A. S. Rodger et al., J. Atmos. Terr.Phys. 48, 715, copyright 1986, with permission from Elsevier Science.)

ions and neutral particles is likely to be an important factor. The rate coefficientsk1 and k2 in the expression for the recombination coefficient

k1[N2]k2[O2] (5.7)

are temperature dependent, as shown in Figure 5.31, and a relative drift betweenthe ions and the neutral species heats the gas. Figure 5.31 shows the relative veloc-ity as a second abscissa scale. The heating increases the rate of loss by recombina-tion and causes an upward flow of plasma that also depletes the F region. It isargued, therefore, that plasma depletion is expected in regions heated by highdifferences in speed between ions and neutral particles.


RA

TE

CO

EF

FIC

IEN

TS

(cm

3 s

1 )

109

5

2

1010

5

2

1011

5

2

1012

5

2

1013

0 2 4 6 8 10 12

ION TEMPERATURE (103 K)

0 1.0 2.0 3.0 4.0

RELATIVE VELOCITY (km s1)

O+ O2

O+ N2

Figure 5.31.

Temperature depen-dences of recombinationreactions in the F region.The relative velocity ofions and neutral speciesis shown on the secondscale. (Reprinted fromA. S. Rodger et al., J.Atmos. Terr. Phys. 54, 1,copyright 1992, withpermission from ElsevierScience.)

5.5 Troughs and holes at high latitude

Depletions occurring poleward of the main trough – that is, within the auroraloval and polar cap – have been observed by incoherent-scatter radar and in satel-lite passes, but in general they have not been so intensively studied as the maintrough. Rodger et al. (1992) have summarized the principal features of thesetroughs as follows.

High-latitude troughs are between 5° and 9° wide, with a poleward edgebetween 67° and 71° magnetic latitude and an equatorward edge between61° and 67°. (Note that this overlaps with the position of the main troughin the afternoon sector.)

They last for 4–8 h, moving to higher latitude towards the end of theperiod.

Their equatorward edge moves equatorward with increasing Kp, and thereis some evidence that the trough forms earlier when Kp is larger.

They are often associated with a reversal of the convection (as a functionof latitude) in the morning sector, but in the evening are on the equator-ward side of the reversal. Figure 5.32 illustrates this point.


Figure 5.32. The average location of the high-latitude trough determined from passes of thesatellite OGO-6 (dotted line), plotted over an electric-field pattern (solid lines). (Reprintedfrom A. S. Rodger et al., J. Atmos. Terr. Phys. 54, 1, copyright 1992, with permission fromElsevier Science.)

The ion temperature (Ti ) and the electric field are usually increased withinthe trough but the electron temperature is not usually affected.

There are relationships between Ti and the field-aligned plasma velocity.

The atomic ions (H, O, and N) are reduced in concentration, but con-centrations of molecular species (NO and O2

) are increased.

Figure 5.33 shows further examples of high-latitude troughs in terms of the iondensities; note the enhancements in concentration of NO.

The polar hole is recognized as a distinct feature. It is a long-lived depletionobserved in years of low solar activity during winter in the Antarctic polar cap(Brinton et al., 1978), occurring shortly after midnight at magnetic latitudes near80º. The electron density (at 300 km) is as low as (1–3)102 cm3, compared withup to 105 cm3 elsewhere in the polar cap. The hole appears sporadically at the

5.5 Troughs and holes 277

Figure 5.33. High-latitude troughs in O and H at 70°–75° north, from OGO-6, 18 March1970, 1830–1854 Kp2. There is an enhancement in concentration of the molecularspecies NO. (Reprinted from J. M. Grebowsky et al., Planet. Space Sci. 31, 99, copyright1983, with permission from Elsevier Science.)

equinoxes and hardly ever in summer. The seasonal variation can be explained byinvoking the movement of the solar terminator, which ensures that the relevantregion is dark in winter but illuminated in summer. The electron temperature isreduced in the polar hole and the ion speeds are low. Concentrations of molecu-lar species are not enhanced there. For reasons that remain unknown, the polarhole has not been observed in the Arctic.

It must be appreciated that it is not unusual for the high-latitude ionosphere tobe irregular where it is not illuminated by the Sun. We have drawn attention inSection 5.3.2 to the phenomena of patches and blobs in the high-latitude iono-sphere, where the emphasis is on the enhancements. A study of the depletionsshould be complementary to this, and it might not always be clear whether it isthe enhancement or the depletion which is abnormal. In some cases the structuremay actually comprise both – that is, the mechanism may remove ionization fromone place and concentrate it elsewhere.

Figure 5.34 summarizes the location of high-latitude depletions (as well as themain trough). Table 5.4 describes the various features.


Figure 5.34. A summary of F-region depletions under steady geophysical conditions whenthe cross-tail electric field is small. The solar terminator is along the line 1800–0600. A,main trough; B, polar hole. C, region of significant frictional heating of ions and neutralspecies. The features are superposed on the polar convection pattern of Figure 5.1(b).(Reprinted from A. S. Rodger et al., J. Atmos. Terr. Phys. 54, 1, copyright 1992, with per-mission from Elsevier Science.)

12h00

18h00 06h00

24h00

65

43

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B

80°

70°

60°

50°

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The reader is referred to the review paper by Rodger et al. (1992) for furtherdetails and for discussion of other high-latitude depletions of the F region.

5.6 Summary and implications

In radio propagation the F region principally affects systems operating at thehigher frequencies, specifically in the HF, VHF, and UHF bands. Effects can bemajor even when the F region is undisturbed, especially during the winter seasonwhen electron densities tend to become very small during the long polar night.During geomagnetic storms and substorms additional effects appear. The precip-itation of energetic charged particles increases, whereupon circuits operating inthese bands may be seriously degraded.

The main ionospheric trough, a region of depleted electron density just equat-orward of the auroral oval, depresses HF operating frequencies when the reflec-tion or control points come within its boundaries. The main trough is asemi-permanent feature at the transition between the mid-latitude and the high-latitude ionospheres, occurring mainly at night and more strongly in winter thanin summer in the northern hemisphere. The incidence is somewhat different in thesouthern hemisphere.

When energetic electrons and protons precipitate into the auroral F regionthey produce field-aligned irregularities of various sizes, which may deviate andscatter HF to UHF signals incident on them. Backscatter may be produced bythe component of irregularity having a wavelength equal to half the radio wave-length when the signal is propagating in a direction essentially normal to the geo-magnetic field-lines. The geometry is such that HF scattering is most likely tooccur when signals propagate from temperate latitudes towards and into theauroral oval. HF radars operated for research purposes make use of this back-scatter to study the structure and dynamics of the polar ionosphere. Over-the-horizon HF radars experience system degradation by field-aligned irregularities,and satellite-to-earth VHF and UHF signals suffer scintillation phenomenacausing a rapid and sometimes severe fading of amplitude and irregular fluctua-tions of phase.

In the polar ionosphere, by which we mean that part poleward of the auroralzone, the particle precipitation is generally not as intense as that into the oval.Nevertheless, some major F-region irregularities do occur. The dominant features,known as arcs or patches, are enhancements of F-region plasma density that prob-ably originate not locally but in the ionosphere at lower latitude, and then driftover the polar cap under the control of the electric field between the dusk anddawn sides of the polar cap. There is now a substantial body of knowledge aboutthese structures, though it is not yet sufficient for prediction purposes.

A detailed description of the effects of the high-latitude F region on the prop-agation of radio signals over the whole spectrum from MF to UHF is given inChapters 8 and 9.



5.1 Circulation of the high-latitude ionosphereBoyle, C. B., Reiff, P. H., and Hairston, M. R. (1997). Empirical polar cap potentials.J. Geophys. Res. 102, 111.

Cowley, S. W. and Lockwood, M. (1997) Excitation and decay of solar wind-drivenflows in the magnetosphere-ionosphere system. Ann. Geophysicae. 10, 103.

Dudeney, J. R., Rodger, A. S., Pinnock, M., Ruohoniemi, J. M., Baker K. B., andGreenwald, R. A. (1991) Studies of conjugate plasma convection in the vicinity of theHarang discontinuity. J. Atmos. Terr. Phys. 53, 249.

Hairston, M. R. and Heelis, R. A. (1995) Response time of the polar ionospheric con-vection pattern to changes in the north-south direction of the IMF. Geophys. Res. Lett.22, 631.

Jayachandran, P. T. and MacDougall, J. W. (1999) Seasonal and By effect on the polarcap convection. Geophys. Res. Lett. 26, 975.

Kelley, M. C. (1989) Section 6.2. In The Earth’s Ionosphere. Academic Press, NewYork.

Lu, G. and 20 others. (1994) Interhemispheric asymmetry of the high-latitude iono-spheric convection pattern. J. Geophys. Res. 99, 6491.

Rich, F. J. and Hairston, M. (1994) Large-scale convection patterns observed byDMSP. J. Geophys. Res. 99, 3827.

Ruohoniemi, J. M. and Greenwald, R. A. (1996) Statistical patterns of high-latitudeconvection obtained from Goose Bay HF radar observations. J. Geophys. Res. 101, 21743.

Todd, H., Bromage, B. J. I., Cowley, S. W. H., Lockwood, M., van Eyken, A. P., andWillis, D. M. (1986) EISCAT observations of rapid flow in the high latitude daysideionosphere. Geophys. Res. Lett. 13, 909.

Willis, D. M., Lockwood, M., Cowley, S. W. H., van Eyken, A. P., Bromage, B. J. I.,Rishbeth, H., Smith, P. R., and Crothers, S. R. (1986) A survey of simultaneous obser-vations of the high-latitude ionosphere and interplanetary magnetic field withEISCAT and AMPTE UKS. J. Atmos. Terr. Phys. 48, 987.

5.2 Behaviour of the F region at high latitudeFarmer, A. D., Crothers, S. R., and Davda, V. N. (1990) The winter anomaly atTromsø. J. Atmos. Terr. Phys. 52, 561.

Muldrew, D. B. and Vickrey, J. F. (1982) High-latitude F region enhancementsobserved simultaneously with ISIS 1 and the Chatanika radar. J. Geophys. Res. 87,8263.

Raitt, W. J. and Schunk, R. W. (1983) Composition and characteristics of the polarwind. In Energetic Ion Composition in the Earth’s Magnetosphere (ed. R. G. Johnson),p. 99. Terra Scientific Publishing, Tokyo.

Robinson, R. M., Tsunoda, R. T., Vickrey, J. F., and Guerin, L. (1985) Sources of F-region ionization enhancements in the night-time auroral zone. J. Geophys. Res. 90,7533.


Walker, I. K., Moen, J., Mitchell, C. N., Kersley, L., and Sandholt, P. E. (1998)Ionospheric effects of magnetopause reconnection observed by ionospheric tomogra-phy. Geophys. Res. Lett. 25, 293.

Whitteker, J. H., Shepherd, G. G., Anger, C. D., Burrows, J. R., Wallis, D. D.,Klumpar, D. M., and Walker, J. R. (1978) The winter polar ionosphere. J. Geophys.Res. 83, 1503.

5.3 Irregularities of the F region at high latitudeAarons, J. (1982) Global morphology of ionospheric scintillations. Proc IEEE 70, 360.

Anderson, D. N., Buchau, J., and Heelis, R. A. (1988) Origin of density enhancementsin the winter polar-cap ionosphere. Radio Sci. 23, 513.

Buchau, J., Reinish, B. W., Weber, E. J., and Moore, J. F. (1983) Structure and dynam-ics of the winter polar cap F region. Radio Sci. 18, 995.

Buchau, J., Weber, E. J., Anderson, D. N., Carlson, H. C., Moore, J. G., Reinisch, B.W., and Livingston, R. C. (1985) Ionospheric structures in the polar cap: their originand relation to 250 MHz scintillation. Radio Sci. 20, 325.

Burns, C. J. and Hargreaves, J. K. (1996) The occurrence and properties of large-scaleelectron-density structures in the auroral F region. J. Atmos. Terr. Phys. 58, 217.

Carlson, H. C., Wickwar, V. B., Weber, E. J., Buchau, J., Moore, J. G., and Whiting, W(1984). Plasma characteristics of polar cap F-layer arcs. Geophys. Res. Lett. 11, 895.

de la Beaujardière, O. and Heelis, R. A. (1984) Velocity spike at the poleward edge ofthe auroral zone. J. Geophys. Res. 89, 1627.

Gussenhoven, M. S., Hardy, D. A., and Heinemann, N. (1983) Systematics of theequatorward diffuse auroral boundary. J. Geophys. Res. 88, 5692.

Hargreaves, J. K., Burns, C. J., and Kirkwood, S. C. (1985a) EISCAT studies of F-region irregularities using beam scanning. Radio Sci. 20, 745.

Hargreaves, J. K., Burns, C. J., and Kirkwood, S. C. (1985b) Irregular structures in thehigh-latitude F-region observed using the EISCAT incoherent scatter radar. Proc.AGARD Conference 382 (Fairbanks, Alaska) p. 6.2-1.

Kelley, M. C., Baker, K. D., Ulwick, J. C., Rino, C. L., and Baron, M. J. (1980)Simultaneous rocket probe, scintillation and incoherent scatter observations of irregu-larities in the auroral zone ionosphere. Radio Sci. 15, 491.

Lockwood, M. and Carlson, H. C. (1992) Production of polar cap electron densitypatches by transient magnetopause reconnection. Geophys. Res. Lett. 19, 1731.

Muldrew, D. B. and Vickrey, J. F. (1982) High-latitude F region irregularities observedsimultaneously with ISIS 1 and the Chatanika radar. J. Geophys. Res. 87, 8263.

Radio Science (1994) Special section on high-latitude structures. Radio Sci. 29,155–315.

Rino, C. L. (1978) Evidence for sheetlike auroral ionospheric irregularities. Geophys.Res. Lett. 5, 1039.

Rino, C. L., Livingston, R. C., Tsunoda, R. T., Robinson, R. M., Vickrey, J. F., Senior,C., Cousins, M. D., and Owen, J. (1983) Recent studies of the structure and morphol-ogy of auroral-zone F-region irregularities. Radio Sci. 18, 1167.


Robinson, R. M., Tsunoda, R. T., Vickrey, J. F., and Guerin, L. (1985) Sources of F-region ionization enhancements in the night-time auroral zone. J. Geophys. Res. 90,7533.

Secan, J. A., Bussey, R. M., Fremouw, E. J., and Basu, S. (1997) High-latitude upgradeto the Wideband ionospheric scintillation model. Radio Sci. 32, 1567.

Sojka, J. J., Bowline, M. D., Schunk, R. W., Decker, D. T., Valladares, C. E., Sheehan,R., Anderson, D. N., and Heelis, R. A. (1993) Modelling polar cap F region patchesusing time varying convection. Geophys. Res. Lett. 20, 1783.

Sojka, J. J., Bowline, M. D., and Schunk, R. W. (1994) Patches in the polar ionosphere:UT and seasonal dependence. J. Geophys. Res. 99, 14959.

Tsunoda, R. T. (1988) High-latitude F region irregularities: a review and synthesis.Rev. Geophys. 26, 719.

Vickrey, J. F., Rino, L. C. and Potemra, T. A. (1980) Chatanika/TRIAD observationsof unstable ionization enhancements in the auroral F-region. Geophys. Res. Lett. 7,789.

Weber, E. J. and Buchau, J. (1981) Polar cap F layer auroras. Geophys. Res. Lett. 8,125.

Weber, E. J. and Buchau, J. (1985) Observations of plasma structure and transport athigh latitudes. The Polar Cusp (eds. Holtet and Egeland) p. 279. Reidel, Hingham,Massachusetts.

Weber, E. J., Buchau, J., Moore, J. G., Sharber, J. R., Livingston, R. C. ,Winningham,J. D., and Reinisch, B. W. (1984) F layer ionization patches in the polar cap. J.Geophys. Res. 89, 1683.

Weber, E. J., Klobuchar, J. A., Buchau, J., Carlson, H. C., Livingston, R. C., de laBeaujardière, O., McCready, M., Moore, J. G., and Bishop, G. J. (1986) Polar cap F-layer patches: structure and dynamics. J. Geophys. Res. 91, 12121.

Yeh, K. C. and Liu, C. H. (1982) Radio wave scintillation in the ionosphere. Proc.IEEE 70, 324.

5.4 The main troughBates, H. F., Belon, A. E., and Hunsucker, R. D. (1973) Aurora and the poleward edgeof the main ionospheric trough. J. Geophys. Res. 78, 648.

Best, A., Best, I., Lehmann, H.-R., Johanning, D., Seifert, W., and Wagner, C.-U.(1984) Results of the Langmuir probe experiment on board Intercosmos-18. Proc.Conference on Achievements of the IMS, Graz, Austria (June 1984). ESA report SP-217, p. 349.

Collis, P. N. and Häggström, I. (1988) Plasma convection and auroral precipitationprocesses associated with the main ionospheric trough at high latitudes. J. Atmos. Terr.Phys. 50, 389.

Halcrow, B. W. and Nisbet, J. S. (1977) A model of F2 peak electron densities in themain trough region of the ionosphere. Radio Sci. 12, 825.

Hargreaves, J. K. and Burns., C. J. (1996) Electron content measurement in the auroralzone using GPS: observations of the main trough and a survey of the degree of irregu-larity in summer. J. Atmos. Terr. Phys. 58, 1449.


Jones, D. G., Walker, I. K., and Kersley, L. (1997) Structure of the poleward wall ofthe trough and the inclination of the geomagnetic field above the EISCAT radar. Ann.Geophysicae 15, 740.

Kavanagh, L. D., Freeman, L. W., and Chen, A. J. (1968) Plasma flow in the magneto-sphere. J. Geophys. Res. 73, 5511.

Kohnlein, W., and Raitt, W. J. (1977) Position of the mid-latitude trough in the topsideionosphere as deduced from ESRO 4 observations. Planet. Space Sci. 25, 600.

Liszka, L. (1967) The high-latitude trough in ionospheric electron content. J. Atmos.Terr. Phys. 29, 1243.

Mallis, M. and Essex, E. A. (1993) Diurnal and seasonal variability of the southern-hemisphere main ionospheric trough from differential-phase measurements. J. Atmos.Terr. Phys. 55, 1021.

Moffett, R. J. and Quegan, S. (1983) The mid-latitude trough in the electron concen-tration of the ionospheric F-layer: a review of observations and modelling. J. Atmos.Terr. Phys. 45, 315.

Muldrew, D. B. (1965) F-layer ionization troughs deduced from Alouette data. J.Geophys. Res. 70, 2635.

Pike, C. P., Whalen, J. A., and Buchau, J. (1977) A 12-hour case study of auroral phe-nomena in the midnight sector: F layer and 6300 Å measurements. J. Geophys. Res. 82,3547.

Rodger, A. S., Brace, L. H., Hoegy, W. R., and Winningham. J. D. (1986) The pole-ward edge of the mid-latitude trough – its formation, orientation and dynamics. J.Atmos. Terr. Phys. 48, 715.

Rodger, A. S., Moffett, R. J., and Quegan, S. (1992) The role of ion drift in the forma-tion of ionisation troughs in the mid- and high-latitude ionosphere – a review. J.Atmos. Terr. Phys. 54, 1.

Rycroft, M. J. and Burnell, S. J. (1970) Statistical analysis of movements of the iono-spheric trough and the plasmapause. J. Geophys. Res. 75, 5600.

Spiro, R. W. (1978) A study of plasma flow in the mid-latitude ionization trough.Ph.D. thesis, University of Texas at Dallas, Richardson, Texas.

Thomas, J. O., Rycroft, M. J., Colin, L., and Chan, K. L. (1966) The topside iono-sphere. 2. Experimental results from the Alouette 1 satellite. In Electron DensityProfiles in Ionosphere and Exosphere, p. 322. Amsterdam, North-Holland.

5.5 Troughs and holes at high latitudeBrinton, H. C., Grebowsky, J. M., and Brace, L. H. (1978) The high-latitude winterF region at 300 km: thermal plasma observations from AE-C. J. Geophys. Res. 83,4767.

Rodger, A. S., Moffett, R. J., and Quegan, S. (1992) The role of ion drift in the forma-tion of ionisation troughs in the mid- and high-latitude ionosphere – a review. J.Atmos. Terr. Phys. 54, 1.


Chapter 6

The aurora, the substorm, and the E region

6.1 Introduction

By “aurora” people usually mean the emission of light from the upper atmos-phere, but in fact there are numerous related phenomena, each being a direct orindirect consequence of energetic particles entering the atmosphere from the mag-netosphere. They include

(a) luminous aurora;

(b) radar aurora, by which is meant the reflection of radio signals from ioniza-tion in the auroral region;

(c) auroral radio absorption, the absorption of radio waves in the auroral ion-ization;

(d) auroral X-rays, which are generated by the incoming particles and may bedetected on high-altitude balloons;

(e) magnetic disturbances, due to enhanced electric currents flowing in theauroral ionization, which may be detected by magnetometers;

(f) electromagnetic emissions in the very-low- and ultra-low-frequency bands,which are generated in the magnetosphere by wave–particle interactions(Section 2.5.6), and which then propagate to the ground where they maybe detected with a radio receiver or a sensitive magnetometer.

Arising as they do from a common cause, the auroral phenomena displayseveral common properties.

(1) They all exhibit a general relationship with solar activity, though oftenthere is no specific association with any obvious solar event. From the

285

1930s the term M region was used to signify a hypothetical and unseensolar region causing aurora and magnetic storms, and this served as a uni-fying hypothesis for some 40 years. It is now well appreciated, of course,that the unseen agent is the solar wind.

(2) They are essentially zonal in occurrence, their occurrence and intensitycoming to a maximum some 10°–25° from the magnetic poles. This prop-erty is treated in Section 6.2.

(3) All the auroral phenomena exhibit substorm behavior. They are greatlyenhanced during bursts of activity lasting perhaps 30–60 min, which areseparated by quieter intervals of several hours. It is now clear that the sub-storm is caused by processes in the magnetosphere. This aspect is dis-cussed in Section 6.4.

The auroral luminosity originates within the ionospheric E region. The parti-cles which excite the emission of light also create additional ionization and therebyenhance the electron density. This in turn increases the ionospheric current atthose heights, which has further consequences. The behavior of the auroral Eregion is therefore closely related to that of the aurora. The high-latitude E regionis considered in Section 6.5.

6.2 Occurrence zones

6.2.1 The auroral zone and the auroral oval

In general the auroral phenomena are highly structured in both space and time,with essentially zonal patterns of occurrence. The classical picture of the occur-rence of aurorae (Figure 6.1) shows a zone centered about 23° from the geomag-netic pole (i.e. about 67° geomagnetic latitude) and about 10° wide in latitude. Theisochasms show the occurrence of discrete aurorae, which is 100% at themaximum and falls off both to the equatorward and to the poleward sides.(“100%” here means that some aurora was seen every clear night, not that it wasvisible all the time.) This pattern, which is a geographic distribution, was firstdefined by Vestine (1944) and is based on reports of visual sightings of the auroraover several decades.

However, in 1963 Y. I. Feldstein, using all-sky camera data from theInternational Geophysical Year of 1957–1958, pointed out that at a fixed time thelocus of the aurora is not circular but oval (Figure 6.2). The maximum is near 67°latitude at midnight, but this increases to about 77° (the latitude of the cusp –Section 2.2.5) at noon. The auroral oval, as it is generally known, is widest at mid-night and narrowest at noon. It is essentially fixed with respect to the Sun, and theclassical auroral zone is the locus of the midnight sector of the oval as the Earthrotates underneath it. The auroral oval is one of the important boundaries of geo-space. In relation to the structure of the magnetosphere it is generally considered

286 The aurora, substorm, and E region

to mark the division between open and closed field-lines. The regions poleward ofthe ovals (one in each hemisphere) are generally taken to be the polar caps inwhich the magnetic field-lines connect to the IMF and circulate under the influ-ence of the solar wind (Section 2.4.1).

Although it was originally just a statistical concept, later work, both ground-based (Feldstein and Starkov, 1967) and using photography from space (Akasofu,1974; Frank and Craven, 1988), has shown that the oval exists virtually continu-ously as a permanent ring of light around the magnetic pole, and also as a ring ofparticle precipitation (Fuller-Rowell and Evans, 1987; Hardy et al., 1985). In thepictures from space the general form of the oval is clearly observed (Figure 6.3) asa continuous band of light around the pole that is nearly always present, thoughits intensity varies greatly from time to time.

The latest auroral pictures from space are adding much detail and have shownthat the oval form is not by any means the whole story. The detailed spatial distri-bution varies considerably from time to time. Sometimes an arc is seen to extendacross the polar cap, connecting the day and night sides of the oval – a configura-tion called the -aurora. Sometimes the morning side of the oval is quiet while the


Figure 6.1. The northern auroral zone, showing the percentage of good observing nightswhen aurorae may be seen. (After E. H. Vestine, Terr. Magn. Atmos. Electricity, 49, 77,1944, copyright by the American Geophysical Union.)

evening is active, and sometimes the morning side is the more active. The behav-ior of more localized brightenings within the oval can also be observed fromspace. Some examples are shown in Figure 6.4. The variety of configurations anddynamics emphasizes the complexity of the auroral distribution and suggests thatpresent classifications are incomplete.

6.2.2 Models of the oval

Without doubt the auroral oval is a special region of the ionosphere. That beingso, it is often convenient to refer observed phenomena to the location (or prob-able location) of the oval at the time of the observation, and thus it is helpful tohave models giving the typical position of the oval under stated conditions. Figure6.5(a) indicates, for typical conditions, the geographic location of the oval every 2h of the UT day. It is usual to quantify the level of disturbance by using one of themagnetic activity indices (see Section 2.5.4), Q being a popular one for thispurpose since it is derived at 15-min intervals. Figure 6.5(b) gives the locations ofthe oval for several levels of Q taken from a set of diagrams developed by Whalen(1970). (Kp being a more common index, the following relations may be used toobtain the appropriate value of Q when one is using Whalen’s results: Q8 ifKp&6; QKp2 if 1Kp6; and Q3Kp if Kp%1.) Since the oval is closer tothe magnetic pole at noon than it is at midnight, it is quite possible for an observeron the Earth to be poleward of the oval at midnight and equatorward of it at noon.

Distributions based not on luminosity but on measurements of particles of


Figure 6.2. The auroral oval in relation to the 40-keV trapping boundary. (S.-I. Akasofu,Polar and Magnetospheric Substorms, Reidel, 1968, with kind permission from KluwerAcademic Publishers.)

FLUX = 104 cm2 s1

TRAPPED ELECTRONS(E ( 40 KeV)(FRANK, VAN ALLEN, & CRAVEN)

AURORAL OVAL (75%–90%)(FELDSTEIN)

SUN

energy 30 or 50 eV to 20 keV on DMSP satellites (Hardy et al., 1985; 1989) showzones of electron and ion influx in terms of magnetic latitude and local time, atlevels of geomagnetic activity quantified by Kp. Figure 6.6 shows the distributionof electrons for Kp3. Figure 6.6(a) is very like the auroral oval, being offset fromthe magnetic pole towards midnight. The particles forming the dayside maximumare relatively soft (i.e. of low energy).

Using data from the same source, Meng and Makita (1986) defined the boun-daries of the precipitation zones for “low-energy” (500 eV) and “high-energy”(500 eV) electrons, for magnetically quiet (AE%150 nT) and disturbed(AE400 nT) conditions, and for the evening and morning sectors – see Table 6.1.The criterion for the boundary was a flux of 107 electrons cm2 s1 steradian1.The transition latitude, where the fluxes of low- and high-energy particles wereequal, was also noted.


Figure 6.3. The auroral oval from space, observed in the ultra-violet (118–165 nm) by theDynamics Explorer I spacecraft on 16 February 1982. The aurora is plainly visible aroundthe northern pole. Airglow bands north and south of the equator, dayglow above the morning(right) limb of the Earth, and resonant Lyman- scattering in the protonosphere are also tobe seen. (L. A. Frank and J. D. Craven, University of Iowa, private communication.)

It is generally agreed that the oval expands equatorward, to lower latitudes, asmagnetic activity increases. The region of low-energy precipitation becomes nar-rower and the high-energy region broadens. According to Chubb and Hicks(1970), the equatorward boundary of the luminous oval moves about 1.7° equat-orward per unit of Kp on the day side of the Earth, and 1.3° on the night side; itmoves by 1°–3° of latitude in individual substorms. (See also Section 6.4.2.) Theoval also varies with the IMF, increasing in size by about 0.5° for each one increase in the southward component of the IMF. According to Meng (1984), thepolar cap can be as small as 12° side to side under quiet conditions and as largeas 50° when it is disturbed, which is not inconsistent with the results in Table 6.1.

Gussenhoven et al. (1983) express the variation of the equatorward boundaryof the oval in terms of the Kp index, as


Figure 6.4. Images of the northern auroral region observed by the Viking satellite. Thecamera had a 20° by 25° field of view and responded to UV of 134–180 nm, mainly emis-sions from nitrogen. Each exposure lasted 1.2 s. The top left-hand image shows the wholeauroral oval including the day side. The one below it shows a substorm in the midnightsector, with activity also around noon and faint arcs in the morning. The top right-handimage is from the last stage of a substorm, when regularly spaced bright spots, lasting 1–5min, may appear along the poleward edge of the oval near midnight. The fourth imageshows a sun-aligned arc extending across the polar region from midnight (at the bottom) tonoon. (Pictures and commentary from G. Enno, private communication. The Vikingproject was managed by the Swedish Space Corporation for the Swedish Board for SpaceActivites. The UV imager was a project of the National Research Council of Canada, andwas operated by the Institute for Space Research, University of Calgary, with support fromthe Natural Sciences and Engineering Research Council of Canada.)

LL0aKp, (6.1)

where L0 and a depend on the local magnetic time (MLT) as in Table 6.2.Figure 6.7 illustrates the position of the oval at three levels of disturbance, and

its magnetic latitude and thickness against Kp.The foregoing results may be expected to apply also to those propagation phe-

nomena which are typical of the auroral oval.

6.3 The auroral phenomena

6.3.1 The luminous aurora

The luminous aurora is a well-known phenomenon of the high-latitude regions,and in fact the most readily observed consequence of the dynamic magnetosphere.Although it is only in the present century that there has been any kind of under-standing of the aurora, it must surely rank amongst the oldest of the known geo-physical phenomena. There are accounts of lights in the night sky going back toGreek and Roman times, when they were frequently given a mystical or propheticinterpretation. The term aurora borealis dates from 1621, and the southern lights,observed by Captain James Cook in 1773, were subsequently called the auroraaustralis. Detailed reports of auroral displays date from 1716 and the first writtenwork devoted entirely to the polar aurora was published in France in 1733.

The first proof that the auroral light is a consequence of the excitation of atmos-pheric gas by energetic particles did not come until the early 1950s, and it was notuntil 1958, when rockets were fired into an aurora, that energetic electrons wereidentified as the primary source. Where those electrons come from and how theyare energized are questions that have not yet been answered in full, but their mag-netospheric origin is beyond doubt and much has been learned about them inrecent years.

6.3.2 The distribution and intensity of the luminous aurora

Auroral investigations before about 1950 tended to fall into one of two areas.Morphological studies were intended to map the occurrence of the aurora in spaceand time and to determine the details of the fine structure of individual auroralforms. Auroral spectroscopy was virtually a separate discipline, concerned withthe emitted light, in particular with its spectrum and its origin in photochemicalprocesses – a topic having strong affinities with airglow.

The luminous aurora is highly structured and dynamic. Some features are asthin as 100 m, and the time changes can be as rapid as 10 s1. The basic record-ing intrument is the all-sky camera which was first used during the 1950s and isparticularly valuable for surveying the occurrence of aurorae. It uses a convex


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mirror to obtain a picture of the night sky from horizon to horizon, and it wouldtypically be operated automatically at regular intervals on every clear night duringthe winter viewing season.

There is a classification of auroral structure based on its general appearence, asin Table 6.3. When structure is present the height of the luminosity may be deter-mined by triangulation. Between 1911 and 1943, C. Störmer made 12000 heightdeterminations with spaced cameras and found that the lower borders of auroralforms were usually at heights of 100–110 km (Figure 6.8(a)). In some of the formsthe luminosity is concentrated into a band only 10–20 km deep and the lower edge,in particular, can be quite sharp. The brightness of a discrete arc typically falls off

by a factor of ten within a few kilometers below the maximum, and by a furtherfactor of ten only 1 or 2 km below that. The vertical distribution of auroral lumi-nosity is illustrated in Figure 6.8(b) for several types of aurora.


Figure 6.6. Electron-precipitation zones forKp3. (a) The total flux of precipitatingelectrons in the energy range 30 eV to 30keV in units of cm2 s1 sr1. Numbers inbrackets are powers of ten. (b) The totalenergy flux, in units of keV cm2 s1 sr1,due to the same flux of electrons. (c) Theaverage energy (keV) of electrons in theband 30 eV to 30 keV. The data are from theDMSP satellites F6 and F7, and the mapsare in corrected geomagnetic latitude(marked every 10° from 50° to 80°) andmagnetic LT. (Private communication fromM. S. Gussenhoven and D. H. Brautigan,Space Hazards Branch, Air Force ResearchLaboratory. Further details are given by D.A. Hardy et al., J. Geophys. Res. 90, 4229(1985) and J. Geophys. Res. 94, 370 (1989).)

(a) (b)

(c)


Table 6.1. The magnetic latitude of the poleward boundary of “low”-energy, andthe equatorward boundary of “high”-energy electron precipitation (after Mengand Makita, 1986)

Quiet conditions Disturbed conditions

Evening Morning Evening Morning

Polewardboundary(low energy) 80°–82° 80°–82° 73°–75° 76°–77°

High-to-low-energytransition 73°–75° 73°–75° 70°–72° 70°–72°

Equatorwardboundary(high energy) 69°–71° 67°–69° 64°–66° 64°–66°

Table 6.2. Values of L0 and a forEquation (6.1)

MLT (h) L0 a

00–01 66.1 1.9901–02 65.1 1.55

04–05 67.7 1.4805–06 67.8 1.8706–07 68.2 1.9007–08 68.9 1.9108–09 69.3 1.8709–10 69.5 1.6910–11 69.5 1.4111–12 70.1 1.2512–13 69.4 0.84

15–16 70.9 0.8116–17 71.6 1.2817–18 71.1 1.3118–19 71.2 1.7419–20 70.4 1.8320–21 69.4 1.8921–22 68.6 1.8622–23 67.9 1.7823–24 67.8 2.07


Figure 6.7. (a) Positions of the auroral oval under three levels of activity. (b) The magneticlatitude and thickness of the oval as functions of Kp. (J. M. Goodman, HFCommunications. Van Nostrand Reinhold, 1992.)

Table 6.3. Classification of auroral forms

Forms without ray structure

Homogeneous ray structure: a luminous arch stretching across the sky in amagnetically east–west direction; the lower edge is sharper than the upper, and there isno perceptible ray structureHomogeneous band: somewhat like an arc but less uniform, and generally exhibitingmotions along its length: the band may be twisted into horseshoe bendsPulsating arc: part or all of the arc pulsatesDiffuse surface: an amorphous glow without distinct boundary, or isolated patchesresembling cloudsPulsating surface: a diffuse surface that pulsatesFeeble glow: auroral light seen near the horizon, so that the actual form is notobserved

Forms with ray structure

Rayed arc: a homogeneous arc broken up into vertical striationsRayed band: a band made up of numerous vertical striationsDrapery: a band made up of long rays, giving the appearance of a curtain; the curtainmay be foldedRays: ray-like structures, appearing singly or in bundles separated from other formsCorona: a rayed aurora seen near the magnetic zenith, giving the appearance of a fanor a dome with the rays converging on one point

Flaming aurora: waves of light moving rapidly upward over an auroral form

The intensity of an aurora as seen from the ground is measured in units of theRayleigh, named in honour of R. J. Strutt (the fourth Baron Rayleigh), who wasa notable amateur scientist of his time and the leading pioneer of airglow studies.The unit (R) is defined as

1 R106 photons cm2 s1. (6.2)

It is a measure of the height-integrated rate of emission, as would be observed byan instrument on the ground looking vertically upward. A more general classifi-cation of brightness uses a scale of I–IV, as in Table 6.4. This also shows the


Figure 6.8. Observations of auroral luminosity. (a) The distribution of 12330 height meas-urements made by Störmer and colleagues. The vast majority lie between 90 and 150 km.(C. Störmer, The Polar Aurora. Oxford University Press, 1955. By kind permission ofOxford University Press.) (b) Profiles of auroral luminosity along various forms. (After L.Harang, The Aurorae. Wiley, 1951.)

Hei

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Light intensity

80

70

60

50

40

30

20

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Draperies

Arcs withray-structure

Arcs

500

400

300

200

100

500

400

300

200

100

0 500 1000

500 1000

Hei

gh

t (k

m)

Number of observations

(a) (b)

standard a visual observer might use for comparison, the equivalent in kiloray-leighs, and the approximate rate of deposition of energy into the atmosphere.

A photometer is needed for exact intensity measurements, and this can either bepointed in a fixed direction, for example to the zenith, or scanned across the sky torecord the spatial distribution of intensity as well. A diagram of the latitudinal vari-ations with time is sometimes called a keogram. Neither scanning photometers norcameras are sufficiently sensitive to record the most rapid fluctuations in the auroralemissions, but TV techniques, both monochrome and color, are more sensitive andhave been applied very successfully to dynamic auroral photography in recent years.In addition to their scientific value, some of these auroral “videos” are possessed ofno little esthetic interest (particularly if they are set to music). Figure 6.9 shows anexample of a keogram composed from TV data.

One significant distinction that should be made is that between discrete anddiffuse (or mantle) aurora. All the earlier studies concentrated on the discrete


Figure 6.9. A keogram from auroral TV in Scandinavia. North (poleward) is at the top.This example shows the main features of auroral activity during a 2-h period on 18February 1993, giving some idea of the complexity of the aurora on an active day (Kp4).Note the dominance of equatorward movements. There are also several poleward

Table 6.4. Intensity classification of the aurora

Intensity Equivalent to Kilorayleighs Energy deposition (erg cm2 s1)

I Milky Way 1 3II Thin moonlit cirrus 10 30III Moonlit cumulus 100 300IV Full moonlight 1000 3000

auroral forms (Table 6.3) which are the more readily observed against the back-ground light of the night sky because of their fine and dynamic structure.However, as was demonstrated in the early 1960s, the aurora may also take theform of a diffuse glow. This contributes at least as much total light as the discreteforms, though it is more difficult to observe from the ground because of its lowintensity per unit area. The night-time discrete and diffuse aurorae are thought tomap along the geomagnetic field into different regions of the magnetotail (Section2.2.6 and Figure 2.6). The diffuse aurora is generally associated with the centralpart of the plasma sheet, and the discrete forms, which tend to appear polewardof the diffuse aurora, are thought to map onto the edge of the plasma sheet or toan X-type neutral line (Section 2.4.2 and Figure 2.20).

Downward-looking satellites, by virtue of their ability to observe a large partor even the whole of the auroral oval at the same time, and which avoid theproblem of poor seeing conditions which so often affects the ground-based tech-niques, have provided much new information about the distribution of the lumi-nous emissions. The diffuse aurora tends to dominate in these pictures, butdiscrete forms are also seen within the diffuse glow or poleward of it; they are notseen on the equatorward side, however.

When the IMF is northward, luminous arcs extending for thousands of kilom-eters and aligned towards the Sun are observed in the polar caps. They are notbright (emitting only tens of rayleighs, against thousands for a normal aurora) butthey can be detected with modern equipment and at that low intensity areobserved about half the time. It appears, therefore, that they are almost always


expansions, either substorm onsets or pseudo-break-ups (Section 6.4.2). The distance scaleassumes that the emission comes from a height of 110 km. A distance of 600 km is equivalent to approximately 5.5° of magnetic latitude. (Data from P. N. Smith, SpacePhysics Group, University of Sussex, via the Auroral TV Database.)

U.T.

present when the IMF is northward. It is believed that these arcs are on closedfield-lines and that they may be magnetically conjugate (i.e. they occur simultane-ously at opposite ends of field-lines in northern and southern hemispheres). TheSun-aligned arcs are associated with velocity shears in the polar-cap convection(Section 5.1.2 and Figure 5.5). (The -aurora, mentioned above, is also associatedwith a velocity shear, but it is much brighter and also much rarer than the commonSun-aligned arcs. It is not at present clear whether it is a different phenomenon.)

6.3.3 Auroral spectroscopy

Aurorae and airglow have similar causes, both being the emission of quanta ofradiation from common atmospheric gases, particularly O and N2. In the first casethe excitation is by energetic particles entering the upper atmosphere from themagnetosphere, and in the second by electromagnetic radiation from the Sun. Theemission lines represent transitions between energy states of the emitting species,but these may be complex and the task of interpreting the auroral spectrum wasfar from trivial. In spectroscopists’ terms the lines are in general “forbidden,”which means in practice that they are generated by transitions that are relativelyimprobable.

Most aurorae are too faint to be seen in color by the naked eye, but a brightaurora appears green or red, the colours being due to atomic-oxygen lines at 557.7nm (the green line) and 630.0 nm (the red line), respectively. The 391.4-nm line fromionized molecular nitrogen (N2

) is also present in the violet. Some aurorae havered lower borders, and, when this occurs, the red light is due to emissions frommolecular oxygen. Such aurorae result from unusually energetic particles that pen-etrate deeper into the atmosphere. The important group of emissions from atomicoxygen and the transitions which cause them are illustrated in Figure 6.10.

Some of the UV emissions, particularly the O emissions near 130 nm, haveproved particularly valuable for mapping the aurora from space vehicles because,at those wavelengths, the aurora may be seen in sunlight. In addition to theirobvious applications to the detection and mapping of the aurora, some of theemission lines can be exploited to provide information helpful to other branchesof upper-atmospheric science. The intensities of the emissions from N2

at 427.8and 391.4 nm are proportional to the rate of ionization by the incoming electrons.The neutral-air wind in the thermosphere may be determined by measuring theDoppler shift of the 630-nm line of oxygen.

6.3.4 Ionospheric effects

The auroral phenomena are all associated with the precipitation of energetic elec-trons into the atmosphere. Although the best known of them, the luminosity isactually a byproduct of ionization by energetic particles and of the subsequentrecombination processes. Other phenomena, more directly related to theenhanced electron density of the auroral region, are of greater direct concern in


radio propagation. They are briefly reviewed here, and some will be treated indetail in later chapters. Figure 6.11 illustrates the connections in a schematic, andadmittedly simplistic, manner.

The E region

In the E region, for example, it is not unusual for the electron density to beincreased to several times 1012 m3 by electron precipitation. Electron densities of


Figure 6.10. Energy levels and transitions in atomic oxygen. (a) Transitions that have beenobserved in airglow or aurorae. (M. H. Rees, Physics and Chemistry of the UpperAtmosphere. Cambridge University Press, 1989.) (b) Details of the most important lines.(After S. J. Bauer, Physics of Planetary Ionospheres. Springer-Verlag, 1973, copyright noticeof Springer-Verlag.) In each case the unit of wavelength is the ångstrom unit.

(a)15

14

12

10

8

6

4

2

0

Ene

rgy

abov

e O

l gro

und

stat

e (e

V)

1218

999 11

52 922

1641

5577

4368

1304

7990

8446

811

878

989

1356

3947 4968

7774

1027

29726300

Excitationenergy

Quantumstate

4.17 eV

1.96 eV

0.00 eV

0.74 s

5577 Green

2972 UV

110s

6364 Red

6300 Red

01

2

1D2

S01

3 p

(b)

Singlets Triplets Quintets

S0

1 S00

1

P1

1 P10

1 D2

1 D20

1

F3

1 F30

1 S10

3 P2,

1,0

3

P2,

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10

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D3,

2,1

3

D4

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5

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this magnitude may reflect vertically incident waves of radio frequency up to 20MHz (Section 3.4.2, Equation (3.64)), and those of higher frequency if they areobliquely incident (Section 3.4.3, Equation (3.73)). The ionization may thereforebe detected by radar as a total reflection if the frequency is not too high. If theobserving geometry is suitable, echoes may also be received at higher frequencies,and these echoes come from electron-density irregularities that are produced byinstabilities arising in the auroral electrojet (Section 2.5.3). Since the irregularitiestend to be field-aligned, the echo intensity is aspect-sensitive and the best observ-ing geometry is when the radar lies in the plane normal to the magnetic field-lines.The radar aurora is described in detail in Section 6.5.5.

The D region

The more energetic electrons penetrate into the D region (See Figure 2.26), andthe ionization they create there acts to absorb radio waves by an amount depend-ing on their frequency. The effect is usually monitored with a Riometer (Section4.2.4), which typically operates in the range 30–50 MHz, at which frequencies theabsorption rarely exceeds 10 dB; but the effect will generally be considerablygreater in the HF band. (The absorption varies approximately with the inversesquare of the frequency – Section 3.4.4, Equation (3.95)). The properties ofauroral radio absorption are detailed in Section 7.2. Figures 7.23 and 7.24 illus-trate E- and D-region electron-density profiles observed by incoherent-scatterradar during electron-precipitation events.

X-rays

Auroral X-rays are generated by the Bremsstrahlung process outlined in Section2.6.2. They have no direct influence on radio propagation, but, because of theirgreater penetrating power, they produce ionization at a lower altitude than dotheir parent electrons. The incidence and morphology of auroral X-rays are inmany ways similar to those of auroral radio absorption, both being due to theharder end of the electron spectrum.


Figure 6.11. Some links between auroral phenomena. Techniques are shown in brackets. (J.K. Hargreaves, Proc. Inst. Electr. Electron. Engineers. 57, 1348, © 1969 IEEE.)

Magnetic effects

Magnetic bays (Section 2.5.3) are essentially a phenomenon of the auroral zone,though, as a magnetic perturbation, they are also detected by magnetometers aconsiderable distance away. The bays are primarily due to the ionospheric currentwhich flows in enhancements of the E-region electron density. VLF and ULFemissions also increase when the auroral zone is active. They have various causes,some involving wave–particle interactions, but they are basically magnetosphericin origin and are not a factor in radio propagation.

6.3.5 The outer precipitation zone

Hartz and Brice (1967) generalized the definition of auroral phenomena by rec-ognizing that they actually fall into two groups having different patterns of occur-rence (Figure 6.12). The inner one, corresponding to the luminous oval, ischaracterized by


Figure 6.12. The two zones of auroral particle precipitation in the northern hemisphere.The density of symbols indicates the average flux, and the coordinates are geomagnetic lati-tude and time. (Reprinted from T. R. Hartz and N. M. Brice, Planet. Space Sci. 15, 301,copyright 1967, with permission from Elsevier Science.)

luminosity,

sporadic-E on ionograms,

spread-F on ionograms,

soft X-rays,

impulsive micropulsations,

negative bays on magnetometers,

soft but intense electron fluxes detected by satellites,

high frequency (4 kHz) VLF hiss, and

rapid fading of VHF scatter signals.

In addition there is a second zone at a lower latitude, which is almost circularand covers approximately 60°–70°, with its center at about 65° geomagnetic lati-tude. This zone displays

diffuse aurorae,

radio absorption,

sporadic-E at 80–90 km altitude,

continuous micropulsations,

hard X-rays of long duration,

harder (40 keV) electrons detected by satellites,

VLF emissions at 2 kHz, and

slow fading of VHF scatter signals.

This second zone of precipitation is generally thought to be connected with theouter Van Allen zone of trapped particles (Section 2.3.4); considering that it is alsothe outer of the two zones when plotted on a polar map, we shall call it the outerprecipitation zone. The ionospheric phenomena in the outer zone are related toelectron precipitation more energetic than that typical of the oval. The phenom-ena tend to be of longer duration in the outer zone. The rate of occurrence is great-est by day, whereas it is greatest by night in the oval. In both zones the phenomenaare sporadic and dynamic, and both exhibit substorm behavior (Section 6.4.2).They occupy much the same latitude at midnight but become increasingly separ-ated towards noon.

In Figure 6.6, which shows properties of electron precipitation (30 eV to20 keV), the total flux of particles (a) is distributed like the luminous oval.However, the dayside particles being relatively soft at the higher latitudes, the totalenergy flux (b) has a night maximum near and just before midnight, as in the inneroval of Figure 6.12. The average energy of the particles (c) is a maximum between60° and 70° in the morning, in the vicinity of the peak of Hartz and Brice’s outerzone.

It is also interesting to compare the Hartz-and-Brice picture, made 30 years agoand based mainly on ground-based observations, with the new satellite-basedresults in Figure 6.13. The inner zone is the luminous intensity recorded by a



Figure 6.13.

Comparison between theinner and outer precipi-tation zones. The auroralimages were taken by theVIS camera on boardPOLAR, 7 May 1996and 13 May 1996 (datacourtesy of L. A. Frank,University of Iowa,USA). The radiation-belt data were obtainedby the HILT electrondetector on boardSAMPEX (data courtesyof B. Klecker, Max-Planck-Institut für extra-terrestrische Physik,Garching bei München).Figure provided by T. I.Pulkkinen (FinnishMeteorologicalInstitute).

camera on the POLAR satellite (averaged over 1 h), and the outer one is a com-posite of fluxes of 1 MeV electrons taken during 15 orbits of the SAMPEX sat-ellite over the course of one day. Quiet (Ap4) and more active (Ap14)conditions are represented. On 7 May the oval is contracted and the outer zoneinactive; on 13 May the oval is expanded and the outer zone intense. The plots donot confirm the morning maximum of the Hartz-and-Brice picture, but this maybe because SAMPEX samples at only two local times.

Recall that the distribution of occurrence of scintillation (Section 5.3.3) alsoexhibits a latitudinal separation from the edge of the precipitation zone that is con-siderably greater by day than it is by night. This would seem to identify the regionof F-region irregularity with the auroral oval rather than with the outer zone.

6.4 The substorm

6.4.1 History

As early as 1837, auroral observers had noted that during a single night therewere times when the aurora was at its most intense, the activity being weakerduring the periods in between (Stern, 1996). The same was true of the relatedmagnetic signature, and it was Birkeland (1908) who first studied this tendency inmagnetic records and identified what he called the “elementary polar magneticstorm.” However, Birkeland’s work in this area, which also involved field-alignedcurrents, fell into disfavor, and the topic made no further progress until the early1960s. It was then that Akasofu and Chapman (1961), in a study of the polar dis-turbance (DP) field, coined the term “DP substorm” for the short periods ofenhanced magnetic disturbance that Birkeland had noted more than 50 yearsbefore. Shortly thereafter, Akasofu noted that these events were often accompa-nied by bursts of auroral activity, which (at Chapman’s insistence, it is said) werenamed “auroral substorms” (Akasofu, 1970). Akasofu subsequently introducedthe term “magnetospheric substorm” to indicate the generality of the phenome-non and to make it clear that, although the consequences of the substorm aremost apparent in the polar regions, its cause lies in the magnetosphere (Rostokeret al., 1980).

6.4.2 The substorm in the aurora

The essence of a substorm as it affects the auroral regions is best described byAkasofu’s analysis of the “auroral substorm” which he developed in the 1960s(Akasofu, 1968; 1977). Akasofu used all-sky camera pictures of aurorae recordedduring the International Geophysical Year (1956–1958), and applied them toderive a convincing description on the global scale of the typical behavior of theluminous aurora during a substorm.

The aurora tends to be active for about an hour at a time, with quiet periods of


2–3 h between, and there is also a dynamic aspect. Akasofu’s representation isillustrated in Figure 6.14. The sequence begins as a quiet arc brightens and movespoleward, forming a bulge. If several arcs are present, it is often the equatorwardone which brightens. Active auroral forms then appear in the bulge, equatorwardof the original arc. This is called break-up or the expansion phase, and the instantwhen it begins is usually called the onset. Near midnight the oval is now broaderthan before, while the polar cap contained within the oval is smaller. At the sametime active auroral patches move eastward towards the morning sector and otherforms travel westward towards the evening. The westward movement is called thewestward-traveling surge. After 30 min to 1 h the night sector recovers and the sub-storm as a whole dies away (the recovery phase). The sequence is likely to berepeated 2–3 h later. By defining a repeating pattern in auroral behavior it was thisanalysis which really established the substorm as the central concept in studies ofthe auroral phenomena.

The period before the break-up is now recognized as a growth phase, which isnot so spectacular in the aurora but which was first studied in the magnetotail(Section 2.2.6), which becomes gradually more tail-like for tens of minutes to an


Figure 6.14. The sub-storm in the luminousaurora: (a) T0; (b) T0–5 min; (c) T5–10min; (d) T10–30 min;(e) T30 min–1 h; and(f) T1–2 h. (S.-I.Akasofu, Polar andMagnetosphericSubstorms. Reidel, 1968,with kind permissionfrom Kluwer AcademicPublishers.)

(a) (b)

(c) (d)

(e) (f)

hour before the onset. During the growth phase the arcs of the auroral oval moveequatorward and the area of the polar cap contained within the oval grows larger.The equatorward motion of the oval during the growth phase is typically severalhundred m s1 (Elphinstone et al., 1991). Arcs form again during the recoveryphase, and these also drift equatorward.

Some brightenings of the aurora do not develop into full substorms. Theyremain limited to a few hundred kilometers (Akasofu, 1964), and are relativelyshort-lived. Such events are called pseudo-breakups. The distinction between sub-storms and pseudo-breakups has been discussed by Pulkkinen (1996).

Satellite observations using downward-pointing photometers have confirmedthis general picture. Figure 6.15 shows an example of a substorm breakupobserved from a DMSP satellite. The auroral satellites (such as DMSP, Viking,Akebono, and POLAR) have also added much detail to the original concepts,both of the auroral oval and of the substorm – and, as so often happens, the topic


Figure 6.15. An aurora observed from space by a DMSP satellite at the maximum of a sub-storm, 9 January 1973 at 2024: (a) a photograph, and (b) interpretation over a map includ-ing the magnetic latitude. (S.-I. Akasofu, Space Sci. Rev. 16, 617, 1974, with kindpermission from Kluwer Academic Publishers.)

turns out to be more complicated than had originally been thought! For example,it now appears that the westward-traveling surge is made up of a number of local-ized brightenings or surges that do not move far as individuals. Each surge lastsfor just a few minutes, and then a new surge appears to its west. Thus the auroraas a whole does indeed move westward toward the evening sector, but it goes in aseries of jumps.

Murphree et al. (1991) have summarized the following details of the opticalsubstorm observed by the VIKING satellite.

(1) The latitudinal width of the auroral activity does not vary systematicallyduring the growth phase.

(2) During this phase the motion of the equatorward boundary of the diffuseaurora is generally equatorward with a speed less than a fewhundred m s1.

(3) The expansion phase is preceded by auroral intensifications lasting up toseveral hours of local time, which fade shortly before onset.

(4) The onset region is very localized, being less than 500 km across in theionosphere.

(5) Auroral observations under moderately active conditions indicate thatauroral emissions can extend several degrees of latitude poleward of thelocation of the onset. This suggests that the onset region can be well awayfrom the boundary between open and closed field-lines.

(6) When the position of the onset is mapped along the geomagnetic field tothe equatorial plane, it is consistent with the location of the inner boun-dary of energetic particle flux, the so-called “injection boundary,”observed in other studies.

In Section 6.3.5 it was pointed out that the auroral phenomena occupy not one“zone” but two. Both zones exhibit substorm behavior, and Figure 6.16 shows anoverall picture illustrating how the substorm develops in each zone, as representedby the fluxes of softer (5 keV) and harder (50 keV) electrons, respectively. The ovaland the outer zone are obviously related to each other in some way. The mostlikely mechanism is that, when the auroral oval is active in a substorm, the outerzone becomes populated with energetic particles that drift in longitude and aresubsequently precipitated. However, not all the physical connections between thetwo zones have been explained fully.

6.4.3 Ionospheric aspects of the substorm

The enhanced precipitation of energetic electrons during a substorm increases therate of ionization of the ionosphere, and of the lower ionosphere in particular, byan amount depending on the particle flux, and over a range of altitudes deter-mined by the particle energies (Figure 2.26). Consequently, the substorm behav-


ior observed in the luminous aurora carries over into the various ionosphericeffects (Section 6.3.4). The E-region reflections called radar aurora, radiowaveabsorption in the D region, X-ray generation, and the occurrence of magneticbays are all, therefore, substorm phenomena. Some of these will be described inmore detail in Section 6.5 and Chapter 7.

6.4.4 Substorm currents

Figure 6.17 shows one of the earlier descriptions of the current flowing during anindividual substorm. This is still an equivalent-current system, because it assumesthat the current flows only horizontally. Note that the intensity is relatively greateron the morning side of midnight. It will be seen that Figure 6.17 is considerablydifferent from Figure 2.23, which showed the auroral electrojets converging on theHarang discontinuity at midnight. The patterns of Figures 6.17 and 2.23 arerelated rather as is the auroral oval to the auroral zone.

However, the acceptance of field-aligned, or Birkeland, currents (Section 2.3.6)has fundamentally changed the approach to current modeling, because currents


Figure 6.16. The typical development of electron precipitation in a substorm. Note that thetwo zones are distinct on the day side. (S.-I. Akasofu, Polar and Magnetospheric Substorms.Reidel, 1968, with kind permission from Kluwer Academic Publishers.)

60°

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T = 0–5 minElectrons

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T = 10–30 min T = 30–1 hr

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within the magnetosphere as well as currents flowing between the magnetosphereand the ionosphere may be included in the circuit. Despite general agreement onthis point, the form of the current system during a substorm is still a topic of inves-tigation.

One influential concept in present-day modeling of substorm currents is that ofthe current wedge. As we shall see, the magnetotail collapses in a limited regionwhen a substorm begins, and the cross-tail current from that region becomesdiverted along field-lines (as Birkeland currents) into the ionosphere. There thecircuit is completed in the E region, probably by an electrojet flowing along an arcor through some other form whose conductivity is enhanced by particle precipi-tation. Figure 6.18(a) shows the magnetospheric part of the circuit, and Figure6.18(b) the “substorm electrojet” in the ionosphere. The electrojet, which flows


Figure 6.17. The equivalent current system of a magnetic substorm. The concentrations ofcurrent lines in the early morning and near 1800 LT would appear as electrojets. (S.-I.Akasofu and S. Chapman, Solar–Terrestrial Physics. By permission of Oxford UniversityPress, 1972.)

6

0

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westward in the midnight sector, connects to an upward field-aligned current atits western end, which also concides with a bright auroral feature.

However, this is only part of the picture. The local collapse in the magnetotailat the onset of a substorm accelerates particles towards the Earth, and somebecome trapped to form a partial ring current (Section 2.3.5) that is completed byBirkeland currents to the ionosphere and currents within the ionosphere. Theseare driven, at least in part, by the electric field due to the general polar convection(Sections 2.4.1 and 2.4.3), which is likely to be enhanced during substorm activ-ity. Various suggestions about the form and relationship of these currents flowingduring a substorm have been put forward. Figure 6.19 indicates one possibility;Figure 6.19(b) shows the “convection electrojet” in the ionosphere.

Kamide (1996) has pointed out that, whereas the enhanced conductivity of theionosphere is the main factor controling the westward electrojet near midnight, theeastward electrojet which flows in the late evening sector, before the Harang discon-tinuity, is dominated by a northward electric field (in the northern hemisphere). Anelectric field, this time southward, also dominates the situation in the westwardelectrojet later in the morning (Figure 6.20). These are the fields generated byplasma convection. The components due to the wedge of current and convection donot vary in the same way during a substorm, however – their typical time constantsare 15 min and 2 h, respectively (Rostoker, 1991) – so that the total behavior cannotbe expected to be simple, or even the same in all cases. The total electrojet shouldbe a combination of Figures 18(b) and 19(b), but in varying amounts.

Although the electrojet is usually conceived in terms of a single wedge ofcurrent, more recent studies (Rostoker, 1991) have shown that it is composed of asequence of short bursts (lasting about 12 min) of westward current (christenedwedgelets!), following one after the other and often appearing sequentially furtherto the west. Thus there is a gradual westward progression of the current, as thereis of the luminosity (Section 6.4.2).


Electrojet FieldAlignedCurrents

NearEarthNeutralLine

TailAxis

TailCurrent

N

S

substorm electrojets

Figure 6.18. (a) The substorm current wedge due to the diversion of tail current to the ion-osphere. (Y. Kamide, Report ESA SP-389, 1996, after McPherron et al., 1973.) (b) The sub-storm electrojet in the auroral zone. (G. Rostoker, in Magnetospheric Substorms, copyrightby the American Geophysical Union, 1991.)

(a) (b)

6.4.5 The substorm in the magnetosphere

If the substorm is the unit of auroral activity, then it is important to discover thedetails of the phenomena through which the substorm is revealed, and thisincludes its appearence in the magnetosphere. Moreover, if we are to predict whenthe high-latitude ionosphere is likely to be affected by substorms we have to under-stand the essential nature of the substorm and the factors that make it happen;these factors concern the dynamics of the magnetosphere and its interaction withthe solar wind. It is another topic that is still being actively researched, bothexperimentally and theoretically. Although the final story has not yet emerged,there are some aspects that seem well established.


Figure 6.19. (a) Magnetospheric currents showing the ring current and associatedBirkeland currents. (Y. I. Feldstein, in Magnetospheric Substorms, copyright by theAmerican Geophysical Union, 1991.) (b) The substorm electrojet in the auroral zone. (G.Rostoker, in Magnetospheric Substorms, copyright by the American Geophysical Union,1991.)

TO SUN

DUSK DAWN

convection electrojets

Figure 6.20. Regions of the electrojet dominated by conductivity and by electric fields. (Y.Kamide, in Auroral Physics, Cambridge University Press, 1991, p. 385.)

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Field-line circulation

The circulation of the magnetosphere, discussed in Section 2.4, is mainly drivenby magnetic merging on the sunward side of the magnetopause, but its continuitydepends on reconnection in the plasmasheet which lies along the central plane ofthe tail. If we select a field-line at high latitude on the day side of the Earth andfollow its progress, we find that it goes through a sequence:

(1) connecting with the IMF which divides it in two;

(2) convecting over the north and south poles as two separate halves;

(3) reconnecting in the tail; and

(4) returning to a more dipolar form and returning to the day side.

In a steady state these stages would be in balance. Substorms occur becauseneither the dayside connection nor the nightside merging are continuous pro-cesses. Thus, energy accumulates in the tail and the substorm marks its suddenrelease.

The Interplanetary Magnetic Field (IMF) reaching the Earth lies principally inthe ecliptic plane, but it generally has some northward or southward componentas well, and it couples most strongly with the geomagnetic field when that com-ponent is southward (Section 2.4.2). When the IMF turns from northward tosouthward, the connection rate increases. For a while more open field is producedthan removed, the total magnetic flux in the polar cap increases, the auroral ovalmoves equatorward, and the tail of the magnetosphere grows fatter, representinga store of energy. This energy is released in the substorm, when the reconnectionrate in the tail exceeds the supply of magnetic flux coming from the polar regions,the tail becomes more dipolar, flux is lost from the polar caps and the auroral ovalshrinks again. This sequence of events in the magnetosphere may be identifiedwith the phases of the auroral substorm observed from the ground: the growthphase, the expansion phase, and the recovery phase.

Behavior in the tail

In the magnetosphere the growth phase corresponds to an increase in erosionfrom the front of the magnetosphere, and the plasma sheet and the current sheetbecome thinner (though not necessarily at the same time) as illustrated in Figure6.21(a).

One concept of the expansion phase begins with the formation of a neutral linenearer to the Earth than that which exists during quiet times (Figure 6.21(b)).Here the magnetotail collapses because the magnetic field has gone to zero in alocalized region, and the cross-tail electric current is diverted into the currentwedge as described above. In fact the collapse and the diversion of current mustgo together because of the Biot–Savart law. At the same time, satellites at geosyn-chronous distance observe an increase in the flux of energetic electrons, and thegeomagnetic field becomes more dipolar.


The region of the tail between the two neutral lines forms a plasmoid, which isejected along the magnetotail as the recovery phase begins; this may be detectedby satellites (20–100)RE down the tail as a burst of energetic particles moving awayfrom the Earth. Satellites near the neutral sheet detect a loss of particle flux duringthe expansion phase, indicating that the plasma sheet becomes thinner at thattime.


Figure 6.21. (a) Changes in the magnetosphere during the growth phase of a substorm. (R.L. McPherron et al., J. Geophys. Res. 78, 3131, 1973, copyright by the AmericanGeophysical Union.) (b) The near-Earth neutral line (N3) and the plasmoid formed in asubstorm. N1 is the merging point at the front of the magnetosphere and N2 the mergingregion in the distant tail. (D. P. Stern, Rev. Geophys. 34, 1, 1996, copyright by the AmericanGeophysical Union.)

REDUCTIONENLARGEMENT

THINNING

INWARD MOTION

EROSION

EQUATORWARDMOTION

N1

N3

N2

PLASMOID

(a)

(b)

Quite recently, spacecraft in the magnetotail (AMPTE and GEOTAIL) haveobserved bursty bulk flows (BBFs) in the plasma sheet. Plasma in the sheet gener-ally flows at speeds of less than 100 km s1, but during a BBF, which typically lastsfor about 10 min, the speed exceeds 400 km s1 and the direction of flow isEarthward. BBFs occur at all observed distances beyond 15RE, and appear to beassociated with the occurrence of substorms (Angelopoulos, 1996). If they areobserved a long way down the tail (e.g. at (90–100)RE) the event appears some 90min after the substorm, suggesting that there is a centre of acceleration thatretreats progressively down the tail away from the Earth. When the fast-flowingplasma gets closer to the Earth, it is stopped by the stronger geomagnetic fieldwhich is dipolar in form. Figure 6.22 illustrates the stopping region. Note that themagnetic field is configured with a Y-type neutral line (compare with Figure 2.20).In this treatment the flow stops at the boundary between the tail field and thedipole field, and this region is also the inner edge of the plasma sheet.

Various theories

The exact configuration of the magnetotail during the phases of a substorm hasnot been established fully, and several models – among which the observationshave not yet been able to distinguish – have been put forward. Some modelsinvolve a local reversal of the tail field, and others include multiple neutral linesto correspond to the multiple arcs seen in aurorae. Liu (1992) has summarized thecontending theories as six models.

(a) Formation of a neutral line in the magnetotail at a distance of (10–20)RE,which allows magnetic reconnection between the lobes of the magnetotail.

(b) Generation of the Kelvin–Helmholtz instability (Section 2.5.6) in the mag-


Figure 6.22. A Y-typeneutral point at theEarthward edge of theneutral sheet where thehigh-speed ion flow isstopped by the dipole-like field of the innermagnetosphere. (K.Shiokawa et al.,Geophys. Res. Lett. 24,1179, 1997, copyright bythe AmericanGeophysical Union.)

netospheric boundary layer, by enhanced reconnection at a neutral linesome 100RE distant down the tail.

(c) A “thermal catastrophe” in the plasma sheet, due to the sheet becomingopaque to Alfvén waves and consequent sudden heating.

(d) Intense field-aligned currents due to an increase in the rate of field recon-nection on the day side of the magnetosphere, leading to the “currentwedge” of the substorm and a collapse of field in the magnetotail.

(e) A disruption of the cross-tail current due to a current instabilty.

(f) A “ballooning instability” invoking a transition between field configura-tions that are essentially dipolar and essentially tail-like, which againdiverts the cross-tail current.

These are illustrated in Figure 6.23, but nothing would be gained by going into alltheir details here. The task of making a synthesis of all the various substormobservations and theories has been considered by Elphinstone et al. (1996).

6.4.6 The influence of the IMF and the question ofsubstorm triggering

The magnetic power of the solar wind

It is clear that the term “substorm” includes a considerable range of phenomena,but the central idea is of a sudden and sporadic episode in the magnetosphere inwhich a large amount of stored energy is released. The energy comes initially fromthe solar wind, and important factors in the occurrence of substorms, therefore,are the energy flux of the solar wind and the efficiency with which the energy iscoupled into the magnetosphere. It is found that the index AE, which indicates thelevel of geomagnetic activity in the northern auroral zone, is well correlated to aquantity

vB2sin4(/2) l 02, (6.2)

where v is the solar wind speed, B is the magnitude of the IMF, l0 is a length relatedto the cross-section of the magnetosphere (7RE), and is the “clock angle” of theIMF seen from the Earth (as defined in Section 5.1.2.).

The magnetic energy reaching the magnetosphere per unit time is proportionalto vB2l0

2: this is the “magnetic power” of the solar wind. The expression sin4(/2)is intended to represent the fraction of this power coupled into the magneto-sphere. Its form gives a gradual transition between full coupling when the IMF isfully southward (sin4(/2)1) and zero coupling when it is fully northward(sin4(/2)0). If BzBy, /245°, and the coupling factor is 0.25. Some otherexpressions based on different combinations of solar-wind parameters also corre-late to the occurrence of substorms, though ) is perhaps the best (Figure 6.24).


The influence of Bz on triggering

Although much is known about the sequence of events in a substorm, it is notclear what causes the event to begin. Clearly, a store of energy must have built upawaiting release, but then we still have to ask whether the substorm is triggered bysome other identifiable event, for instance in the solar wind, or whether it mightbe a spontaneous phenomenon without apparent cause. This point is clearly avital element in any substorm theory, and (at the time of writing) it is not yetsettled.


Figure 6.23. A selection of substorm-triggering ideas. (A. T. Y. Lui, in MagnetosphericSubstorms, copyright by the American Geophysical Union, 1991.)

Some things can be said, however. That substorms occur most frequently whenBz is southward has been known for many years, and it is found that the begin-ning of the substorm often coincides with a southward turning of the IMF.However, there are also cases when the substorm begins as the IMF turns north-ward, having previously been southward for an hour or two. In such a case itappears that a southward IMF puts energy into the magnetosphere and then theshock of the northward turning in some way triggers its release. Indeed, in manycases the growth phase begins as the IMF turns south. It is possible that there maybe more than one kind of trigger.

The substorm rate

If the speed of the solar wind is 440 km s1 (Figure 2.2(a)), the substorm rate is inthe range 800–1500 per annum, or about 2–4 a day on the average (Borovsky etal., 1993; N. Flowers, private communication). Also, the rate of occurrence of sub-storms increases with the speed of the solar wind. Observations of substorms inauroral absorption give a median rate of 3.8 per day at 450 km s1 increasing to7.2 per day at 700 km s1, approximately a v2

sw dependence (Hargreaves, 1996).

6.4.7 Relations between the storm and the substorm

Storms and substorms are defined in different ways, the former mainly from mag-netic observations at low latitude where the greatest influence is the ring current,


Figure 6.24. Correlation between the AE index of magnetic activity and the parameter during a storm in July 1974. (Reprinted from S.-I. Akasofu, Planet. Space Sci. 27, 425,copyright 1979, with permission from Elsevier Science.)

and the latter from observations at a higher latitude where the greatest contribu-tion comes from the auroral electrojet. Their effects at the ground are usually rep-resented by the magnetic indices Dst and AE (Sections 2.5.2 and 2.5.4). It is wellknown that during a significant storm there will almost certainly be one or moresubstorms. On the other hand, substorms may very well occur when there is nostorm.

Because it is the more frequent occurrence, the substorm has usually beenregarded as the fundamental element, which, it is commonly supposed, leads toan increase in the population of trapped particles in which an increased ringcurrent may then flow. This view is supported by direct observations in the mag-netotail, in which a significant difference between those substorms occurringduring a storm and those occurring at other times is found (Baumjohann, 1996).In a “storm-time substorm” the magnetic field moves from tail-like to dipole formin a matter of 15–30 min, whereas in “non-storm substorms” the change is bothslower and less complete. There is a greater reduction of magnetic pressure in thestorm-time substorm and the ion temperature in the tail is larger throughout.These results suggest that there are two kinds of substorm (perhaps arising atdifferent distances down the tail), one of which is the more effective at populatingthe ring current (and thus promoting the signature of the classical magneticstorm).

There are also contrary arguments, notably from studies showing that the ringcurrent is as likely to grow before the auroral activity begins as it is to follow it.Furthermore, there is a good correlation between the magnitude of the ringcurrent and the electric field across the magnetosphere, showing that the solarwind affects the magnetic storm directly, as well as indirectly via substorm activ-ity (Clauer and McPherron, 1980).

Plainly, the nature of the storm–substorm relationship is not yet fully under-stood.

6.5 The E region at high latitude

6.5.1 Introduction

At middle latitude the E region is easily the most boring part of the ionosphere. Itbehaves as an -Chapman layer (Section 1.3.3) and supports the (Sq) current gen-erated by atmospheric tides. Sporadic-E (Section 1.4.2) adds some interest, butbeyond that there is little more to be said. The same is true at high latitude whilegeophysical conditions are quiet, but, when the Sun is active, the high-latitude Eregion becomes arguably one of the most exciting parts of the ionosphere. It thendiffers markedly from the mid-latitude and equatorial E regions – in terms of ion-ization sources, plasma processes, and radio-propagation characteristics. It isoften the case that precipitating particles are the dominant source of ionization.


When thus enhanced the E region supports the auroral electrojet, in whichinstabilities may arise. Ionograms exhibit sporadic-E of the auroral variety.

6.5.2 The polar E layer

The most benign part of the high-latitude E layer is over the polar cap – that is,poleward of the auroral zone. Here it is essentially under solar control; it varieswith the solar zenith angle and exhibits strong seasonal effects, as does the mid-latitude E region.

6.5.3 The auroral E layer under quiet conditions

When Kp is small the auroral oval retreats poleward (see Figure 6.7), and, underquiet conditions, there is relatively little disturbance by precipitating particles inthe nominal auroral zone, from, say, 60° to 70° magnetic latitude. The normalionospheric layers occur much as at middle latitude and are subject to the samediurnal, seasonal, and sunspot-cycle variations. Figure 6.25 shows typicalelectron-density profiles from the Chatanika incoherent-scatter radar (ISR) com-pared with ionograms from College, Alaska, sites both near 65° magnetic latitude.

These results are for magnetically quiet conditions near sunspot minimum.Because of its high latitude the site was illuminated by the Sun throughout the day.Thus, even the ionogram at 0215 LT shows a strong E layer that masks the Fregion, giving the G condition (Section 5.2.1). Observing by ionosonde alonewould suggest, falsely, that the F layer was absent. Note that TeTi in the Fregion, which is usual.

6.5.4 The disturbed auroral E layer

The main disturbances affecting the auroral E layer are geomagnetic storms andsubstorms (Section 6.4). With increasing activity, and particularly if Kp3, theauroral oval expands both poleward and equatorward, and the auroral structureand motion become much more dynamic.

The precipitating electrons of energy 1–10 keV which create the visual auroraalso create the auroral-E ionization. As pointed out in Section 2.6, fast electrons(and protons) entering the atmosphere produce one ion pair (an ion plus an elec-tron) for each 36 eV of energy lost, most of which is deposited towards the end ofthe path. Since the average ionization potential of the ionospheric atoms andmolecules is about 15 eV, approximately 40% of the energy goes into ionizationand 60% goes into the motion of the electron produced, which subsequently ther-malizes. In the E region the neutral air is dense relative to the higher levels, andthe recombination between electrons and ions proceeds rapidly.

Altitude profiles of the rate of ionization due to a flux of 108 electrons cm2 s1

at several initial values of energy Ep (keV) precipitating along geomagnetic field


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Ele

ctro

n D

ensi

ty (

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Te

Ti

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Height (km)

1211

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219

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2333

— 2

247

Kp

: 2—

lines into the auroral ionosphere are shown in Figure 6.26. It is instructive tocompare these ionization profiles with luminosity profiles (for example Figure6.8), as a general indication of the energies of the particles which cause the auroralluminosity and the disturbed E region.

The regions of enhanced electron density are also regions of high conductivityand this is where the auroral electrojet flows, its magnitude increasing with theauroral luminosity. The form of these current systems is discussed in Sections 2.5.3and 6.4.4. Plasma waves generated in the electrojet produce the various types ofradar-backscatter signature discussed in Section 6.5.5.

The relation between the visual aurora and auroral-E ionization has beenstudied in a semi-quantitative fashion by Hunsucker (1975). Figure 6.27 showssimultaneous all-sky camera, ionosonde, and incoherent-scatter radar data duringthe passage of an auroral arc through the fields of view of the instruments. The E-region electron density is greatly increased within the arc.

The rapid changes of electron density that may be observed by incoherent-scatter-radar over an hour are illustrated in Figure 6.28. Note that, during theelectron-density spike, the ionogram shows intense sporadic-E. The enhancementof the E region may be surprisingly large during major disturbances. During the


Figure 6.26. Profiles of the rate of ionization due to monoenergetic fluxes of 108 electronscm2 s1, incident from above, for the energy range (2–100 keV) having greatest effect in theE (and D) regions. (M. H. Rees, Physics and Chemistry of the Upper Atmosphere,Cambridge University Press, 1989.)

great magnetic storm of August 1972 the E-region electron density exceeded1.8106 cm3 (Figure 6.29), one of the higher values of electron density observedin the E region.

The morphology, structure, and dynamics of the auroral-E layer have beendescribed in some detail by Hunsucker (1975) and others referred to therein.

6.5.5 Auroral radar

Most of our knowledge of irregularities in the auroral E layer is based on dataobtained by direct backscatter from field-aligned irregularities within the auroral


Figure 6.27. E-region electron density profiles and ionograms associated with the passageof an auroral arc across the field of view, 2 March 1973. The incoherent-scatter radar wasat Chatanika, Alaska, and the all-sky camera and ionosonde were at College nearby. Theradar beam was just south of the arc in (a) but in the centre of the arc in (b). The maximumelectron density and the penetration frequency of the E layer were greatly increased withinthe arc. It was late evening. (R. D. Hunsucker, Radio Sci. 10, 277, 1975, copyright by theAmerican Geophysical Union.)

(a)

(b)

oval using radars operating in the VHF/UHF frequency range. The term radioaurora is often used for the phenomena so observed, and radar aurora is equallyvalid. There is now a voluminous literature on the subject.

There is an enormous difference between the scattering cross-sections forcoherent and incoherent radar, the former being the stronger by 50–80 dB.Backscatter from auroral E-layer irregularities has been classified into four groupsin terms of their line-of-sight Doppler velocities, as shown in Figure 6.30, whichalso summarizes their essential properties.


Figure 6.28. The variation of electron density at three heights, showing a sharp spike in theelectron density at 0800 UT (2200 LT) on 2 March 1973. At the same time the ionosonderegistered sporadic-E above 7 MHz. Observations from Alaska, using the Chatanika ISRand the College ionosonde. (H. F. Bates and R. D. Hunsucker, Radio Sci. 9, 455, 1974,copyright by the American Geophysical Union.)

Theory

The two most generally accepted theories to explain the observations are the two-stream plasma instability and the gradient-drift mechanism. These plasma instabil-ities are generated in the auroral electrojet and produce electrostatic ion waves thatmay scatter incident radio waves as discussed in Section 3.5.1. A necessary condi-tion for the occurrence of these instabilities is a sufficiently large relative velocitybetween the electrons drifting in an electric field and the ions whose motion isdominated by collisions. The waves travel nearly perpendicular to the geomagneticfield lines. The latter property implies that the backscatter cross-section ismaximum when the radar is pointing almost perpendicular to the field line,although there have been several instances of auroral backscatter occurring atlarge aspect angles.

Other physical mechanisms for producing the auroral irregularities have alsobeen proposed. A critical review of plasma irregularities in the auroral electrojet,covering both theory and experiment, has been given by Sahr and Fejer (1996).

Polarization

Investigations of the polarization of backscatter from auroral E-region irregular-ities have concluded that coherent scatter of spectral classes 1, 2, and 3 has similarpolarization characteristics. For most of the observations, the scattering of a lin-early polarized incident wave produced an essentially linear and highly polarizedscattered wave, implying that there was a small scattering volume and/or a smallnumber of discrete scatterers located close to one another. This also confirms thatthe scattering process is a weak coherent one. There were, however, some signifi-cant exceptions.

Observing geometry and occurrence

The region which may be observed is restricted by the aspect sensitivity. As Figure6.31 illustrates, the radar must be situated at middle latitude if the beam is to inter-sect field-aligned irregularities in the auroral zone.


Figure 6.29. An electron-density profile of the auroral E region during a great magneticstorm in August 1972. (R. D. Hunsucker, Radio Sci. 10, 277, 1975, copyright by theAmerican Geophysical Union.)

Broadly speaking, the diurnal and seasonal occurrence of the radio aurora issimilar to that of the visual aurora, except, of course, during daylight when thevisual aurora cannot be seen. The strongest echoes occur near 65° latitude, andduring disturbances the echoing region extends equatorward. The echoes can bedetected at any time of day, but are most pronounced near local magnetic midnight.

The phenomenon of E-region irregularities is closely related to the behavior ofthe auroral electrojet. Sahr and Fejer (1996) draw attention to the problem ofmodeling them globally, to their importance in radio propagation, and to gaps inmethods of data analysis.


Figure 6.30. Four types of echo in auroral radar. The observations were with a 50-MHzradar in March 1989, and each analysis was based on 20 s of data. Cs is the local speed ofsound. (J. D. Sahr and B. G. Fejer, J. Geophys. Res. 101, 26893, 1996, copyright by theAmerican Geophysical Union.)

6.5.6 Auroral infrasonic waves

Coming within the acoustic-wave domain of atmospheric waves (Section 1.6),auroral infrasonic waves (AIWs) are an interesting, though not very well known,feature in the spectrum of high-latitude phenomena (Wilson, 1969). They origi-nate in the supersonic horizontal motion of the large-scale electrojets that flowwithin auroral arcs. The motion produces an infrasonic “bow wave,” whose prop-agation is highly anisotropic and which reaches the Earth’s surface about 6 minafter the zenith passage of the arc. AIWs can sometimes be detected at the groundas much as 1000 km from the source. The period will be in the range 10–100 s – afrequency range of 0.01–0.1 Hz – with maximum spectral power density at aperiod of about 70 s, the pressure wave having amplitude from 0.5 to 20 bars.They are detected with microphone arrays and are found to be highly coherentacross arrays whose sensors are spaced by up to 6 km. The speed at which an AIWcrosses the array will be between about 300 and 1000 m s1, the average beingabout 500 m s1 (Wilson et al., 1976). An example observed with a microphonearray at Fairbanks, Alaska, is shown in Figure 6.32.

AIWs occur on the night side of the Earth, having a diurnal occurrencemaximum near local midnight and a seasonal maximum around the equinoxes.Episodes of AIW activity, AIW substorms, are highly correlated to the occurrenceof negative magnetic bays (Section 2.5.3). The horizontal component of the AIW’svelocity is parallel to the motion of the supersonic auroral arc and also parallel tothe horizontal magnetic-disturbance vector associated with the westward electro-jet within the arc.

It is an interesting fact that AIWs are generated only by arcs moving equator-ward; those moving poleward have no such effect. It has also been noted (Wilsonand Hargreaves, 1974) that, statistically speaking, their direction of movement issimilar to that of peaks in auroral radio absorption (Section 7.2.4) at a similar lat-


Figure 6.31. The geometry of coherent radar-scatter at high latitude. For significant scat-tering the beam must be normal to the field-aligned irregularites to within about 2°. In theabsence of refraction (at VHF and UHF) the range will be between 400 and 1200 km. (J. D.Sahr and B. G. Fejer, J. Geophys. Res. 101, 26893, 1996, copyright by the AmericanGeophysical Union.)

itude, having, in addition to the equatorward motion, a westward componentbefore midnight and an eastward one after midnight.

AIWs should be considered as part of the total energy budget of terrestrialauroral phenomena and as a sensitive sensor of dynamic auroral arcs.

6.5.7 The generation of acoustic gravity waves

Another consequence of the electron-precipitation events, which create large elec-tron densities supporting electrojets, is that the auroral E region is a source ofacoustic-gravity waves (AGWs). Strictly, there are two mechanisms. One is intenseJoule heating (J•E), where J is the current density and E is the electric field, whichoccurs in localized regions. The other is the Lorenz force (JB), where B is theflux density of the geomagnetic field. AGWs were introduced in Section 1.6 and ithas been shown that those in the “large-scale” category originate in the auroralregions, probably from one of these sources. In the ionosphere the AGW is recog-nized as a traveling ionospheric disturbance (TID), which propagates in the Fregion, primarily equatorward, for distances that may exceed 10000 km.


Figure 6.32. An auroral infrasonic wave observed with a detector array at Fairbanks,Alaska. The upper panel shows the power spectrum of the event, and the lower one showsthe waveform. The wave traveled at 502 m s1, arriving from azimuth 27.6°. (R. D.Hunsucker, private communication.)

In an investigation of TIDs in electron content at L4, most of which were inthe “medium-scale” range with period 20–60 min, Hunsucker and Hargreaves(1988) noted that the waves were present almost continuously during daylighthours at the level of 1–4% of the electron content. Although no specifc source wasidentified, it must be significant that the incidence of these waves was far greaterat L4 than it is at middle latitudes.

Some of the energy deposited in the auroral ionosphere from the magneto-sphere may be transported to other latitudes by the action of AGWs, as well as byneutral-air winds and tides. It is estimated that 5–10% of this redistribution is dueto AGWs.


Except for the very large seasonal variability, the polar E region is relativelybenign, compared with the auroral region. Precipitation of 1–10 keV electronsalong geomagnetic field lines through the magnetospheric plasma sheet into theauroral ionosphere produces several very important effects: the luminous aurora,anomalously high E-region electron densities (conductivity), and localizedregions of intense Joule heating and Lorenz forces. These phenomena are orga-nized by the geomagnetic field into the northern and southern auroral ovals,which are stationary in space in Sun–Earth coordinates, with the Earth rotatingunderneath. The ovals are centered at approximately 67° geomagnetic latitudenear magnetic midnight and 77° geomagnetic latitude near geomagnetic noonunder “quiet conditions,” and the latitudinal “thickness” of the oval increases asKp increases. The most used auroral oval models are those derived from visualauroral observations, which give a reasonable estimate of auroral-E ionization forKp values up through Kp7. Other ovals based on the TIROS and DMSP satel-lite particle measurements, which may give a more accurate mapping both of theelectron precipitation which produces the auroral-E ionization and of the particleprecipitation which produces D-region absorption and F-region irregularities, arealso available.

The altitude profiles of auroral luminosity and of electron density in the Eregion are almost identical in shape. Electron densities as high as 4.4106 elec-trons cm3 have been measured by the Chatanika ISR during a large geomagneticstorm, and densities from 5105 to 106 electrons cm3 are quite commonaround magnetic midnight near 65° north geomagnetic latitude (Fairbanks,Alaska). The College (Fairbanks) ionosonde has also measured an auroral-E topfrequency of 13 MHz, which corresponds to an oblique frequency of 57 MHz ona 1000-km long, curved-Earth-limited one-hop propagation path. There is somequestion, however, regarding whether the auroral-E top frequency measured byan ionosonde is really a true plasma frequency.

The temporal and spatial behavior of the auroral-E layer is dynamic and isprobably best demonstrated by observing the visual aurora and realizing that the


regions of highest intensity (brightness) are, indeed, regions of high auroral-Eelectron density

The regions of high intensity in the visual aurora are also regions of intensifiedconductivity (hence electric current) of the auroral electrojet in the E region.These enhanced currents can produce intense Joule heating and Lorenz forces,which in turn generate atmospheric gravity waves (AGWs) that couple with theelectron plasma to produce traveling ionospheric disturbances (TIDs). The large-scale TIDs may travel equatorward in the F region for over 10000 km, creatinganomalous propagation in mid-latitude HF systems.

Another effect of auroral-oval E-region dynamics is the generation of auroralinfrasonic waves (AIWs), which occur when an auroral arc travels supersonicallytowards the equator. For certain conditions of electron density, auroral bright-ness, particle-precipitation energies, Mach number, and orientation of the arc withrespect to the geomagnetic field, a “bow shock wave” is formed, launching AIWsthat may then be detected by a suitable array of acoustic sensors on the ground.


6.2 Statistical distribution of the auroraAkasofu, S.-I. (1968) Polar and Magnetospheric Substorms, Reidel, Dordrecht.

Akasofu, S.-I. (1974) The aurora and the magnetosphere; the Chapman memoriallecture. Planet. Space Sci., 22, 885.

Chubb, T. A. and Hicks, G. T. (1970) Observations of the aurora in the far ultravioletfrom OGO 4. J. Geophys. Res. 75, 1290.

Feldstein, Y. I. and Starkov, G. V. (1967) Dynamics of auroral belt and polar geomag-netic disturbances. Planet. Space Sci. 15, 209.

Frank, L. A. and Craven, J. D. (1988) Imaging results from Dynamics Explorer 1. J.Geophys. Res. 26, 246.

Fuller-Rowell, T. J. and Evans, D. S. (1987) Height-integrated Pedersen and Hall con-ductivity patterns inferred from TIROS-NOAA satellite data. J. Geophys. Res. 92, 7606.

Goodman, J. M. (1992) HF Communications – Science and Technology. Van NostrandReinhold, New York.

Gussenhoven, M. S., Hardy, D. A., and Heinemann, N. (1983) Systematics of theequatorward diffuse auroral boundary. J. Geophys. Res. 88, 5692.

Hardy, D. A., Gussenhoven, M. S., and Holeman, E. (1985) A statistical model ofauroral electron precipitation. J. Geophys. Res. 90, 4229.

Hardy, D.A., Gussenhoven, M. S., and Brautigan, D. (1989) A statistical model ofauroral ion precipitation. J. Geophys. Res. 94, 370.

Meng, C.-I. (1984) Dynamic variation of the auroral oval during intense magneticstorms. J. Geophys. Res. 89, 227.

Meng, C.-I. and Makita. K. (1986) Dynamic variations of the polar cap. SolarWind–Magnetosphere Coupling (eds. Kamide and Slavin), p. 605. Terra Scientific, Tokyo.

Vestine, E. H. (1944) The geographic incidence of aurora and magnetic disturbance,Northern Hemisphere. Terr. Magn. Atmos. Electricity 49, 77.


Whalen, J. A. (1970) Auroral oval plotter and nomograph for determining geomag-netic local time, latitude and longitude in the Northern Hemisphere. Report AFCRL-70-0422, Environmental Research Paper 327. (From Defense Technical InformationCenter, Cameron Station, Alexandria, VA 22314, USA)

6.3 The auroral phenomenaBauer, S. J. (1973) Physics of Planetary Ionospheres. Springer-Verlag, Berlin.

Gazey, N. G. J., Smith, P. N., Rijnbeek, R. P., Buchan, M., and Lockwood, M. (1996)The motion of auroral arcs within the convective plasma flow. Third InternationalConference on Substorms, Versailles, France. Report ESA SP-389, p. 11.

Harang, L. (1951) The Aurorae. Wiley, New York.

Hargreaves, J. K. (1969) Auroral absorption of HF radio waves in the ionosphere – areview of results from the first decade of riometry. Proc. Inst. Elect. ElectronicsEngineer 57, 1348.

Hartz, T. R. and Brice, N. M. (1967) The general pattern of auroral particle precipita-tion. Planet. Space Sci. 15, 301.

Rees, M. H. (1989) Physics and Chemistry of the Upper Atmosphere. CambridgeUniversity Press, Cambridge.

Störmer, C. (1955) The Polar Aurora. Oxford University Press, Oxford.

6.4 The substormAkasofu, S.-I. (1964) The development of the auroral substorm. Planet. Space Sci. 12,273.

Akasofu, S.-I. (1968) Polar and Magnetospheric Substorms. Springer-Verlag, New York.

Akasofu, S.-I. (1970) In memoriam Sydney Chapman. Space Sci. Rev. 11, 599.

Akasofu, S.-I. (1974) A study of auroral displays photographed from the DMSP-2 sat-ellite and from the Alaska meridian chain of stations. Space Sci. Rev. 16, 617.

Akasofu, S.-I. (1977) Physics of Magnetospheric Substorms. Reidel, Dordrecht.

Akasofu, S.-I. (1979) Interplanetary energy flux associated with magnetosphericstorms. Planet. Space Sci. 27, 425.

Akasofu, S.-I. and Chapman, S. (1961) The ring current, geomagnetic disturbance andthe Van Allen radiation belts. J. Geophys. Res. 66, 1321.

Akasofu, S.-I. and Chapman, S. (1972) Solar–Terrestrial Physics. Oxford UniversityPress, Oxford.

Angelopoulos, V. (1996) The role of impulsive particle acceleration in magnetotail cir-culation. Third International Conference on Substorms, Versailles, France. Report ESASP-389, p. 17.

Birkeland, K. (1908) The Norwegian Aurora Polaris Expedition 1902–3, vol. 1, section1. H. Aschehoug, Christiana.

Baumjohann, W. (1996) Storm–substorm relationship. Third International Conferenceon Substorms, Versailles, France. Report ESA SP-389, p. 627.

Borovsky, J. E., Nemzek, R. J., and Belian, R. D. (1993) The occurrence rate ofmagnetospheric-substorm onsets. J. Geophys. Res. 98, 3807.

Clauer, C. R. and McPherron, R. L. (1980) The relative importance of the interplane-


tary electric field and magnetospheric substorms on partial current development. J.Geophys. Res. 85, 6747.

Elphinstone, R. D., Murphree, J. S., Cogger, L. L., Hearn, D., and Henderson, M. G.(1991) Observations of changes to the auroral distribution prior to substorm onset.Magnetospheric Substorms (eds. J. R. Kan, T. A. Potemra, S. Kokubun, and T. Iijima),p. 257. American Geophysical Union, Washington DC.

Elphinstone, R. D., Murphree, J. S., and Cogger, L. L. (1996) What is a global auroralsubstorm? Rev. Geophys. 34, 169.

Feldstein, Y. I. (1991) Substorm current systems and auroral dynamics.Magnetospheric Substorms (eds. J. R. Kan, T. A. Potemra, S. Kokubun and T. Iijima),p. 29. American Geophysical Union, Washington DC.

Hargreaves, J. K. (1996) Substorm effects in the D region. Third InternationalConference on Substorms, Versailles, France. Report ESA SP-389, p. 663.

Kamide, Y. (1991) The auroral electrojets: relative importance of ionospheric conduc-tivities and electric fields. Auroral Physics (eds. C.-I. Meng, M. J. Rycroft, and L. A.Frank), p. 385. Cambridge University Press, Cambridge.

Lui, A. T. Y. (1991) Extended consideration of a synthesis model for magnetosphericsubstorms. Magnetospheric Substorms (eds. J. R. Kan, T. A. Potemra, S. Kokubun,and T. Iijima), p. 43. American Geophysical Union, Washington DC.

Lui, A. T. Y. (1992) Magnetospheric substorms. Phys. Fluids B, 4, 2257.

McPherron, R. L., Russell, C. T., and Aubry, M. P. (1973) Satellite studies of magnet-ospheric substorms on August 15, 1968: 9. Phenomenological model for substorms. J.Geophys. Res. 78, 3131.

Murphree, J. S., Elphinstone, R. D., Cogger, L. L., and Hearn, D. (1991) Vikingoptical substorm signatures. Magnetospheric Substorms (eds. J. R. Kan, T. A. Potemra,S. Kokubun, and T. Iijima), p. 241. American Geophysical Union, Washington DC.

Pulkkinen, T. I. (1996) Pseudobreakup or substorm? Third International Conference onSubstorms, Versailles, France. Report ESA SP-389, p. 285.

Rostocker, G. (1991) Some observational constraints for substorm models.Magnetospheric Substorms (eds. J. R. Kan, T. A. Potemra, S. Kokubun, and T. Iijima),p. 61. American Geophysical Union, Washington DC.

Rostoker, G., Akasofu, S.-I., Foster, J., Greenwald, R. A., Kamide, Y., Kawasaki, K.,Liu, A. T. Y,. McPherron, R. L., and Russell, C. T. (1980) Magnetospheric substorms– definitions and signatures. J. Geophys. Res. 85, 1663.

Shiokawa, K., Baumjohann, W., and Haerendel, G. (1997) Braking of high-speedflows in the near-Earth tail. Geophys. Res. Lett. 24, 1179.

Stern, D. P. (1991) The beginning of substorm research. Magnetospheric Substorms(eds. J. R. Kan, T. A. Potemra, S. Kokubun, and T. Iijima), p. 11. AmericanGeophysical Union, Washington DC.

Stern, D. P. (1996) A brief history of magnetospheric physics during the space age.Rev. Geophysics 34, 1.

6.5 The E region at high latitudeBates, H. F. and Hunsucker, R. D. (1974) Quiet and disturbed electron density profilesin the auroral zone ionosphere. Radio Sci. 9, 455.


Hunsucker, R. D. (1975) Chatanika radar investigation of high-latitude E-region ion-ization structure and dynamics. Radio Sci. 10, 277.

Hunsucker, R. D. and Hargreaves, J. K. (1988) A study of gravity waves in ionosphericelectron content at L4. J. Atmos. Terr. Phys. 50, 167.

Rees, M. H. (1989) Physics and Chemistry of the Upper Atmosphere. CambridgeUniversity Press, Cambridge.

Sahr, J. D. and Fejer, B. G. (1996)Auroral electrojet plasma irregularity theory andexperiment: a critical review of present understanding and future directions.J. Geophys. Res. 101, 26 893.

Wilson, C. R. (1969) Auroral infrasonic waves. J. Geophys. Res. 74, 1812.

Wilson, C. R. and Hargreaves, J. K. (1974) The motions of peaks in ionosphericabsorption and infrasonic waves. J. Atmos. Terr. Phys. 36, 1555.

Wilson, C. R., Hunsucker, R. D., and Romick, G. J. (1976) An auroral substorm investi-gation using Chatanika radar and other geophysical sensors. Planet. Space Sci. 24, 1155.

General reading on the aurora and related topics

BooksAkasofu, S.-I. (1968) Polar and Magnetospheric Substorms. Reidel, Dordrecht.

Akasofu, S.-I. (1977) Physics of Magnetospheric Substorms. Reidel, Dordrecht.

Akasofu, S.-I. and Chapman, S. (1972) Solar–Terrestrial Physics. Oxford UniversityPress, Oxford.

Brekke, A. (1997) Physics of the Upper Polar Atmosphere. Wiley, Chichester.

Brekke, A. and Egeland, A. (1983) The Northern Lights – From Mythology to SpaceResearch. Springer-Verlag, Berlin.

Chamberlain, J. W. (1961) Physics of the Aurora and Airglow. Academic Press, New York.

Eather, R. H. (1980) Majestic Lights. American Geophysical Union, Washington, DC.

Kamide, Y. and Baumjohann, W. (1993) Magnetosphere–Ionosphere Coupling.Springer-Verlag, Berlin.

Kan, J. R., Potemra, T. A., Kokubun, S., and Iijima, T. (eds.) (1991) MagnetosphericSubstorms. American Geophysical Union, Washington,DC.

Kennel. C. F. (1995) Convection and Substorms. Oxford University Press, Oxford.

Omholt, A. (1971) The Optical Aurora. Springer-Verlag, Berlin.

Störmer, C. (1955) The Polar Aurora. Oxford University Press, Oxford.

Vallance Jones, A. (1974) Aurora. Reidel, Dordrecht.

Conference reportsAkasofu, S.-I. (ed.) (1980) Dynamics of the Magnetosphere. Reidel, Dordrecht.

McCormac, B. M. (ed.) (1967) Aurora and Airglow. Van Nostrand Reinhold Co., NewYork.

McCormac, B. M. and Omholt, A. (eds.) (1969) Atmospheric Emissions. Van NostrandReinhold Co., New York.

McCormac, B. M. (ed.) (1971) The Radiating Atmosphere. Reidel, Dordrecht.

Meng, C.-I., Rycroft, M. J., and Frank, L. A. (eds.) (1991) Auroral Physics. CambridgeUniversity Press, Cambridge.


Chapter 7

The high-latitude D region

7.1 Introduction

The differences between the E and D regions in middle latitudes hold also at highlatitude. The E region is characterized by relatively simple photochemistry andhigh electrical conductivity, whereas the D region below it has a complex and lesswell-known chemistry, the electric currents and plasma motions being inhibitedby the higher atmospheric pressure. What they have in common at high latitude isthe importance of ionization by energetic particles. Typical spectra include parti-cles with energies such that they are stopped and ionize in both regions, the lowerenergies (for example, electrons of a few kilo-electron volts) affecting the E regionand the higher ones (e.g. electrons with energies of tens of kilo-electron volts) pen-etrating into region D. Figure 7.1 shows electron-density profiles between 65 and110 km due to representative spectra of ionizing electrons incident on the atmos-phere from above. Increasing the characteristic energy of the spectrum lowers thepeak of the layer, increasing the electron density in the D region but reducing it inthe E region.

At middle latitude the D region’s role in radio propagation is a secondary one.The main parameters of HF propagation are determined by the E and the Fregions, and the D region acts mainly as an absorbing layer, reducing the strengthof the signal but seldom preventing communications for any long period. At highlatitude the D region may be much enhanced and then absorption becomes a con-siderable problem. There are two principal phenomena, each peculiar to high lat-itude. The first is auroral radio absorption (AA), which occurs only in the auroralregions and is due to fluxes of energetic electrons precipitated from the magneto-

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sphere sporadically during periods of auroral activity. The second is polar-capabsorption (PCA), which is caused by energetic protons emitted from the Sun,usually at the time of a major solar flare.

These two kinds of phenomenon have rather different properties. The PCA isrelatively infrequent, there being only about one event per month on the averagein a year of high solar activity; many less when the Sun is quiet. However, whenan event does occur, the absorption may be very strong. The absorbing region isrelatively uniform over the whole polar cap, leading to HF black-out over a widearea. Auroral absorption is more common, but it is confined to the auroral zonesand is generally more structured. Though the amount of absorption does not riseto the intensity sometimes seen in PCA, the spatial structure, which is generallynot known in detail, adds to the difficulty of predicting the effects on HF propa-gation.

AA and PCA are discussed in Sections 7.2 and 7.3, respectively. The chapterconcludes with an introduction to a phenomenon that is still not well understood,the polar mesosphere summer echo.

7.2 Auroral radio absorption

7.2.1 Introduction – history and technique

Auroral radio absorption was discovered by Appleton and colleagues (Appletonet al., 1933) during an expedition to Tromsø during the International Polar Year(1932–1933), when it was observed that ionosondes were blacked out duringperiods of auroral and magnetic activity. The earliest studies of the phenomenonwere performed using ionosondes, but this method is not entirely satisfactorybecause all that can be measured is the incidence of black-out, which in any casemight not always be due to absorption; furthermore, ionosondes are not allequally sensitive. The absorption due to proton events, which has different prop-erties, could also have confused the early results since the proton event was not atthat time recognized as a separate entity.

Since the International Geophysical Year of 1957–1958, auroral absorption(AA) has generally been studied with a riometer – and preferably with a group ofriometers covering a range of latitude and/or longitude. Since it uses trans-ionospheric propagation, the riometer does not say at what height the absorptionoccurs, but various studies have left little doubt that most of the absorptiondetected in the auroral regions arises in the D region of the ionosphere and iscaused by energetic electrons arriving from the magnetosphere.

Riometer technique is outlined in Section 4.2.4. We should recall here that theabsorption is not measured directly but requires first the determination of a quiet-day curve (QDC), an estimate of the signal level in the absence of absorption.


Although the idea is simple enough, the accurate derivation of the QDC can bethe most difficult part of absorption measurement by the riometer technique.

Most riometer-based absorption data come from instruments using a simpleantenna, which, therefore, has a wide beam – e.g. 60° between half-power points– projecting onto a region about 100 km across in the D region. Therefore astandard riometer installation has limited spatial resolution. In recent years,however, there has been an increase in narrow-beam work and the use ofimaging riometers. We shall quote results from both wide-beam and narrow-beam instruments.

Since the absorption is strongly frequency-dependent (an inverse square lawin most circumstances), the observing frequency must also be stated. The reduc-tion of absorption with increasing frequency is one factor determining theoptimum observing frequncy. At the lower frequencies the antenna is larger andalso there is more interference from ionospherically propagated signals. Thecompromise has generally led to use of the band 30–50 MHz. When data areobtained at several frequencies, it is usual to reduce them to 30 MHz for com-parison purposes:

A(30 MHz)A( f )(30)2/f 2. (7.1)

7.2.2 Typical auroral-absorption events and their temporaland spatial properties

One notable fact about auroral absorption is its temporal structure, distinguish-ing it from other major varieties of radio absorption, which generally vary moregradually. In the example of Figure 7.2 it is seen that the absorption tends to occurin bursts (or events); these show preferences for certain times of day, and they

340 The high-latitude D region

Figure 7.2. Auroral radio absorption observed with a 30-MHz riometer on 15 October1963 at Byrd Station, Antarctica. The descriptions below the axis refer to the typical behav-ior; the evening minimum was not respected on the day shown! Note the difference of struc-ture between the night and day activity. (After J. K. Hargreaves, Proc. Inst. Electr. Electron.Engineers 57, 1348, 1969, © 1969 IEEE.)

change character between day and night. While there is no general classificationof auroral absorption that covers all events, there are some recognized types thatoccur frequently.

Sharp-onset and spike events at night

Occurring near magnetic midnight (and more before than after) are sharp-onsetevents. Here the event rises in a couple of minutes or less (Figure 7.3). The dura-tion is tens of minutes to an hour. Some of these events appear isolated, but othersare followed by continuing activity lasting for several hours. The continuingactivity tends to be less prominent at the higher latitudes. At some other latitudeswhat appears to be the same event may begin with a more gradual onset. Manysharp onsets, though not all, coincide with the beginning of a substorm.

At the beginning of the event there may be a “spike,” as in the examples ofFigure 7.3, in which case that feature is a spike event. The occurrence of spikeevents at L5.6 is shown in Figure 7.4. At that location, half occurred in a 3-hperiod up to local magnetic midnight. Usually, an individual spike is seen over a


Figure 7.3. Examples of sharp-onset night events. (a) Skibotn (L6.0), 4 November 1975(J. K. Hargreaves et al., J. Geophys. Res. 84, 4225, 1979, copyright by the AmericanGeophysical Union.) (b) Kilpisjärvi (L5.9), 6 October 1994. (Reprinted from J. K.Hargreaves et al., J. Atmos. Solar–Terr. Phys. 59, 872, copyright 1997, with permission fromElsevier Science.). The first is in the form of a traditional riometer chart, with the level pro-portional to the received power. The lower one was reconstructed from digital data and thesignal power is plotted on a scale of decibels. The absorption is reckoned from the markedquiet-day curve.

(a)

(b)

more limited range of latitude (probably less than 200 km) than the onset itself,which in some cases may be tracked over a wide range of L values if some timedifferences are allowed.

Figure 7.5 illustrates the spatial confinement of the spike event determined withan imaging riometer. At L5.9 the typical spike event is elliptical in shape, themajor axis being generally east–west. Typical dimensions are 190 km by 80 km,and the axial ratio is about 2.5 (Hargreaves et al., 1997). The properties of spikeevents at the South Pole (L13) have been found to be remarkably similar in formthough they are generally smaller in magnitude. The spike event lasts for 1–2 minonly, and is dynamic (Section 7.2.4).

The main part of the night event is considerably more widespread than the spike.As an example, Figure 7.6 shows the distribution before, at, and after the intensepeak (2132 UT) in the event of Figure 7.3(b). During the main part of the event(which peaked at about 9 dB at 38.2 MHz) the absorption covers the whole area ata substantial level, although spatial structure is also present. The properties ofthese events have not yet been worked out in full, but they are clearly distinct fromthose of spikes and arcs (see below).


Figure 7.4. The occurrence of spike events (1 dB) at Abisko, 1980–1985. Magnetic mid-night is about 2130 UT. The riometer frequency was 30 MHz. (Reprinted from J. K.Hargreaves et al., J. Atmos. Solar–Terr. Phys. 59, 872, copyright 1997, with permission fromElsevier Science.)


Figure 7.5. Examples of spike events at (a) the South Pole, 22 July 1988 at 2042:50 UT, and(b) Kilpisjärvi, 14 November 1994 at 2015:10 UT showing typical dimensions. A height of90 km is assumed. Contours are of absorption in decibels at 38.2 MHz, and the time reso-lution was 10 s. This South-Pole (L13) event was an exceptionally intense one, but that atKilpisjärvi (L5.9) is more typical for that latitude. ((a) J. K. Hargreaves et al., Radio Sci.26, 925, 1991, copyright by the American Geophysical Union; (b) reprinted from J. K.Hargreaves et al., J. Atmos. Solar–Terr. Phys. 59, 872, copyright 1997, with permission fromElsevier Science.)

175.0

125.0

75.0

25.0

–25.0

–75.0

–125.0

–175.0

N

E

km

–175.0 –125.0 –75.0 –25.0 25.0 75.0 125.0 175.0

km

(b)

0.5

1.01.5

0.5

1.0

0.51.0

1.50.5

0.0

175.0

125.0

75.0

25.0

–25.0

–75.0

–125.0

–175.0

km

–175.0 –125.0 –75.0 –25.0 25.0 75.0 125.0 175.0km

(a)S

W100 km

2

4468 2

22

4

26

Apparently distinct from the spike is another short-duration feature, whichappears for only a short time because it rapidly crosses the field of view in a west-ward direction, namely the westward surge. It extends 75–85 km north–south butits east–west dimension is not known. It may be related to the westward-travelingsurge in the luminous aurora (Section 6.4.2).

Daytime spike events

The larger spike events are never observed in the day sector, but smaller ones havebeen observed by day at high latitude in the northern hemisphere (Stauning andRosenberg, 1996). At Sondrestrom (invariant latitude 73.7º, L13) they havedurations of less than 5 min, with the most probable value 1–2 min. The distribu-tion of magnitude (at 38 MHz) peaks at 0.2–0.3 dB, and most occur between 1200and 1800 local magnetic time with the mode at 1500–1600. Their spatial extent is50–100 km. On present evidence these events are distinct from the larger spikestypical of the night sector both at this site and at lower latitudes.


Figure 7.6. Absorption distributions during the main part of a night event at Kilpisjärvi on6 October 1994. The highest contours are 2.7, 8.2, and 3.1 dB at 2128:20, 2132:20, and2146:30 UT, respectively. The frequency is 38.2 MHz. The maxima are marked x and thecontours at half the maximum are dotted. These are 10-s averages. The event came intoview from the north-west, peaked overhead, and then drifted westward.

Preceeding bays

Starting 1–1.5 h before the onset there may be observed a weak absorption eventlasting 40–60 min (pre-onset absorption or the preceding bay), and there can belittle doubt that this feature is in some way related to the main event which follows.Imaging riometers have identified the form of the preceeding bay as an arc,extended east–west but only 60–100 km wide north–south (Figure 7.7). The wholefeature tends to be weak, and it contains embedded structure. The arc normallyundergoes a slow equatorward drift, and, as we shall see (Section 7.2.4), there arecases in which a sharp-onset event appears to grow from it. It is possible that thisabsorption feature is connected with the luminous auroral arc.

Slowly varying events and pulsations

Then, in the morning sector, between about 0600 and 1200 local magnetic time,there occur slowly varying events (SVAs). These last for an hour or two and are


Figure 7.7. An absorption arc observed at Kilpisjärvi (L5.9) on 11 April 1995, at 1745,1817, and 1826 UT. Each picture is a 1-min average and the dotted lines show the absorp-tion arc defined by absorption half that at the peak. There is marked spatial structure alongthe arc. Each picture is 240 km on the side, and a height of 90 km is assumed. The featurewas visible for an hour, and it drifted equatorward at less than 10 m s1 initially but thenmore rapidly at 130 m s1. At 1845 UT it was followed by a sharp-onset event.

smooth, with little structure (Figure 7.8(a)). Some of these are modulated withquasi-periodic pulsations having a period of several minutes (Figure 7.8b), in therange of Pc4 and Pc5 (Section 2.5.6). The SVA is spatially more widespread thanthe spike and the preceeding bay.

Relativistic electron-precipitation events

As long ago as 1965 it was realized that some of the absorption events affectingradio circuits (particularly in VHF forward-scatter communications) were due toelectrons of unusually high energy and relativistic speed (Bailey, 1968).Relativistic electron precipitation (REP) is a daytime phenomenon, and moreevents are observed at the equinoxes than at the solstices. The events may beintense, and they are geographically widespread according to the reports of the1960s.

Since a riometer does not determine the height of the absorption, it is notimmediately apparent which of the events detected are in the REP category, butsimultaneous incoherent-scatter measurements have shown that, in some cases,the absorption was indeed at unusually low altitude (Collis et al., 1996), and theseare almost certainly due to relativistic electrons. (An electron of energy 100 keVtravels at just over half the speed of light and one of energy 500 keV travels at0.86c. Electrons more energetic than 250 keV penetrate below 67 km and producemaximum ionization at heights below 75 km – Figure 2.26.) In at least some ofthese cases the event is confined to a small area in the D region, less than 100 kmnorth–south though more extended east–west. Figure 7.9 is an example. In this


Figure 7.8. Events in the morning sector: (a) a slowly varying event at Abisko (L5.6), 23March 1985 and (b) an event with pulsations at Andøya (L6.2), 23 August 1985(Reprinted from J. K. Hargreaves and T. Devlin. J. Atmos. Terr. Phys. 52, 193, copyright1990, with permission from Elsevier Science.)

3.02.52.01.51.00.5

00200 0300 0400 0500 0600

Abs

orpt

ion

(dB

)

U.T.

Abisko

Andøya3

2

1

00200 0300 0400 0500 0600

Abs

orpt

ion

(dB

)

U.T.0700

(a)

(b)


Figure 7.9. Properties of a co-rotating daytime event observed at 1204 UT on 1 March1995 by an imaging riometer and the EISCAT radar simultaneously. (a) The spatial struc-ture, assuming an altitude of 90 km. The contours are in decibels and the time resolution is2 min. The radar beam intersected the event at R, somewhat away from the maximum of3.5 dB. (b) The vertical profile of electron density, at 1-min resolution. (c) The verticalprofile of the incremental absorption computed from (b). The electron-density peak below90 km and the absorption peak below 70 km identify this as an event due to electron pre-cipitation of unusually high energy. (P. N. Collis et al., Ann. Geophysicae 14, 1305, 1996,copyright notice of Springer-Verlag.)

175

125

75

25

–25

–75

–125

–175–175 –75 25 125

0.50

0.75

0.50

0.50

0.50R

km

(a)

km

(b)

(c)

0 0.5 1.0 1.5

0 0.05 0.10 0.15

Absorption (dB km–1)

110

100

90

80

70

60

120

110

100

90

80

70

60

50

Alti

tude

(km

)

Alti

tude

(km

)

Electron density (1011 m–3)

event the electron density due to electron precipitation peaked below 90 km, andthe absorption peaked at 67 km. These are unusually low altitudes for an auroralabsorption event (see Section 7.2.6).

It is interesting to note the similarity in the size and shape of some of these typesof event, even when they are seen in different circumstances. This suggests thatthey have a common underlying physical cause.

7.2.3 General statistics in space and time

Latitude and longitude distributions

Two versions of the overall global occurrence of AA are shown in Figure 7.10 withrespect to magnetic latitude and time. Most obvious in the second diagram is themorning peak around 0600–1000 magnetic time, where 1 dB (at 30 MHz) isexceeded for 8% of the time. This does not mean that AA is only a daytime phe-nomenon, however. We have already described some night-time events, and thereis just as much absorption activity in the night as there is in the day sector, a factthat shows up more clearly in the first diagram which plots the median intensityof those events which happen to peak at a given time and latitude. The daytimeevents dominate in the other kind of statistics because they tend to be of longerduration. However, the night events can be just as intense. There is a deepminimum in the pattern of occurrence at around 1600–1700 magnetic time.


Figure 7.10. (a) The median intensity of AA events in decibels at 30 MHz. (Reprinted fromJ. K. Hargreaves and F. C. Cowley, Planet. Space Sci. 15, 1571, copyright 1967, with per-mission from Elsevier Science.) (b) the percentage occurrence of 30-MHz absorptionexceeding 1 dB. (After T. R. Hartz et al., Can. J. Phys. 41, 581, 1963.) The diagrams differbecause the night events are shorter than the day events.

50°

60°

70°

80°

90°

0.5

0.5

1.0

1.0

1.5

1.52.0

12

10

08

06

04

02

00

22

20

18

16

14

50°

60°

70°

80°

90°

12

10

08

06

04

02

00

22

20

18

16

14

4

48

0.5

4

4

GEOMAGNETIC LATITUDE

INVARIANT LATITUDE

GEOMAGNETIC TIME GEOMAGNETIC TIME

(a) (b)

These distributions reveal a zonal phenomenon, having a maximum at about67° geomagnetic latitude (corresponding to L7), though the details vary some-what. Hartz et al. (1963) found the maximum at 67° in Canada, but Holt et al.(1961) found it at 62° in Norway. Using data from Canada and conjugate stationsin the Antarctic, Hargreaves and Cowley (1967a) found small daily variations inthe latitude of the maximum, in particular a decrease of 2° or 3° over the few hoursup to about midnight, a recovery to 67°–68° during the morning, and a furtherdecrease after noon. The latitudinal distribution of absorption may be approxi-mated by a Gaussian curve,

AmA0exp (7.2)

where Am is the median absorption, the invariant latitude, and the half width(or “standard deviation”) of the absorption zone. The half width is severaldegrees: for example 4.5° (Hartz et al., 1963) or 3.7° (Holt et al., 1961).

Despite its daily variation, it is clear that the absorption zone is not the sameas the auroral oval defined by the occurrence of luminous aurorae, but corre-sponds to the more circular zone – the “outer zone” – discussed in Section 6.3.5.The luminous oval and the absorption zone coincide (or at least are very closetogether) near midnight, but the absorption zone is more circular than the ovaland lies at lower latitude on the day side. It is instructive to compare the incidenceof AA in Figure 7.10 with the distribution of energetic electron precipitationobserved from satellites in Figure 6.6.

The spatial extent

The horizontal extent of individual absorption events in kilometers is obviouslyan important matter practically as well as scientifically. Should they be very small,their effect on HF propagation could be reduced by space-diversity reception. If,on the other hand, they should blanket very large areas, it is difficult to see whatcould be done. (The same is the case with the polar-cap absorption due to protons– Section 7.3). Various measurements using groups of wide-beam riometers (Table7.1) do not agree with each other completely, but the general indication is that theevents cover a few hundred kilometers. Some reports suggest that there is a degreeof elongation in the east–west direction, but other studies have come out in favorof more or less circular patches of absorption.

The results of Table 7.1 come from the earlier period of observations, and wemust remember that the broad beam of the antenna prevents structure smallerthan about 100 km being detected. As pointed out in Section 7.2.2, work usingnarrower beams is finding events narrower than 100 km. Spike events, which areonly some tens of kilometers across, tend to appear in isolation and their magni-tude is considerably underestimated in broad-beam measurements. However,since they occur for such a short time, they will not have much effect on the general

( 0)2

2 2


Ta

ble

7.1

.S

pati

al p

rope

rtie

s of

Aur

oral

Abs

orpt

ion

(Har

grea

ves,

1969

)

Shap

e of

co

rrel

atio

n A

utho

rL

Dat

aV

alue

s co

rrel

ated

Res

ults

patt

ern

Lit

tle

and

Lei

nbac

h5.

53

days

, sum

mer

H

ourl

y va

lues

R

egio

ns a

t le

ast

200

km N

–S a

nd(1

958)

5.5–

91

mon

th, M

arch

Hou

rly

valu

es90

km

E–W

Day

:

0.57

at

800

km N

–SN

ight

:

0.43

at

800

km N

–S

Hol

t et

al.

(196

1)6

12 s

elec

ted

peri

ods

?

0.

5 at

380

km

Cir

cula

r

Kav

adas

(19

61)

4?

?H

igh

corr

elat

ion

over

10

km N

–S

Jelly

et

al.(

1961

)4–

8?

—C

an b

e si

mila

r ov

er 3

80 k

m o

r di

ffer

ent

over

35km

, N–S

Lei

nbac

h an

d B

asle

r5.

549

day

s, J

anua

ry–M

arch

Hou

rly

valu

es

0.

70 a

t 25

0 km

N–S

E

llipt

ical

(196

3)

0.

74 a

t 80

0 km

E–W

Lit

tle

et a

l.(1

965)

4D

ayti

me

even

ts, 5

4 da

ys,

2-m

in v

alue

s if

abs

orpt

ion

0.5

at 6

50 k

m N

–S a

ndC

ircu

lar

Dec

embe

r–F

ebru

ary

&0.

3 dB

700

km E

–W

Par

thas

arat

hy a

nd B

erke

y5.

5Su

dden

-ons

et e

vent

sP

eak

abso

rpti

on

0.

26 a

t 25

0 km

N–S

(90

eve

nts)

E

llipt

ical

(196

5)

0.

41 a

t 80

0 km

E–W

(35

eve

nts)

Ans

ari (

1965

)5.

5Sl

owly

var

ying

eve

nts,

Pea

k ab

sorp

tion

Fai

r ag

reem

ent

over

350

km

N–S

abou

t 60

exa

mpl

es

Ber

key

(196

8)5.

5D

istu

rbed

nig

hts

?&

0.5

dB

0.

9 at

20

km N

–S

Eck

lund

and

Har

grea

ves

417

mon

ths,

Hou

rly

valu

es if

&0.

3 dB

Nig

ht:

0.

5 at

750

km

E–W

and

E

llipt

ical

(196

8)A

ugus

t–Ja

nuar

y15

5 or

465

km

aN

–S

Day

:

0.5

at 3

65 k

m E

–W a

ndE

llipt

ical

or

170

or 3

00 k

ma

N–S

circ

ular

Bew

ersd

orff

et a

l.(1

968)

5–7

Slow

ly v

aryi

ng e

vent

s,–

Lit

tle

vari

atio

n ov

er 3

00–4

00 k

m

four

exa

mpl

esE

–W

Har

grea

ves

and

Eck

lund

712

mon

ths

Hou

rly

valu

es if

&0.

3 dB

Nig

ht:

0.

5 at

160

km

Cir

cula

r(1

968)

Day

:

0.5

at 2

50 k

mC

ircu

lar

Not

e:a

Mea

sure

men

ts p

olew

ard

and

equa

torw

ard

of t

he c

entr

al s

tati

on, r

espe

ctiv

ely.

statistics. At other times, particularly during the longer-lived activity, whether bynight or by day, the smaller structures are only a component of the total distribu-tion, and the results of Table 7.1 still have significance in showing that, even whenfiner structure is present, at least some part of the absorption extends for 200–300km.

Durations

It is not as easy as one might imagine to determine the durations of individualabsorption events. Some events are isolated and thus easily recognized, whereasothers run into each other and it might not be obvious whether any such caseshould be described as one long event or several short ones. Furthermore, manyevents fade away rather gradually, so the end is not always clear. (Onsets tend tobe sharper.)

Some values and the variation of duration from event to event are indicated byFigure 7.11, which shows the relative distribution of duration in the day and nightgroups at one station near the occurrence maximum and one just equatorward ofit. Note that the durations are shorter at the higher L value, and are shorter bynight than they are by day at both sites (Table 7.2). The groups of day and nightevents are specifed in UT. Add 2.5 and 0.5 h to get local magnetic time at Kirunaand Siglufjordür, respectively.

7.2.4 Dynamics

The dynamic nature of AA events is a property that is often not appreciated. Themovements have been investigated using chains or groups of geographically sep-arated riometer stations, supplemented more recently by imaging riometers. Theresults reveal a good degree of consistency in the movement of events of a giventype, implying that the motion has some physical significance – even if we are notyet sure what that significance is! Some examples are given below.

The onset and main event in the night sector

The sharp-onset event occurs in the pre-midnight sector. As pointed out above, itmay but need not include a spike, and it may appear as a more gradual onset atsome latitudes. The onset of this type of event usually appears first at an L valuebetween 5 and 6, which is somewhat equatorward of the statistical absorptionmaximum (at L7). From that latitude it spreads both poleward and equator-ward. The poleward section is the more often observed; the velocity is between 0.5and 3 km s1 in most cases. There is a clear demarkation between the polewardand the equatorward sections, as may be seen in Figure 7.12(b).

However, this is not the case for absorption peaks subsequent to the onset(Figure 7.12(a)): they can move in either direction, rather more than half movingequatorward. In the example of Figure 7.6 the event arrives over Kilpisjärvi


40 30 20 0 10 20 30

36

912

Tim

e (h

)

Occurrence (%)

10

Q

MQ

QM

Q

Nig

ht (

16–

23 U

T)

138

even

ts

Kiru

na

40 30 20 0 10 20 30

36

912

Tim

e (h

)

10

Q

M

Q

QM

Q

Nig

ht (

18–

24 U

T)

92 e

vent

s

Sig

lufjo

rdür

Mor

ning

(02

–09

UT

)10

4 ev

ents

Mor

ning

(04

–11

UT

)12

2 ev

ents

M: M

edia

nQ

: Qua

rtile

Fig

ure

7.1

1.

Dur

atio

ns o

f ev

ents

sta

rtin

g in

the

nig

ht a

nd m

orni

ng s

ecto

rs a

t K

irun

a (L

5.

4) a

nd S

iglu

fjor

dür

(L

6.9)

.


Table 7.2. Medians and quartiles of the distributionsof durations of events at Kiruna (L5.4) andSiglufjordür (L6.9) (durations are given in hours)

Kiruna Siglufjordür

Day Night Day Night

Lower quartile 1.5 0.7 0.8 0.3Median 2.2 1.5 1.5 0.9Upper quartile 3.5 2.9 2.9 1.3

Figure 7.12. Relative fre-quencies of polewardand equatorward move-ments along a meridianthrough Alaska: (a)peaks, and (b) onsets. Atall latitudes most of thepeaks move equator-ward, whereas the onsettends to move equator-ward at L5 but pole-ward at L6.(Reprinted from J. K.Hargreaves. Planet.Space Sci. 22, 1427,copyright 1974, withpermission from ElsevierScience.)


Figure 7.13. Maximum absorption, and distances in the X (west–east) and Y (south–north)directions of the location of the maximum from overhead at Kilpisjärvi, for the spike eventat 2046–2051 UT on 6 October 1994 (see Figure 7.3). The time resolution is 1 s. Despite thetime structure revealed at this resolution and erratic movement west–east, the polewardprogression is remarkably persistent. These features are typical of night-time spike events.

(L5.9) from the north and peaks almost overhead, but then moves off to thewest. It is not clear whether this is typical, but previous observations (Hargreaves,1970) using wide-beam riometers over a 250-km baseline at L7 have revealed awestward component in the night sector up to about 2 h after midnight.

When an event begins with a spike, this invariably moves poleward, as inthe example from the Kilpisjärvi imaging riometer in Figure 7.13. This details thespike event at 2046–2051 UT in Figure 7.3(b), and shows the value of themaximum absorption and its location within the field of view, all at 1-s resolution.Note that the magnitude of the absorption varies with quasi-period 30–60 s.East–west motions are rather irregular, but there is a poleward progressionoverall. This is typical of spikes occurring at the beginning of a night event. Figure


Figure 7.14. The spike event of Figure 7.13 at five selected times. The absorption wasmaximum at the points marked by black circles, and the shaded areas show where theabsorption was greater than half the maximum. Note the tendency towards an ellipticalshape.

120

80

40

0

–40

–80

–120–120 –80 –40 0 40 12080

S

km

N

W km E

2050:13 UT

2049:13 UT

2047:45 UT

2047:06 UT

2047:13 UT

7.14 shows the position of the absorption patch at five selected times (which arealso marked on Figure 7.13). The maximum moved by 200 km in just over 3 min,an average speed of 1 km s1, though the speed was greater to begin with. Thedimensions of the patch are changing, but the tendency towards east–west exten-sion is maintained.

Motions on the global scale

The onset also propagates eastward and westward from its first appearence in thenight sector. There is some variation among published results, partly due, nodoubt, to actual variability in the movements from one instance to another, andperhaps also to observational selection. For stations near the centre of the absorp-tion zone, observations between stations separated by some thousands of kilom-eters (for example over 90° longitude) indicate median speeds of about 4°longitude min1 (or 2.8 km s1), eastward between midnight and 1400 LT, west-ward otherwise (Hargreaves, 1967; Pudovkin et al., 1968; Jelly, 1970). The figureof 4° min1 is for specific features recognized at both the stations. If absorptionevents are compared without consideration of form, the median speed comesout smaller by a factor of three. Hajkowicz (1990) found westward speeds of2.7–4.5 km s1 for pre-midnight sudden onsets at L values of 5.2–6.1.

A simple model of the longitudinal movements at L7 is given in Figure 7.15.From its first appearance near midnight it takes (on average) about 20 min for anonset to travel 5 h of local time and 30–40 min to reach the morning sector. Theeastward and westward sections are not alternatives; observations have verifiedthat they occur simultaneously. It is generally thought that the energetic electronswhich are precipitated by day actually originated in the night sector and thendrifted eastward as particles trapped in the geomagnetic field (Section 2.3.4 andFigure 2.14). This cannot explain the westward motion before midnight, which ispresumably governed by other factors in the magnetospheric tail.

Combining results from studies of latitudinal and longitudinal movementsgives the overall global picture of Figure 7.16. This makes no attempt to distin-guish among different types of event.

The most comprehensive investigation of absorption movements on the globalscale was performed by Berkey et al. (1974), who analyzed 60 substorm events at40 riometer stations. Some of the main points, which confirmed the earlier workand added some new results, are as follows.

(a) The activity most frequently begins near midnight.

(b) The onset is earlier and at lower latitude when the level of magnetic distur-bance is greater.

(c) The longitudinal velocity is in the range 0.7–7 km s1.

(d) The westward part of the expansion (usually seen before midnight) some-times follows the auroral zone (i.e. the outer zone) and sometimes followsthe auroral oval.



Figure 7.15. Longitudinal time delays. (J. K. Hargreaves. Proc. Inst. Electr. Electron.Engineers 57, 1348, 1969a, © 1969 IEEE.) (a) A simple model of delay with respect to anonset at midnight. (b) The delay over 5 h of local time, compared with observations fromHargreaves (1967), the basis of the model. (c) Time delays between events at College andMurmansk (Pudovkin et al., 1968) compared with predictions from the model.

(e) The speed of the westward expansion is about 1 km s1 when it expandsalong the oval and about twice that when it expands along the zone.

(f) There is much variability among individual substorms.

(g) Some maps show morning activity (pre-noon) following a midnight onset,but with little or no activity between the midnight and day regions.

The drift of the pre-onset bay

The weak absorption bay that may precede an onset moves equatorward at atypical speed of a few hundred m s1. (See also Figure 7.27 later.) Many of theseare so weak that they may be detected only by the practiced eye, but Figure 7.17shows one that was unusually strong.

That bay was clearly seen in the sectors of Finland, Sweden/Norway, and


Figure 7.16. The progression of the onset of absorption projected onto the equatorial plane(assuming a dipole field). The wavefronts are drawn at 10-min intervals. (Reprinted from J.K. Hargreaves. J. Atmos. Terr. Phys. 30, 1461, copyright 1968, with permission fromElsevier Science.)

Iceland. It turns out that the imaging riometer is able to observe these moving arcsin greater detail (as in Figure 7.7). The speed is not always uniform, and the arcmay fade and strengthen during its passage across the field of view. It is temptingto relate the movement to that of auroral arcs (Section 6.4.2), which is alsoequatorward in most cases.


Figure 7.17. Equatorward motion of a bay preceeding an onset in the European sector on 4May 1977. The diagram includes chains of stations at various longitudes, and the motion isclearly seen in the data from Finland, Norway/Sweden and Iceland. The related onsetwhich followed is marked by arrows. (Reprinted from H. Ranta et al., Planet. Space Sci. 29,1287, copyright 1981, with permission from Elsevier Science.)

A (dB)

13

1

3

1

3

1

4

2

4

2

3

1

1

1

1

13

1

3

1

1

1

1

13

1

17 18 19 UT

DIXON ISLAND

NORILSK

KEVO

IVALO

SODANKYLÄ

ROVANIEMI

OULU

JYVÄSKYLÄ

BJÖRNÖYA

TROMSØ

ANDØYA

ABISKO

THORSHAVN

GODTHÅB

NARSSARSSUAQ

SIGLUFJORDÜR

LEIVORGUR

FAGURHOLSMYRI

The relation between the bay and the onset

Ranta et al. (1981) have studied the incidence of these bays in relation to the sharp-onset events which follow. Most of the bays occur between L values of 4 and 9,and individual examples cover between one and five or six L units. They have notbeen reported from the South Pole (at L13). In longitude they can extend morethan 90°. The onset may be seen over a larger range of L, from 4 to 16 or more,and individuals have been observed to cover ten units of L. It can exceed 150° inlongitude.

The onset often appears first at or near the eastern end of the preceeding bay,which means that, statistically, the bays occur earlier in the day (in the afternoonand evening sectors) than do the sharp onsets whose preference is for the hours justbefore and up to midnight. The event following the bay of Figure 7.17 wasobserved in the sectors of Finland and Sweden/Norway, and exhibited polewardmotion.

The relationship between bay and sharp-onset event may be summarized as the“reversed-y” event of Figure 7.18.

Figure 7.19 illustrates what appear to be the typical dynamics of a night-timeevent at L5.9, somewhat equatorward of the zone maximum. The record is froma wide-beam 38.2-MHz riometer but the movements have been identified using animaging riometer at the same site. The spike had a rapid poleward movement, butan arc preceeding it and patches following drifted equatorward. The main event,which was widespread, came into view from the poleward side, but then driftedout of view to the west.


Figure 7.18. The con-nection between preceed-ing bay and sharp onsetidealized as a “reversed-y” event. (Reprintedfrom J. K. Hargreaves etal. Planet. Space Sci. 23,905, copyright 1975,with permission fromElsevier Science.)

The slowly varying event

The slowly varying event in the morning sector typically exhibits an eastwardmotion when it is observed by riometers 250 km apart (Hargreaves, 1970;Hargreaves and Berry, 1976). Each of these studies gave a median eastward speedjust below 40 km min1 (about 620 m s1), but with a large variation in individualcases. (Half the eastward speeds were between 20 and 80 km min1.) It will benoticed that these speeds are considerably below those determined from widelyspaced observations.

Co-rotation

A tendency towards co-rotation has been noted in some events of the morning anddayside (Hargreaves et al., 1994; Collis et al., 1996). In particular, it appears thatthe spatially restricted, very energetic type of event described in Section 7.2.2. canremain virtually fixed with respect to the rotating Earth for a long period. Theevent shown in Figure 7.9 remained in the field of view for more than 1.5 h.According to radar measurements, the meridional F-region ion drift during theevent varied from 100 m s1 westward to approximately zero – in agreement withthe motion of the absorption event. This clear example, taken with the evidence

S

1703

1736

–39

1748

–180

5

1830

1938

Arc

mov

ing

eq

uat

orw

ard

Sp

ike

Mai

n e

ven

t fr

om

no

rth

Pat

ches

dri

ftin

gto

war

ds

equ

ato

r

gap

No

thin

g

2

0

2

4

6

Lev

el (

dB

)

Time (UT)

16 17 18 19 20

Figure 7.19. Night activity on 30 January 1995 observed with a wide-beam 38.2-MHzriometer at Kilpisjärvi, noting the main features and their movement. Excepting the spike,the dominant movement was equatorward.

from the previous paragraph, indicates that more than trapped-particle drift isinvolved in the longitudinal motion of auroral absorption.

7.2.5 The relation to geophysical activity, and predictions ofauroral absorption

A relation to Ap

Auroral absorption (AA) is more frequent and more intense when geomagneticactivity is high. One might also expect that AA would be stronger and occurmore often at times of high sunspot number, but that is not necessarily the case.In some solar cycles the magnetic activity does not rise and fall in the same wayas the number of sunspots, and in such a case the AA is seen to go with themagnetic activity rather than with the sunspot number. We shall return to thispoint.

In the shorter term, it is possible to show relations between the absorption onsingle days and the Ap index (Section 2.5.4). For example, the probability of therebeing at least one event of at least 1 dB during a period of 24 h rises almost line-arly with Ap, becoming virtually unity at Ap15 for a station at L5.6 (Figure7.20(b)). The rate of occurrence is smaller at higher latitude (Figure 7.20(a)) butstill increases approximately linearly with Ap. The average number of events perday also increases with Ap (Figure 7.20(c)), rising from one to three over the Ap

range 8–25.One observation that this association explains is the tendency for AA to be

intense for several days at a time, often then followed by a week or more when itis very low. The pattern tends to repeat from one month to the next. This behav-ior just mirrors that of magnetic disturbance, and is due to the rotation of the Sunwhich carries active regions out of view after a few days and tends to bring themround again a month later.

The latitude of the absorption zone (Section 7.2.3) also shifts with the intensityof magnetic activity (Hargreaves, 1966). During that period of the day when AAis most significant, the latitude of the maximum (0) decreases from approximately70° to 66° as Kp increases from 0 to 5, and at values of 6 or 7 it may be as low as60°. At the same time the half width of the absorption zone () increases some-what (from about 4° to 5.5°), with even greater broadening at the largest values ofKp.

A relation to HF radio propagation

This brings us to the topic of predictions. First, though, to get some idea of theimportance of AA in HF propagation, we must consider the magnitude ofthe absorption involved. The intensity of the absorption, which is usuallymeasured in decibels, depends inversely on the square of the radio frequency. In a



Figure 7.20. Effects of magnetic activity on the incidence of AA events. (a) The probabilitythat at least one event of at least 0.3 dB at 51.4 MHz occurs at the South Pole (L13)within 24 h. This level is equivalent to 0.88 dB at 30 MHz. The data covered the three years1990–1992 inclusive. (b) Similarly for 1-dB events at 30 MHz at Abisko (L5.6). The datacovered the two years 1976 and 1977. (c) The average number of 30-MHz, 1-dB events perday at Abisko. (South Pole data from T. J. Rosenberg, private communication, and Abiskodata compiled from Hargreaves et al., Report UAG-84, 1982.)

100

80

60

40

20

00 10 20 30 40

Ap

(a)

100

80

60

40

20

00 10 20 30

Ap

(b)

0

0

10 20 30Ap

(c)

Pro

babi

lity

of a

t lea

st o

ne e

vent

(%

)

0

1 2 3 4 5Kp

Num

ber

of e

vent

s pe

r da

yP

roba

bilit

y of

at l

east

one

eve

nt (

%)

1

2

3

4

5

radio-communication circuit it also depends on the geometry (specifically, theangle at which the ray passes through the D region). Nonetheless, in roundnumbers, if a 30-MHz zenithal riometer detects 1 dB absorption, an oblique HFpath will suffer about 20 dB (Agy, 1970). Thus, absorption greater than 1 dB on a30-MHz riometer is likely to be of practical significance to HF propagation (espe-cially between 3 and 15 MHz), and the statistics regarding the occurrence ofabsorption are often presented in terms of the 1-dB level. At some latitudes thereare many days per month when this level is reached or exceeded at least once.

AA predictions

Clearly, predictions of AA are going to be statistical in nature because the phe-nomenon is essentially sporadic; but at least there is a good base for the statisticsbecause large quantities of riometer data are available. If the predictions arerequired for radio propagation, the task has two parts: first, to specify from exist-ing riometer data the statistics regarding the occurrence of absorption and theeffects of independent variables such as latitude, season, time of day, and solar andmagnetic activity; and second, from propagation experiments to observe how theevents detected by the riometer are related to circuit effects. Here we consider thematter of absorption statistics, in which there have been some useful develop-ments. Relations to communications circuits are considered in Sections 8.2 and8.4.

The representation of absorption statistics can be taken in two stages. First,having decided our significant absorption level – for instance, 1 dB at 30 MHz –we can then inspect the data from various riometer stations and count up theprobability of 1 dB being exceeded as a function of the various external parame-ters. The probability that A dB will be exceeded is generally called Q(A).

Calculation of Q(1)

Foppiano and Bradley (1985) published a formula (Table 7.3) for calculating Q(1),based on an extensive study involving many sources and taking in data fromseveral longitude sectors and different years. The formula is written as the sum ofday and night contributions, each comprising the product of terms for the varia-tion with magnetic latitude, time of day, solar activity, longitude, and season. Thelatitude variations are of Gaussian form (similar to Equation (7.2)) with the nightpeak at 67° and the day peak at 68° at low sunspot number. The time-of-day termsare also Gaussian, the night activity peaking at midnight and the day activity at1000 local time – compare with Figure 7.10. The dependence on solar activity isexpressed in a table, and there are empirical formulae for the dependences on lon-gitude and season.

Some of these terms are better established than others. Some seasonal varia-tion probably occurs (see Section 7.2.6), but the question of a longitudinal effecthas not been investigated thoroughly. However, the greatest problem with theformula of Table 7.3 is in the assumed dependence on solar activity expressed by



Table 7.3. The Foppiano–Bradley formula for Q(1)

Total Q(1)Q1dQ1s

Day component Q1dKd ddT dRd

dM

Night component Q1sKsssT sRs

sM

magnetic latitude; T local time in hours; R sunspot number; geomagnetic longitude;

Mmonth; Kd and Ks are constants

Latitude terms

dexp[(m)2/(2

2)] s

exp[(m)2/(2

2)]

m and mgeomagnetic latitudes of maxima for day and night components:

m68(10.0004R) for R%100;

m65.28 for R&100;

m67(10.0006R)0.3(10.012R) | t |

where t(T3) for 0%T%15; t(T27) for 15T24

and widths of latitude distributions for day and night components:

3(10.004R) for R%100;

4.2 for R&100.

Time-of-day terms

dTexp[(TTm)2/(22T)] sTexp[(TTm)2/2T

2]

Tm and T m local times of maxima for day and night components:

Tm10(10.002R); T m0

T and Twidths of time distributions for day and night components:

TT2.8

Solar-activity terms

dRsR(1aR), the values of a being from the following table:

T (h) 00 02 04 06 08 10 12 14 16 18 20 22

a 0.0032 0.0025 0.0141 0.0048 0.0149 0.0146 0.0142 0.0090 0.0037 0.0156 0.0206 0.0092

Longitude terms

ds

0.580.42sin[0.947(85)] for 0°%10°

0.16 for 10°%80°

0.580.42sin[1.80(130)] for 80°%180°

0.580.42sin[0.947(275)] for 180°%360°

where corrected geomagnetic longitude in °E

Seasonal terms

dM10.3sin(3.86); sm1

where solar declination angle in degrees (positive in summer, negative in winter).

Constants

Kd21; Ks12

These give Q(1) values in percent

Notes:

The use of d and s for the day and night components derives from Hartz and Brice’s (1967)

“drizzle” and “splash” terminology.

the sunspot number. In a subsequent study of absorption over Finland during awhole solar cycle (1972–1983), Hargreaves et al. (1987) concluded that the sunspotterm of the formula was not very accurate, and they proposed an alternative basedon the monthly mean value of Ap (Ap):

Q(1)(Ap30cos2)exp[(65)2/25]. (7.3)

This formula gives the average Q(1) over all times of day.The significance of Equation (7.3) is that, over the long term, the absorption

probability is proportional to the mean Ap (Ap) above a threshold. The results mayalso be represented by a Gaussian variation with latitude in which the peak value(Q0), the latitude of the peak (0), and its width () depend on Ap as in Table 7.4.Note in particular that, with increasing Ap, the maximum probability increaseslinearly and the position of the maximum moves equatorward. (The foregoinganalysis is based on observations covering only the equatorward side of theabsorption zone.)

The log-normal distribution

The second stage is to consider the form of Q(A). That is, if we can predict Q(1),can we say what Q(2) or Q(0.5) will be? This means knowing the probability dis-tribution for the occurrence of absorption. Foppiano and Bradley (1984) assumeda log-normal distribution for the occurrence of absorption; that is, that the loga-rithm of the absorption follows a normal distribution:

f (logA)d(logA) exp d(logA), (7.4)

where A is the absorption in decibels, Am is the median absorption (logAm beingthe mean of logA), and is the standard deviation of logA. The probabilility ofA being exceeded is then

Q(A) f (logA)d(logA). (7.5)$

log A

(log A log Am)2

2 2 12


Table 7.4. Parameters of Gaussian curvesfitted to latitudinal variations of Q(1) in theFinland sector

Ap 0 (degrees) (degrees) Q0 (%)

0–10 68.1 3.8 5.710–15 67.8 3.9 9.315–20 66.9 3.6 13.320 65.6 3.6 17.4

The cumulative distribution Q(A) should come out as a straight line on log-prob-ability paper. In most cases this appears to be so (Figure 7.21), at least if the rangeof absorption is restricted to values between a few tenths of a decibel and a fewdecibels – that is, to the range for which riometer data are accurate and most plen-tiful. (Whether very large or very small values obey the same distribution is notreally known.)

The log-normal distribution is described by just two parameters, one of whichcan be Q(1). There is something to be said, also, for using Am and , the medianand the standard deviation (the second giving the slope on the log-probabilityplot), since both of these appear explicitly in the formula. It is not wise to extrap-


Figure 7.21. The log-normal distribution of Q(A), South Pole, March 1982. (Data from T.J. Rosenberg, private communication.)

99.99

99.90

99.00

90.00

50.00

10.00

1.00

0.10

0.01

0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0

30 MHz Absorption, A (dB)

% P

rob

abili

ty o

f ab

sorp

tio

n >

A d

B

olate the log-normal law below 0.1 or 0.2 dB. In most sets of riometer data thesesmall values are much affected by any error in the quiet-day curve, and also thereare theoretical reasons why the log-normal form cannot continue indefinatelytowards ever smaller values.

Predicting events

The foregoing approach aims to predict the likehood of a certain level of absorp-tion at a given site if the level of geomagnetic disturbance (or solar activity) isknown – the latter, of course, also being a quantity requiring prediction. Noaccount is taken of the event aspect: the fact that the absorption occurs in bursts;that, once started, it is likely to continue for some time but that there are also longperiods with no significant absorption at all.

Relatively little appears to have been done on prediction of AA using an eventapproach, though elements are implicit in some of the foregoing account. A com-prehensive event description would specify the magnitude, duration, structure,etc., of which (apart from magnitude) the statistical approach takes no account.An event description would also include an element of short-term forecasting.One example, based on the data of Berkey et al. (1974), is shown in Table 7.5.Medians and deciles are given for the absorption at various local times for every15 min after the onset of a substorm (Elkins, 1972). These are interesting forshowing how the distribution of absorption develops in a substorm as a functionof the local time, and also for what seems to be the first use of the log-normal dis-tribution to describe absorption statistics. The actual magnitudes depend, ofcourse, on the original selection of substorms. In this set the maximum absorp-tion was found to be related to the AE index by the empirical formula

(Absorption)max0.008(AE)max, (7.6)

but this is not of much help in a prediction because AE is not a predicted quan-tity. It is better to relate absorption to the daily index Ap, predictions of which arepublished a month in advance.

7.2.6 The wider geophysical significance of auroralabsorption events

The immediate implication of auroral radio absorption for high-latitude propaga-tion is simply the resulting loss of signal. However, since we know that the absorp-tion is due to additional ionization in the lower ionosphere, which in turn isproduced by energetic electrons entering the atmosphere from above, these eventsclearly have deeper implications. In this section we review some contributions ofriometer studies to geophysical topics.



Table 7.5. Medians and deciles of absorption at various local times during asubstorm (Elkins, 1972)

Substorm time LT Median (dB) Upper decile (dB) Lower decile (dB)

T15 00 0.75 2.6 0.2203 0.70 2.2 0.2206 0.54 1.8 1.1709 0.28 1.25 0.06612 0.10 0.56 0.01915 (0.14) (0.38) (0.052)18 0.10 0.56 0.01921 0.37 1.5 0.090

T30 00 0.94 3.7 0.2403 1.1 4.0 0.3206 1.1 4.5 0.3009 0.64 2.8 0.1412 0.42 1.7 0.1015 (0.20) (0.78) (0.052)18 (0.17) (0.58) (0.050)21 0.50 1.9 0.13

T45 00 1.1 3.5 0.3403 1.3 4.0 0.4306 1.6 4.5 0.5409 1.4 6.0 0.3212 0.67 2.8 0.1615 0.28 1.3 0.06418 (0.20) (0.84) (0.049)21 0.44 1.8 0.11

T60 00 1.0 3.2 0.3003 1.3 3.8 0.4006 1.6 4.5 0.5609 1.6 4.5 0.5612 1.1 3.6 0.3515 0.38 1.5 0.09618 (0.25) (1.0) (0.063)21 0.50 2.0 0.070

Notes:1. Parentheses ( ) indicate values with large uncertainties due to the small sizes of

statistical samples.2. Time is to be interpreted as follows: local time “00” means the hours 0000–0259 and

so forth.

Electron-density profiles

That there are, indeed, relationships between total absorption and electron densityat various heights has been shown by direct measurements. Friedrich and Torkar(1983) collected electron-density data from rockets flown into absorption eventsand thus “calibrated” the riometer in terms of the electron-density profile from 70to 110 km. This has recently been extended (Friedrich and Kirkwood, 2000) usingelectron densities from the EISCAT incoherent-scatter radar in Scandinavia(Figure 7.22). This comparison provides an estimate of the electron density atgiven height for a given intensity of AA, though with considerable scatter aboutthe average. Some 50% of values lie within a factor of two of the average plotted.

Some of the scatter is no doubt due to real changes in the profile during events,and from one event to another. Figure 7.23 illustrates the changing electron-density profile during a morning event of the slowly varying type in which theheight of the peak lifts as the event decays. (The growth was more complicated.)

Absorption profiles

Since the electron–neutral species collision frequency is known (for Equation(3.95)), the absorption profile may be computed from the electron-density profile.In most cases the computed and observed absorptions agree well enough to serveas confirmation that the reductions in signal recorded by the riometer are indeeddue to non-deviative absorption.

The height of the absorbing layer and its thickness are of direct interest in HFpropagation. The heights of absorption maxima computed from rocket profiles ofelectron density range over 90 to 95 km at night, but may be lower (75 km) by day(Hargreaves, 1969a). The calculated absorption peaked between 88 and 95 km inthe event of Figure 7.23 which occurred during the early morning, and at 67 kmin the hard, daytime event of Figure 7.9. The absorbing layer is quite thick: gen-erally 15–20 km between points where the incremental absorption is half themaximum. About 80% of the total absorption is produced in this slab. The spe-cific absorption coefficient increases downward, and the absorption peaks some5–15 km (depending on the spectrum) lower than the electron density; as a roughguide it would be true to say that the absorption layer occurs in the underside ofthe electron-density layer, starting just below the peak.

Incoming electron fluxes

Since the AA event is due to precipitating electrons, the calibration can in princi-ple extend back a stage further so that we may infer something about the inten-sity and spectra of the electrons precipitated during AA events. Proceedures forinverting the electron-density profile (Kirkwood, 1988; Hargreaves and Devlin,1990; Osepian et al., 1993) involve routines that give the rate of production froman assumed incoming electron spectrum (Section 2.6.1 and Figure 2.26), and bysome means adjust the spectrum until the computed electron-density profile



Figure 7.22. A riometer “calibration” against electron-density profiles measured by rocketsand incoherent-scatter radar. (a) with the Sun below the horizon and (b) solar zenith angles90° (dashed line) and 60° (solid line). In each case the curves are given for every 0.5 dBfrom 0 to 2.5 dB at 27.6 MHz. All seasons and times of day are included. (M. Friedrich andS. Kirkwood, Advances in Space Research, 25, 15 (2000).)

108 109 1010 1011 1012

Electron Density (m3)

Alt

itu

de

(km

)

150

140

130

120

110

100

90

80

70

60

108 109 1010 1011 1012

Electron Density (m3)

Alt

itu

de

(km

)

150

140

130

120

110

100

90

80

70

60

(a)

(b)

matches the observed one. An essential element is the effective recombinationcoefficient as a function of altitude (Section 1.3.3), which relates the rate ofelectron–ion production to the resulting electron density. This may be taken fromother experimental results or computed from the known chemistry of the D region(Section 1.4.3) – neither approach being entirely satisfactory. Figure 7.24 showselectron-density profiles measured by incoherent-scatter radar and the corre-sponding spectra of incoming electrons computed from them. Note that thedaytime spectrum is “harder” (contains a greater proportion of more energeticparticles) and that the resulting electron-density profile peaks at a lower altitudethan does the night-time spectrum.

Some direct comparisons have also been made. Using particle fluxes measuredon low-orbit satellites, Jelly et al. (1964), Hargreaves and Sharp (1965), andParthasarathy et al. (1966) obtained, respectively, the following empirical relations:

A4103J 1/2, (7.7a)


Figure 7.23. Electron-density profiles measured by the EISCAT incoherent-scatter radarduring a slowly varying event in the morning of 23 March 1985. The heights of maximumion production for electrons of the stated initial energy are marked on the right-hand axis.(Reprinted from J. K. Hargreaves and T. Devlin, J. Atmos. Terr. Phys. 52, 193, copyright1990, with permission from Elsevier Science.)


Figure 7.24. Electron-density and absorption profiles for typical night and morning events,and the estimated spectra of incoming energetic electrons: (a) 14 December 1994 at 2056:10UT; (b) 23 March 1985 at 0310 UT. In the upper panel the solid lines show the electron-density profiles computed from the spectra in the lower panel, the black circles beingobserved values. (Flux is in units of cm2 sr1 s1 keV1.) The morning event has some tentimes the flux of the night event between about 40 and 80 keV, whereas the night event hasa greater flux of softer (25 keV) particles. The daytime absorption peaks at 87–88 km, thenight absorption some 5 km higher.

105

100

95

90

85

80

7510–2 10–1

104 105 106

Incr. abs. (dB km–1)

Ne (cm–3)

ELECTRON DENSITY AND ABSORPTION PROFILESDURING PRECIPITATION EVENTS

(a) (b) (a) (b)

Hei

ght (

km)

7

6

5

4

3

2

10 50 100 150 200 250

Energy (keV)

Log

(Flu

x)

(a) (b)

Input values

Matched profile

Incremental absorption at 30 MHz

(a) 1984 Dec 14, 2056:10 UT(b) 1985 Mar 23, 0310 UT

A2103J 1/2, (7.7b)

A0.40Q1/2, (7.8)

A3.3103J 1/2 (7.9)

where A is the 30-MHz absorption in decibels, J is the flux of electrons of energiesabove 40 keV in cm2 s1 sr1, and Q is the total energy (above 80 eV) in erg cm2

s1. Equation (7.7a) is for day and (7.7b) is for night.The absorption is also significantly correlated to the energy flux over some

energy ranges of electrons detected at geosynchronous orbit (Figure 7.25). Herethe flux was taken only when the detector pointed into the loss cone; at otherangles the electrons would mirror before reaching the D region. These, and other(Penman et al., 1979), comparisons have indicated that the absorption correlatedbest to the energy influx in the bands 40–80 keV and 80–160 keV. As might beexpected, the rate of production calculated from the particle flux also correlatedwell at some heights. Schematic production-rate profiles derived from that com-parison are shown in Figure 7.26.

It should be stressed that these results and those of Figures 7.25 and 7.22 andEquations (7.7)–(7.9) make no distinction between types of event and should betaken as no more than indicative in any single instance.

The onset and dynamics of the substorm

The night event which often begins with a sharp onset, and probably a spike too,is a consequence of the substorm in the magnetosphere. The riometer is thereforea useful monitor of the occurrence of substorms at the site of the riometer. Thisaspect was referred to in Section 6.4.6. Furthermore, the dynamics of absorptionevents which may be observed using a network (Section 7.2.4) relate to the devel-opment of particle precipitation in the substorm.

The equatorward movement of the absorption arc preceeding an onset prob-ably reflects the inward drift of an active region in the magnetosphere. Hargreaveset al. (1975) suggested that the motion may be EB drift due to the magnetos-pheric electric field, in which case the value of the field can be estimated from therelation (Ranta et al., 1981)

E(mV m1) 5.88104

1

. (7.10)

Equatorward drifts measured using a chain of riometers in Alaska (Figure 7.27)had speeds of several hundred m s1, greatest at the highest latitudes.Interpretation in terms of a magnetospheric electric field gives a median value of1.3 mV m1. That the deduced field is independent of L supports the hypothesis,but the procedure has yet to be verified by direct comparisons with the electricfield measured by other means. A later study using data from the Scandinaviansector revealed speeds mostly in the range 0–300 m s1 with a peak at 100–200 m

dtd(1/L2)



Figure 7.25. Relations between energy flux in selected bands measured on the GEOS-2 geo-synchronous satellite, and radio absorption at 30 MHz observed in the auroral zone. Thecorrelation is best in the two middle bands, indicating that the greatest contribution to theabsorption comes from electrons of energy 40–160 keV. (Reprinted from P. N. Collis et al.,J. Atmos Terr. Phys. 46, 21, copyright 1984, with permission from Elsevier Science.)

200

175

150

125

100

075

050

025

0

EN

ER

GY

FL

UX

(10

9 keV

cm

2 s

1 s

r1 )

0 1 2 3 4 5 6 7ABSORPTION (dB)

160–320 keV

5

4

3

2

1

0

EN

ER

GY

FL

UX

(10

9 keV

cm

2 s

1 s

r1 )


40–80 keV

10

8

6

4

2

0

EN

ER

GY

FL

UX

(10

9 keV

cm

2 s

1 s

r1 )


20–320 keV

5

4

3

2

1

0

EN

ER

GY

FL

UX

(10

9 keV

cm

2 s

1 s

r1 )


20–40 keV

200

175

150

125

100

075

050

025

0E

NE

RG

Y F

LU

X (

109 k

eV c

m

2 s

1 sr

1 ) 0 1 2 3 4 5 6 7

ABSORPTION (dB)

80–160 keV

s1. Table 7.6 lists some values of the cross-tail electric field derived from this setof data. In this case the median value is 0.63 mV m1.

On the other hand, the poleward motion of the spike event is not an EB drift(Nielsen, 1980).

The morning events referred to in Section 7.2.2 typically move eastward, fromthe night towards the day side of the Earth. Velocities measured over wide base-lines are not inconsistent with the concept that the electrons precipitated in themorning sector were originally injected into the closed magnetosphere near mid-night and then moved eastward by gradient-curvature drift: an 80-keV electronwould drift eastward at 2.6° min1 and cover 90° of longitude in 35 min. (Comparewith Figure 7.16.) However, motions over smaller baselines tend to be significantlyslower (even to the point of mere co-rotation) and some other mechanism isplainly at work as well.

The westward movement before local midnight also requires some other explanation.

Conjugate behavior

Auroral radio absorption is particularly well suited to studies of the relative behav-iors of auroral phenomena at magnetically conjugate points: that is, at the north-ern and southern ends of the same field-line. In the first instance, one would expectto see the same intensity of absorption and the same patterns of variation. In factthese expectations are rarely met. For instance, the absorption tends to be strongerin the winter hemisphere (Figure 7.28), and there is considerable variation in indi-vidual cases, even to the extent that an event is seen at one station but not at all at


Figure 7.26. Schematic production-rate profiles for a range of values of 30-MHz radioabsorption. The error bars are for one standard deviation. (Reprinted from P. N. Collis etal., J. Atmos Terr. Phys. 46, 21, copyright 1984, with permission from Elsevier Science.)

100

90

80

70

102 103 104 105

0.5 dB 1.0 2.0 3.0 5.0A

ltit

ud

e (k

m)

Production rate (cm3 s1)

the other. The night-time events, particularly at the higher latitudes, exhibit timedifferences between the peaks of events in conjugate regions, the event whichappears first being of greater intensity than its counterpart in the conjugateregion.

One particularly interesting result, which so far lacks an explanation, is that theinterhemispheric ratio depends on the direction of the interplanetary magneticfield carried by the solar wind (Figure 7.29).


0 0.5 1.0 1.5 2.0

70

65

60

(

deg

rees

)

BI

FY

C

P

SM

A

W

V (km s1)

BI

FY

C

P

SMAW

9

8

7

6

5

4

L-v

alu

e

0.3 0.5 0.7 1.0 1.5 2.0 3.0 4.0 6.0 8.0E (mV m1)

(a)

(b)

Figure 7.27. Equatorward drift of absorption bays preceeding onsets in Alaska. The loca-tions of the riometer stations are shown by letters. (a) The speeds determined between pairsof stations (increasing with latitude). (b) The deduced magnetospheric electric field on theassumption that the motion is EB drift. The estimated field is independent of L.(Reprinted from J. K. Hargreaves et al., Planet. Space Sci. 23, 905, copyright 1975, withpermission from Elsevier Science.)


Table 7.6. Values of the cross-tail magnetotail electric field deduced fromthe equatorward drift of absorption arcs (Ranta et al., 1918)

Date UT E(mV m1)

27 March 1975 1430–1530 2.42 May 1975 1300–1500 0.74

1720–1900 0.453 November 1975 1700–1800 1.12 March 1976 1600–1730 0.43

1930–2030 0.942 May 1976 1900–2000 0.9422 May 1976 1800–1900 0.44

2000–2100 0.6229 May 1977 1900–2000 0.584 May 1977 1800–1930 0.63

Figure 7.28. The seasonal variation of the ratio of absorption in northern and southernconjugate regions at L values of 14, 7, and 4. At both of the higher latitudes the absorptiontends to be larger in the winter hemisphere. There is some difference between the day andnight events. (Reprinted from J. K. Hargreaves and F. C. Cowley (1967b), Planet. Space Sci.15, 1585, copyright 1967, with permission from Elsevier Science.)

During the 1960s, spaced riometers were deployed around Byrd station in theAntarctic and its computed conjugate point (Great Whale River) in the CanadianArctic, and these produced evidence that the conjugate point may be displaced byup to 85 km north–south with respect to the computed conjugate point, depend-ing on the season and time of day (Hargreaves, 1969b).

The absorption pulsations in the Pc4 and Pc5 bands, which appear as a mod-ulation of slowly varying events in the morning sector, are observed to be in phasein magnetically conjugate regions (Chivers and Hargreaves, 1964). See Figure7.30. This indicates that the modulation is symmetrical between hemispheres andis imposed in the magnetosphere. From the electron-density profiles observedduring pulsations, it appears that the modulation involves the energy of the par-ticles, not just their flux (Hargreaves and Devlin, 1990).

7.3 The polar-cap event

7.3.1 Introduction

In the history of ionospheric studies the polar-cap event is a relatively recent dis-covery. On 23 February 1956 there occurred a major solar flare that was followedby polar radio blackouts lasting for several days. At the same time, cosmic-raymonitors detected a large increase in the intensity of cosmic rays at ground level.The effects on VHF forward-scatter circuits operating at that time were studied byD. K. Bailey, who showed that the cause of the blackout was an enhancement ofionization in the D region of the polar ionosphere (Bailey, 1959). He deduced


Figure 7.29. The variation of the north-to-south absorption ratio between the conjugate sta-tions Frobisher bay and South Pole (expressed as the “ratio function” (r1)/(r1)). Inaddition to the seasonal variation, note that the ratio is greater when the interplanetary mag-netic field is pointed away from the Sun. (Reprinted from J. K. Hargreaves and F. C. Cowley(1976b), Planet. Space Sci. 15, 1585, copyright 1967, with permission from Elsevier Science.)

further that the most likely cause of this added ionization was a flux of energeticprotons released from the Sun at the time of the flare.

As with auroral absorption, studies based on the occurrence of the blackoutcondition have their limitations. Most scientific studies of the phenomenon havetherefore made use of riometers, which give a quantitative measure of the absorp-tion. These showed that the effects were confined to high magnetic latitudes, but,unlike AA, covered the whole polar cap. Thus they became known as polar-capabsorption (PCA) events.

PCA events are much less frequent than auroral events, there being several eachyear on average. However, their effects, when they do occur, are more severe bothbecause they blanket a large region of the Earth and because of the magnitude ofthe absorption. The most energetic events are also detected at the ground bycosmic-ray counters, and there is about one of these events each year on average.(In total 34 were noted in the 30 years between 1955 and 1985 – Smart and Shea,1989.) A PCA that is also recorded by a cosmic-ray detector at the ground is calleda ground-level event (GLE). The first recognized GLE occurred on 28 February1942; it was identified in retrospect, of course, since the PCA was not yet a knownphenomenon. The flare responsible for that event has another claim to fame as thesource of the first solar radio noise to be recorded at the Earth.

Since the early 1960s it has been possible to observe solar protons in space, and


Figure 7.30. Conjugate pulsations at Great Whale River (– – –) and Byrd (——) (L7).The mean trend has been removed. The pulsations are essentially in phase at the conjugatestations. (Reprinted with permission, from J. K. Hargreaves and H. J. A. Chivers, Nature203, 963, copyright 1964, Macmillan Magazines Limited.)

2.0

1.6

1.2

0.8

0.4

0

20.4

20.8

1640

1650

1700

1710

1720

1730

1740

1750

1800

1810

1820

1830

1840

UT

Ab

sorp

tio

n d

iffe

ren

ce (

dB

)

the monitoring of energetic protons from satellites has now become a matter ofroutine. As might be expected, the satellite monitors find some events that are notseen by ground-based methods. In fact, most solar flares emit protons at the lowerenergies – that is, up to 10 MeV. At energies of several tens of mega-electron voltsthe flux of protons reaching the Earth’s vicinity far exceeds that from galacticcosmic rays, though at the highest energies, greater than 1 GeV, the galactic par-ticles dominate.

7.3.2 Observed properties of PCA events

Occurrence and duration

There is no doubt that the PCA event is due to energetic (1–1000 MeV) protonsemitted from the Sun, usually during a solar flare. The occurrence of PCA there-fore depends strongly on the sunspot cycle. There can be more than ten events inan active year, and none at all – though more often one to three – near solarminimum. The long-term average is about six events per year. The numbersdetected depend, of course, on what detection threshold has been selected.

As an example, the occurrence of PCA events that reached at least 1 dB on a30-MHz riometer situated within the polar cap is shown in Figure 7.31(a). Thiscovers the period 1962–1972, which included the end of solar cycle 19 and thefirst eight years of cycle 20. Some of these events were much larger than 1 dB;12% of them reached 10 dB or more. Those events recorded as &5 dB are indi-cated. It will be noted that none of these larger events occurred during the quietyears 1962–65.

The durations of the events of magnitude &1 dB are shown in Figure 7.31(b).The median was about 2.5 days. The main group in the histogram spans 12–108 h.Those occurring within the narrow range 120–132 h appear as a separate group,but an alternative explanation may be that these long events were actually com-posed of several shorter ones. Be that as it may, we can summarize the distribu-tion by saying that, once a PCA event has started, it is most likely to last for 1–4days but in some cases may continue for a week or more.

Figure 7.32(a) shows the occurrence of proton events in relation to themaximum flux of protons with energy at least 10 MeV. The plot covers the years1976–1989, which included solar cycle 21 and the beginning of cycle 22. Thegeneral influence of the sunspot cycle is seen again, except that there is a remark-able dearth of events near the peak of the cycle in 1979–1980. This looks like anextreme case of a well-reported effect. The correlation between the solar cycle andthe occurrence of PCA events is imperfect, but it has often been noticed that thereare fewer events than might be expected at the maximum of the solar cycle.(Alternatively, there might be too many as the cycle begins and during its decline.)The pattern of occurrence varies from cycle to cycle, but this may be due in partto the statistics of small numbers. Although the number of sunspots is a guide, itis not safe, therefore, to try to predict from previous cycles too exactly.



Figure 7.31. Occurrence and duration of PCA events producing at least 1 dB of absorptionon a 30-MHz riometer in the polar cap. The period covered is 1962–1972. (a) The annualrate of occurrence, related to the 12-month running mean sunspot number. The incidenceof events exceeding 5 dB is indicated. (b) Durations of&1-dB events. In some cases it wasonly recorded that the duration exceeded some value, and these are indicated by shading.The median duration was 62 h (about 2.5 days). It is possible that some of the longer eventswere composed of several shorter but overlapping events. (After M. A. Shea and D. F.Smart, Solar–Terrestrial Physics and Meteorology: SCOSTEP Working Document II, 1997;SCOSTEP Working Document III, 1979.)

16

14

12

10

8

6

4

2

062 63 64 65 66 67 68 69 70 71 72

Num

ber

of e

vent

s pe

r ye

ar

Sun

spot

num

ber

120

100

80

60

40

20

0

Year (19 – –)

Absorption ≥5 dB

Duration of ≥1 dB events, 1962–72

0 24 48 72 96 120 144 168

>16

8

Num

ber

of e

vent

s

Duration (hrs)

7

6

5

4

3

2

1

0

May exceedstated value

(a)

(b)


Figure 7.32. Some properties of solar-proton events recorded by a geosynchronous satelliteand a riometer in the auroral zone. (a) The incidence of proton events according to themaximum proton flux at synchronous orbit. The sunspot numbers are shown as the 12-month running mean. The data cover the years 1976–1989. (b) The relation between 30-MHz absorption at Kilpisjärvi and the flux of protons of energy&10 MeV atgeosynchromous orbit. 60% of the points lie between the straight lines marked, represent-ing J37A2 and J200A2 – compare with equation (7.12). (Data from H. Ranta et al., J.Atmos. Terr. Phys. 55, 751 (1993).)

24

22

20

18

16

14

12

10

8

6

4

2

076 77 78 79 80 81 82 83 84 85 86 87 88 89

Num

ber

of e

vent

s pe

r ye

ar

Year (19 – –)

160

140

120

100

80

60

40

20

0

Sun

spot

num

ber

Flux >10 MeV (cm–2 s–1 str–1)(a)

10987

6

5

4

3

2.5

2

1.5

1.0

0.7

≤0.5

30 MHz absorption at Kilpisjärvi andflux of ≥10 MeV protons at

geosynchronous orbit(b)

Abs

orpt

ion

(dB

)

10 20 50 100 1000200 500 2000 5000 ≥10,000

Proton flux (cm–2 s–1 str–1)

A ∝ J1/2

<100100–1000>1000

The direct connection between proton flux and PCA is confirmed by Figure7.32(b), which plots the absorption at Kilpisjärvi, Finland, against the proton fluxdetected on a satellite in synchronous orbit. The straight lines indicate the law

absorption (flux)1/2, (7.11)

which is to be expected if the electron-production rate is proportional to the par-ticle flux. Note that fluxes greater than 100 cm2 s1 sr1 are likely to produce asignificant PCA. Since Kilpisjärvi is in the auroral zone (at L5.9) rather thanthe polar cap, the absorption recorded there may at times be reduced by the prox-imity of the edge of the polar cap.

An approximate rule that is sometimes used to deduce the proton flux from theradio absorption (Smart and Shea, 1989) is

J10A2, (7.12)

where J is the flux (in cm2 s1 sr1) of protons with energy exceeding 10 MeV, andA is the absorption (in decibels) measured with a 30-MHz riometer in the sunlitpolar cap.

The statistics of the occurrence of PCA is complicated by episodic behavior. Anindividual proton event is generally recognized by noting an increase in protonflux or by virtue of radio absorption having the established PCA characteristics(i.e. a smooth event of long duration). However, an active solar region may wellpersist long enough to produce two or more proton flares and it is not unusual,therefore, for two or more PCAs to occur within a few days of each other. Sincean event may last for several days, some events run into each other. The data setshown in Figure 7.31 contained 63 events. Of these, 25 occurred within one of tengroups of events, the criterion for a group being that events occurred within 5 daysof each other. The count of groups is of course less than the count of individualevents. To take an example, 1968 had 11 PCA events, but eight of them occurredin three groups and only three events were isolated. Perhaps 1968 should be cred-ited with six PCA-producing regions, therefore, instead of with 11 PCAs. 1969 wasalso significantly affected in this way: in February of that year four events occurredon four successive days! Beyond a general impression that more events fall withingroups in the more active years, it is difficult to draw general conclusions becauseof the small numbers involved.

Variation from month to month

One of the puzzles regarding the occurrence of PCA which came to light early waswhat appeared to be a seasonal effect: it was observed that fewer events occurredduring the northern hemisphere winter than at other times of year. There is noreason to suppose that the Sun becomes less active in December and January, but,taking type-IV radio bursts (see Section 7.3.3) as a reference, there is evidence thatthe protons were taking longer to reach the Earth at those times – see Figure 7.33.


It has also been argued that the effect may be artificial and due to some observa-tional bias. The most likely cause of bias is that, since the absorption is weaker ina dark ionosphere (Section 7.3.6), the ionosphere is dark for more of the time inwinter, and more of the early riometer stations were in the northern hemisphere,then the detection of PCAs by radio would be less sensitive overall in the north-ern winter.

Supporting this view (which probably holds sway at present) is the fact that theanomaly in the seasonal occurrence seems to be one of those effects whichbecomes less convincing the more intensively they are studied. It has tended tovanish as the data base has grown with the passing of the years! Thus the sets ofdata used for Figures 7.31 and 7.32 both show the incidence varying considerablyfrom month to month, but they contain no evidence for any significant seasonaleffect. Indeed, the monthly distribution of proton events measured on a satelliteappears to show some preference for the equinoxes (Smart and Shea, 1989). Sincethe question of seasonal effects remains in doubt, it is probably best to assume forprediction purposes that the incidence of PCA has no seasonal dependencebeyond ordinary statistical variations.

That assumption being made, the probability that a stated number of eventswill occur in one month may be calculated from the Poisson distribution which


Month

Strong events Weak events

Rel

ativ

e o

ccu

rren

ce

Delay time (h)

Del

ay t

ime

(h)

1.0

0.5

0

10

5

0

J F M A M J J A S O N D

J F M A M J J A S O N D

Figure 7.33. Seasonaleffect in PCA: (a) thefraction of flares havingType-IV radio burstswhich also producePCA, and (b) the sea-sonal variation of thetime delay between aradio burst and com-mencement of PCA.(After B. Hultqvist,Solar Flares and SpaceResearch (eds. de Jagerand Svestka), p. 215,North-Holland, 1969.)

(a)

(b)

describes the frequency of occurrence of independent events within a given periodof time. We have to know (or assume) the average rate of occurrence. Table 7.7gives the monthly statistics for annual rates of occurrence of two, six, and ten, cor-responding approximately to low, average, and high PCA activity. Since eventsoccurring in a group (as defined above) are probably not independent, a groupshould be counted as one event for this purpose.

Magnitude

Not surprisingly, there are more small PCA events than large ones. Table 7.8,taken from the data of Shea and Smart (1977; 1979), shows how many eventsexceeded various absorption thresholds during the 11-year period 1962–1972.Note that, of the events of magnitude &0.5 dB, about half reach 1 dB, about onefifth of those reach 5 dB, and about one third of those reach 15 dB. An approxi-mate rule that appears to satisfy the limited information available is that thenumber of events exceeding a stated threshold varies in inverse proportion to thatthreshold value.

The review by Smart and Shea (1989) discusses the incidence of proton eventsin some detail.

7.3.3 The relation to solar flares and radio emissions

In fact, not all large flares give rise to proton events and there are some protonevents that have not been associated with any known flare. However, although thecorrelation might not be 100%, there is no doubt that, as a general rule, proton


Table 7.7. The probability that the stated number of PCA events will occur in onemonth, given the annual rate

Expectedannual

Probability of the stated number occurring in one month

rate 0 1 2 3

2 0.846 0.141 0.012 0.0016 0.607 0.303 0.076 0.013

10 0.435 0.362 0.151 0.042

Table 7.8. The distribution of magnitude of PCA events

Threshold (dB) (30-MHz riometer) 0.5 1.0 2.0 5.0 10.0 15.0

Total number of events exceeding threshold 113 63 36 13 8 3Percentage of total 100 56 32 11.5 7.1 2.7

events are associated with the larger solar flares. Those flares that produce protonsare often called proton flares and they are recognized as a distinct class in the flarepredictions which are issued regularly by various national and internationalwarning agencies.

The solar radio emissions known as type IV are useful for predicting whichflares emit protons. The type-IV emission is a radio burst of long duration thatfollows some flares and covers a wide band of radio frequency. (It is attributed tosynchrotron radiation from high-energy particles gyrating in the solar magneticfield.) The bursts associated with proton emission are characterized by aU-shaped spectrum in which the intensity is smaller in the middle than it is at theends. For example, if the spectrum covers the range from a few hundred megahertzto 10 GHz, it will be relatively strong at the high- and low-frequency ends butweaker at the middle frequencies around 1 GHz. From the spectral characteristicsof the radio burst it is possible to predict the flux of protons with energy exceed-ing 10 MeV (Castelli et al., 1967) and also the proton spectrum (Bakshi andBarron, 1979). Since the radio burst is received at the Earth some time before theprotons are due to arrive, the association obviously has some practical impor-tance.

The association between proton ejection and the radio burst is also useful foridentifying the flare responsible and for timing the flight of the proton cloud toEarth. This time appears to be shorter (about 1 h) for strong events and longer(about 6 h) for weak ones.

7.3.4 Effects arising during the proton’s journey to Earth

The production and release of energetic protons appears to be a normal part ofthe solar-flare phenomenon, and flares causing PCA and GLE at the Earth prob-ably differ from others more in degree than in nature. The journey from Sun toEarth may be considered in three parts:

(a) propagation from Sun to Earth – i.e. from the Sun to the boundary of themagnetosphere;

(b) motions within the magnetosphere; and

(c) the interaction between protons and the atmosphere.

Effects in interplanetary space

The propagation of charged particles in the space between the Sun and the Earthis affected by the form of the interplanetary magnetic field. As illustrated in Figure2.3, the field has a spiral form due to the rotation of the Sun, and this affects thepropagation of solar protons despite the weakness of the field and the high energyof the protons. The gyroradius of a 1-GeV proton in a field of 5 nT is less than ahundredth of the distance between the Sun and the Earth; hence there is time forthe IMF to act on even an energetic proton. Those with less energy gyrate in


smaller loops (gyroradius energy1/2) and are more tightly controlled, respondingto irregularities in the IMF as well as to its general form. Hence there is scatter-ing, and the protons appear to be coming from all directions by the time they reachthe Earth.

Scattering in the interplanetary medium, since it also provides a mechanism forstoring particles in space, can account for the observed time delay between a flareand the beginning of the PCA, and for the duration of PCA events. A proton ofenergy 10 MeV would reach the Earth in only 1 h if it traveled in a straight line,and the duration of a flare is typically some tens of minutes only. In fact the delaybefore an event begins is typically several hours, and the event due to one flare maylast for several days (Figure 7.34).

Further evidence for the role of the IMF is as follows.

(a) Flares near the eastern limb of the Sun rarely give rise to PCA events,whereas some events seem to be associated with flares that are out of sightaround the western limb. This is illustrated by Figure 7.35, which gives thepositions of solar flares associated with those proton events energeticenough to be detected at ground level (i.e. GLEs). (Note that the western


Ab

sorp

tio

n (

dB

)

0

5

10

15

2010 11 12 13 14 15 16 17 18 19 20 21 22

May 1959

Thule, Greenland

Ab

sorp

tio

n (

dB

)

0

5

10

15

2010 11 12 13 14 15 16 17 18 19 20 21 22

May 1959

College, Alaska

Figure 7.34. A PCArecorded by riometers at(a) Thule, Greenlandand (b) College, Alaska.Thule is in the polar capand College in theauroral zone. The eventlasted for a week at bothsites, but was modulatedat the lower latitude. (G.C. Reid, in Physics ofGeomagnetic Phenomena(eds. Matsushita andCampbell), AcademicPress, 1967.)

(a)

(b)

limb of the Sun is on the right-hand side as seen from the terrestrial north-ern hemisphere.) It is obvious that the distribution of these flares withsolar longitude is significantly biased towards one side of the centralmeridian. The heliolongitudinal distribution of the source flares broadensfor protons of lower energy (Smart and Shea, 1995).

(b) The time delay between a flare and the related PCA increases with theeastern longitude of the flare.

(c) The delay between flare and PCA is greatest at times of high solar activity,and this is also when the IMF is most irregular.

Recent improvements in detecting structures in the interplanetary mediumhave focused attention on the role of coronal mass ejections (Section 2.2.2). It isfound that some PCAs may be attributed not directly to flares but to the shockwave related to a coronal ejection of mass from the Sun (Shea and Smart, 1995).

Effects in the magnetosphere

On reaching the magnetopause the protons must then pass through the geomag-netic field to reach the atmosphere. To a first approximation this problem may betreated by Störmer theory. The theory describing the trajectories of charged par-ticles in a dipolar magnetic field, which C. Störmer worked out in connection withhis studies of the aurora, does not actually apply to auroral particles (because theirenergies are too low)! However, the theory is valid for cosmic rays and for solarprotons.

In a magnetic field a charged particle tends to follow a spiral path whose radius


Figure 7.35. Solar longi-tudes of flares associatedwith ground-level events.The Sun is happy aboutthis. (D. F. Smart and M.A. Shea. J. SpacecraftRockets, 26, 403, 1989.)

of curvature (rBmv/(Be)) is directly proportional to its velocity and inverselyproportional to the magnetic flux density. Because solar protons are of relativelyhigh energy, the magnetic field changes significantly over one gyration, and there-fore trapping theory (as outlined in Section 2.3.4) does not apply. Nonetheless, itis still true that particles traveling almost along the magnetic field undergo theleast deviation. To reach the equator, which is the least accessible region, a protonhas to cross field-lines all the way down to the atmosphere. Charged particles maydo this only if they are sufficiently energetic, and the equatorial region is effectivelyforbidden to typical protons of solar origin. However, most of the particles in aproton event can penetrate into the atmosphere over a polar cap extending downto about 60° magnetic latitude.

Since the radius of gyration in a given magnetic field depends on the momen-tum per unit charge (mv/e), it is convenient to discuss particle orbits in general interms of a parameter called rigidity:

RPc/(ze), (7.13)

where P is the momentum, c the speed of light, z the atomic number, and e theelectronic charge taken positive. The advantage of this parameter is that all parti-cles with the same value of R will follow the same path in a given magnetic field.

Although the trajectory of a proton in the geomagnetic field can be very com-plicated, Störmer’s analysis simplified matters by defining “allowed” and “forbid-den” regions that could and could not, respectively, be reached by a chargedparticle approaching the Earth from infinity. To reach magnetic latitude c in adipole field, the rigidity of the particle must exceed a cutoff rigidity, Rc:

Rc14.9cos4c, (7.14)

where Rc is measured in gigavolts (109 V). That is, particles of rigidity Rc reach lat-itudes c and above. Conversely, a place at latitude c would receive only those par-ticles with rigidities equal to and greater than Rc. Figure 7.36(a) plots the Störmercutoff latitude against energy both for protons and for electrons.

To perform an exact calculation of the trajectory of a proton through the geo-magnetic field, the procedure is to imagine that a proton with negative charge isprojected upward from the point of impact, since the trajectory of such a particleis exactly the reverse of that of an incoming positively charged particle having thesame rigidity. From a set of computations of this kind it is possible to work outthe directions in space from which the particles reaching a given place at a giventime must have come. Results confirm other evidence that, whereas most protonsare isotropic near the Earth, the more energetic ones (those exceeding 1 GeVwhich are responsible for ground-level events) originate from the western side ofthe Sun. (See Figure 7.35.)

During the main part of a typical PCA event the absorption region is essentially


uniform and symmetrical over the polar caps down to about 60° geomagnetic lat-itude. According to Störmer theory these protons should have energies exceeding400 MeV, but direct observations of the particles have shown that the cutoff rigid-ity at the edge of the polar cap is significantly less than the Störmer value. The sit-uation appears to be that there is a main polar cap surrounding the geomagneticpole that is open to solar protons of all energies, and then at slightly lower latitudethe cutoff reverts fairly abruptly to the Störmer value. Much of this effect (thoughperhaps not all) may be explained by taking account of the tail of the magneto-sphere which connects directly to the polar caps and presumably provides an easy


Figure 7.36. (a) TheStörmer cutoff latitudefor protons and elec-trons. (S.-I. Akasofu andS. Chapman,Solar–TerrestrialPhysics. OxfordUniversity Press, 1972,by permission of OxfordUniversity Press. After T.Obayashi, Rep.Ionosphere Space Res.Japan, 13, 201, 1959.) (b)Cutoff latitudes fordipole and realistic geo-magnetic fields (G. C.Reid and H. H. Sauer, J.Geophys. Res. 72, 197,1967, copyright by theAmerican GeophysicalUnion.)

(a)

(b)

route even for protons of low energy. Figure 7.36(b) shows the difference in cutoff

energy between dipolar and more realistic geomagnetic fields. The cutoff isreduced still further if a magnetic storm (Section 2.2.3), which enhances the ringcurrent (Section 2.3.5) and moves the magnetopause inward, occurs while a PCAevent is in progress. The geographic regions most affected by PCA are illustratedin Figure 7.37 in general terms. The boundaries may be several degrees nearer theequator during a magnetic storm.

7.3.5 Non-uniformity and the midday recovery

Non-uniformity

The spatial distribution of radio absorption is not always uniform, particularlyduring the early and late phases of a PCA. The absorption usually appears firstnear the geomagnetic poles and then spreads to cover the polar caps some hourslater. Towards the end of the event there is likely to be contamination by auroralelectrons related to a magnetic storm, and a concentration of absorption into theauroral zone is then to be expected. In addition, the polar cap expands during thestorm, moving the PCA equatorward.

Midday recovery

Some events exhibit a reduction in the absorption for several hours near localnoon. This effect is known as the midday recovery (MDR), and its main proper-ties are as follows (Leinbach, 1967).

(a) They occur during about 20% of PCA events.

(b) They are usually pronounced on the first day of the event only.

(c) They peak between 0800 and 1500 LT, most of them between 1000 and 1200.

(d) They may last as long as 6–10 h, most being remarkably symmetricalabout the peak.

(e) They are strongest near the equatorward boundary of the polar cap, andare not evident at locations well within the polar cap.

(f) When the polar cap expands during a magnetic storm, the recovery regionremains at its equatorward edge.

Figure 7.38 illustrates some of these features during a PCA observed at theAlaskan stations College (L5.5), Farewell (L4.3) and King Salmon (L3.3).The time scale is given in UT, from which 10 h should be subtracted to obtainAlaskan time. MDRs occurred between 0800 and 1000 LT at the first two stationson the first day of the event. On the second day a magnetic storm extended thepolar cap to lower latitude and a MDR was observed at King Salmon, but wasnot (College) or was barely (Farewell) seen at the higher latitudes. (The horizon-tal bars on Figure 7.38 indicate night-time recoveries; these are different and willbe considered in Section 7.3.6.)



Figure 7.37. The polar areas normally affected by polar-cap absorption. The regions insidethe inner curves may be considered as “polar plateaux”, whereas regions outside the outercurves are usually not affected except during severe geomagnetic disturbance. The outeredges of the diagrams are at latitude 45°. (G. C. Reid, Physics of the Sun (ed. P. A.Sturrock), 3, 251, Reidel, 1986, with kind permission from Kluwer Academic Publishers.)

In a recent case study using data from 25 stations including some in the south-ern hemisphere (Uljev et al., 1995) the maximum effect was found slightly beforelocal noon, covering a range of magnetic latitude approximately from 60° to 70°(Figure 7.39). The effect seems to occur simultaneously and with the same mag-nitude in magnetically conjugate regions, and it is confirmed that the effect is notseen at stations well inside the polar cap (at latitudes greater than 70°).

Two possible explanations were put forward by Leinbach (1967): a local changeof cutoff, and the development of anisotropy in the pitch-angle distribution of theincoming protons. More recent studies have suggested that both effects may occur.There is evidence that a change of cutoff near noon is indeed one factor(Hargreaves et al., 1993), and modeling studies (Uljev et al., 1995) suggest thatanisotropy of the pitch-angle distribution also occurs but only over the latituderange 65°–70°.


Figure 7.38. The PCA event of 7 July 1958, seen at College (L5.5), Farewell (L4.3),and King Salmon (L3.3). The horizontal bars indicate night recoveries and MDR marksmidday recoveries. All observations were at 27.6 MHz. (H. Leinbach. J. Geophys. Res. 72,5473, 1967, copyright by the American Geophysical Union.)

7.3.6 Effects in the terrestrial atmosphere

Upper-atmosphere ionization during a proton event

Energetic protons entering the terrestrial atmosphere lose energy in collisions withthe neutral molecules and leave behind an ionized trail. In order to reach an alti-tude of 50 km, a proton must have an initial energy of 30 MeV, and to reach theground (to cause a GLE) the energy must be over 1 GeV. (Refer to Figure 2.28.)An example of proton spectra observed at geosynchronous orbit during a protonevent in 1984 is shown in Figure 7.40(a). Despite the name solar proton event, itshould be appreciated that other particles, -particles and heavier nuclei, alsoarrive (in proportions typical of the solar atmosphere). However, their contribu-tion to the ionization is small relative to that of the protons. The computation ofionization by protons and -particles was discussed in Section 2.6.3.

Having computed the rate of production of electrons at a given height, knowl-edge of the effective recombination coefficient allows one to calculate the resulting


Figure 7.39. The region affected by midday recovery during the event of 20 March 1990.The coordinates are invariant latitude and magnetic LT. The broken line between 10 and12 h marks the times of minimum absorption at each station. (Reprinted from V. A. Uljevet al., J. Atmos. Terr. Phys. 57, 905, copyright 1995, with permission from ElsevierScience.).


Figure 7.40. (a) Protonfluxes measured by thegeosynchronous satelliteGOES-5 on 16 February1984, fitted by spectra ofform E. (Data from F.C. Cowley, NOAA,Boulder, Colorado,private communication.)(b) Electron-density pro-files measured by inco-herent scatter radarduring the same event.(Reprinted with permis-sion from J. K.Hargreaves et al., Planet.Space Sci. 35, 947, copy-right 1987, with permis-sion from ElsevierScience.)

(a)

(b)

electron density. If an event contains particles of energy 1–100 MeV, the effectsshould appear within the height range 35–90 km (Figure 2.28). Effects due to thehigher energies tend to be smaller because the flux is smaller and the rate of recom-bination is greater at lower height. Nevertheless, in some events substantial ion-ization is created down to 50 km.

Determination of the recombination coefficient

In fact, the recombination coefficient in the lower ionosphere is not a well-established quantity, and one use of PCA events is to measure the recombinationcoefficient and its variations over a range of heights in the mesosphere. The protonspectrum may be measured from a geosynchronous satellite, and from it the pro-duction rate can be computed over a range of heights using a model of the neutralatmosphere. Electron-density profiles can be determined from rocket measure-ments or by incoherent-scatter radar. An example of the latter is shown in Figure7.40(b). Some studies have used riometer data, which are more readily available,though in that case only the integrated absorption can be compared.

Values of the effective recombination coefficient obtained using electron den-sities from incoherent-scatter radar are shown in Figure 7.41. Most striking aboutthese values is their large spread. There is a major difference between day andnight, and also between different determinations of daytime values. It is possiblethat there are seasonal variations caused by seasonal changes in the concentra-tions of minor species (Reagan and Watt, 1976). These differences will be consid-ered in the next section.

Day–night variation and twilight effects

Because the proton influx during a PCA decays relatively slowly, the effects ofdaily variations in the complex chemistry of the region may also be detected. Themost obvious effect is a large diurnal variation in the absorption, which is typi-cally about five times as large by day as it is by night, though the ratio can be assmall as three or as large as ten. The critical factor in this is whether the lower ion-osphere is sunlit. Night-time recoveries were marked on Figure 7.38, and they alsoaccount for the daily absorption recoveries in Figure 7.34(b). (Thule, Figure7.34(a), was illuminated continuously and the recoveries did not occur there.)

The effect is perhaps seen most clearly by comparing the absorption at magnet-ically conjugate stations, one in the summer and the other in winter, as in Figure7.42. Over the Spitzbergen station the ionosphere was illuminated continuously,whereas at Mirnyy the Sun was above the horizon for only a few hours of the day.We expect the proton fluxes at each place to be similar, and, indeed, the absorp-tion was almost the same when both stations were sunlit. However, the absorptionfell to a considerably smaller value at Mirnyy during each night period.

The cause of the day–night modulation is without doubt a variation in the ratioof the concentrations of electrons and negative ions, (defined in Section 1.3.3).In a dark ionosphere, electrons become attached to oxygen molecules to form


90 80 70 60 5010

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negative oxygen ions (O2), as Equation (1.61), but in sunlight the electrons are

detached again by visible light (Equation (1.62)) or through other chemical reac-tions. (See Section 1.4.3.) Since only the ionospheric electrons contribute to theabsorption, a variation of leads to a variation of absorption even though the pro-duction rate, q, remains constant.

The changes between night and day take place over the twilight periods atsunrise and sunset, and the details are of particular interest. The timing of thechange in relation to the elevation angle of the Sun indicates the presence of ascreening layer, probably ozone. Since ozone does not absorb in the visible, thesolar radiation that detatches electrons from negative ions must be in the ultra-violet rather than the visible region of the spectrum (Reid, 1961). The effect is con-fined to altitudes below 80 km (Figure 7.43), which explains why it does notappear in AA (most of which occurs at a higher level).

When the details are examined it becomes apparent that some other factors arealso at work.

(a) There is an asymmetry between the sunrise and sunset changes. Theincrease of absorption over sunrise is slower than the decrease over sunset(Chivers and Hargreaves, 1965). This means that the absorption is larger atsunset than it is at sunrise for the same solar zenith angle. The effect maybe seen by plotting the absorption at a station passing through twilightperiods as a ratio to that at one that is constantly illuminated. The result isa hysteresis curve like Figure 7.44, in which the curve is described counter-clockwise. The same effect is present in the profiles of Figure 7.43, where


Figure 7.42. Polar-cap absorption at magnetically conjugate stations 12–16 July 1966,Spitsbergen in the northern hemisphere and Mirnyy in the Antarctic. (Reprinted from C. S.Gillmor, J. Atmos. Terr. Phys. 25, 263, copyright (1963), with permission from ElsevierScience.)

Solar elevation at Spitsbergen: within 10° and 34° throughout

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the smaller recombination coefficients at given zenith angle at sunset implygreater electron densities. Collis and Rietveld (1990) showed that thetiming of the day–night transition depends also on the altitude, and sug-gested by way of explanation that different processes control the electrondensity above 70 km (photodetachment from O2

) and below 66 km (colli-sional detachment due to O2(

1g)) during the twilight period.

(b) There is evidence that the effective recombination coefficient varies withtime even within the day and night periods. Reagan and Watt (1976) foundthat its value declined gradually during the sunlit period (i.e. betweensunrise and sunset) by as much as a factor of two (at some heights). On theother hand, Hargreaves et al. (1993) reported a gradual increase of theeffective recombination coefficient throughout the night. The reasons forthese slow changes are not known, though they presumably lie in thechemistry of the mesosphere.


Figure 7.43. Effective recombination coefficients at various solar zenith angles over sunriseand sunset during the major proton event of August 1972. (J. B. Reagan and T. M. Watt. J.Geophys. Res. 81, 4579, 1976, copyright by the American Geophysical Union.)

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Assuming that the day–night change which occurs rapidly over twilight isindeed due entirely to a variation in the ratio of negative ions to electrons (), andthat recombination of positive and negative ions is negligible, then a simple appli-cation of Equation (1.39) – remembering also that the effective recombinationcoefficient eff

q/N e2 by definition – gives a relation between night and day values

of at a given height:

. (7.15)

If we take typical estimates of (day) of 1, 0.25, and 0.68 at 80, 75, and 70 km,respectively, the results of Hargreaves et al. (1993), to take an example, give(night) values of 1.7, 20, and 100 at the same heights. There is, however, no gen-erally agreed set of values for this quantity.

Effects on the neutral-species composition

Influxes of energetic particles have another important effect in that they mayproduce changes in the chemical composition of the atmosphere. As long ago as1969 it was observed in rocket measurements that the ozone in the mesosphere (atheights of 54–67 km) was depleted during a PCA event by a factor between two andfour depending on the height (Weeks et al., 1972). The mechanism is as follows. Oneconsequence of the ionization processes is the formation of hydrated ions (O2

.H2O),which then undergo further reactions leading to “odd hydrogen” species such as Hand OH. These radicals then react with ozone to produce molecular oxygen:

HO3 →OHO2

OHO3 →HO2

O3O3 →2O2 (7.16a)

OHO3 →HO2O2

HO2O3 →OH2O2

2O3 →3O2 (7.16b)

2OHO3 →HO2O2

HO2O3 →OHO2

HO3O3 →2O2 (7.16c)

In each case the odd-hydrogen radical is a catalyst; it is destroyed in the first reac-tion of the pair but regenerated in the second. These processes require a sufficientconcentration of water vapor and therefore they are confined to the region belowthe mesopause. They are thought to be important over the height range 50–90 km.Several hours to a day after the precipitation event, the odd-hydrogen speciesreform into stable molecules; then the above reactions cease and the concentration

1 (night)1 (day)

eff(night)eff(day)


of ozone recovers. However, since the H atoms tend to recombine to form H2

rather than H2O, the water vapor may remain depleted for some time. There maybe an increase in the concentration of ozone during this period.

More serious from the point of view of ozone is the effect of “odd-nitrogen”species. These have a much longer lifetime (amounting to several years) in thestratosphere, which is also the site of most of the ozone. The ionization processesproduce secondary electrons with energies of tens and hundreds of electron volts,and these can dissociate and ionize molecular nitrogen to produce atoms and ionsof atomic nitrogen. The N and N then react with O2 to give nitric oxide, NO,which in turn acts to destroy ozone as follows:

NOO3→NO2O2

NO2O → NOO2

O3O → 2O2 (7.17)

Here the NO is the catalyst. This reaction is important up to 45 km, and the longlifetime of NO at those levels means that a given molecule may pass through thereaction many times, converting one O3 at each pass.

The above reactions do not depend on the nature of the primary ionizing radi-ation, but they are of particular importance in PCA because the more energeticprotons ionize at particularly low altitudes and down into the stratosphere. Theprocesses actually go on continuously with the arrival of galactic cosmic rays, butit has been estimated that the total production of NO during one major PCA eventcan be very great, even exceeding the annual production by cosmic rays. The greatproton event of August 1972 had a measurable effect on the ozone concentrationin the stratosphere, which fell by 15%–20% at latitudes 75°–80°. In the event ofJuly 1982 ozone was depleted between 55 and 85 km. This was a relatively “soft”event, which explains why the effects were higher up. A series of PCA events thatoccurred in 1989 is also thought to have affected the ozone content. A computa-tion of the effect of the events during that year is shown in Figure 7.45. The O3

was depleted by more than 10% over a limited height range for several months in1989, and small effects continued for a year or more. Significant though theseeffects are, they have no known effect on high-latitude radio propagation. Furtherinformation is given in papers by Reid (1986) and Jackman (2000).

7.4 Coherent scatter and the summer mesopheric echo

Incoherent and coherent scattering of radio waves in the ionosphere exploit differ-ent phenomena, as a result of which the second process is much the stronger (seeSection 4.2.2). Given the utility of incoherent-scatter radar in ionospheric studiesat high latitude, it would be a great pity if the weak signals which it uses were to beswamped by coherent echoes from the same region. Yet this is just what may occur.

Coherent echoes from the high-latitude D region were first detected in the VHF


band (at 50 MHz) in Alaska (Ecklund and Balsley, 1981), and subsequently inNorway at 53.5 MHz (Czechowsky et al., 1989) and with the EISCAT 224-MHzradar (Hoppe et al., 1988). They have also been observed, though less frequently,with the EISCAT UHF system at 933 MHz (Röttger et al., 1990). Other observa-tions cover the range 2.27 MHz to 1.29 GHz (Röttger, 1994). These strong echoesoccur only in summer and are now usually called polar mesosphere summer echoes(PMSEs). They are a nuisance to IS radar but constitute an interesting topic intheir own right, particularly since they have proved to be something of a mystery.

Their characteristics are very different from those of the incoherent echoesreceived from the D region during particle precipitation. Not only are they muchmore intense, but also they are much narrower, usually less than 1.5 km deep,though there can also be multiple layers (Figure 7.46). The height range is morerestricted, too, peaking at 84–86 km (Figure 7.47), an altitude close to the meso-pause. When the echoes are present their height fluctuates (Figure 7.48), which isthought to indicate the passage of acoustic-gravity waves (Section 1.6). The spec-trum of PMSE is considerably narrower than that of IS returns (Figure 7.49); evenwithout the other evidence this point alone would be sufficient proof that quitedifferent mechanisms are responsible.

7.4 Coherent scatter 407

Figure 7.45. Computed variation of NOy and O3 concentrations at 75° north due to thesolar proton events of 1989. The contours for NOy are 0, 1, 2, 10, 20, 100 and 200%. For O3they are 2, 1, 0.2, 0, 0.2, 1, 2, 10 and 20%. The concentration of NOy isincreased but that of O3 is decreased. Note the long duration of the effects. (C. H. Jackmanet al., J. Geophys. Res. 105, 11659, 2000, copyright by the American Geophysical Union.)

Most strikingly, the echoes are clearly a summer phenomenon, occurring fromJune to August only, with a maximum in July in the northern hemisphere (Palmeret al., 1996). The occurrence also varies during the day. There are maxima nearnoon and midnight, and minima in the morning and the evening hours. The per-centage occurrence, though not very well established, is some 50%–75% of days at


Figure 7.46. An example of PMSE observed at 224 MHz on 29 June 1988 using theEISCAT VHF radar. The density of blob suggests the strength of the echo. Note the heightvariations and the multiple layers. (Reprinted from P. N. Collis and J. Röttger, J. Atmos.Terr. Phys. 52, 569, copyright 1990, with permission from Elsevier Science.)

Figure 7.47. A histogram of the height distribution of PMSE observed with the EISCATVHF radar. (Reprinted from J. R. Palmer et al., J. Atmos. Terr. Phys. 58, 307, copyright1996, with permission from Elsevier Science.)

the maxima and 10%–50% at the minima. The daily variations are most markedin June and August.

The polar mesosphere is particularly cold in the summer, and this may be thekey to the mechanism. It has been proposed (Kelley et al., 1987) that water-clusterions, whose formation is favored by low temperature, reduce the diffusion coeffi-cient of electrons and so extend the scale of turbulence, allowing coherent scatterto occur at shorter wavelengths. However, other mechanisms have also been pro-posed. The development of PMSE studies and the relevent theories have beenreviewed by Cho and Kelley (1993) and by Röttger (1994).


At middle and equatorial latitudes D-region absorption has only a minor effecton HF propagation, but at high latitude it can affect the signal strength pro-foundly. There are two basic types at high latitude, each having a separate causeand morphology. In its effect on radiowave propagation, auroral absorption (AA)


Figure 7.48. Rapid height fluctuations in PMSE, consistent with acoustic-gravity waves, onvarious dates in 1988 and 1991. The dashed curves show the rate of change of altitude, andthe solid curves the vertical velocity derived from the Doppler shift of the echoes.(Reprinted from J. R. Palmer et al., J. Atmos. Terr. Phys. 58, 307, copyright 1996, with per-mission from Elsevier Science.)

is of first-order importance. It may occur over a range of geomagnetic latitudefrom below 60° to above 75°, with a statistical maximum near 67°, and is patchyin its horizontal extent. The patches are tens to hundreds of kilometers in extent,and any elongation tends to be east–west. The diurnal occurrence peaks justbefore magnetic midnight and again in the morning sector between 0700 and 1000magnetic LT.

Most of our knowledge about high-latitude absorption has come from severaldecades of observation by standard riometers (having beams about 60° betweenhalf-power points), though the earliest studies were based on ionosonde data.This information probably describes AA sufficiently well for the purposes of thoseHF communication systems which also use relatively broad beams. However,some modern HF systems (such as over-the-horizon radars and direction finders)require information on the finer structure of D-region absorption. The imagingriometers developed during the 1980s (and further deployed in the 1990s) haveimproved the spatial resolution considerably, and have the potential to provideinformation relevent to the high-resolution HF systems.

AA is a dynamic phenomenon, related, at least in part, to the auroral substorm;though almost certainly involving particle precipitation from the outer Van Allenbelt in the day sector. The particles are electrons with energies from tens to hun-


Figure 7.49. Spectra of incoherent scatter (IS) and PMSE obtained with the 224-MHzEISCAT VHF radar. The left-hand panels show typical IS spectra fitted by Lorentziancurves, and the centre and right-hand panels show broad and narrow PMSE spectra. Eventhe broadest PMSE spectra are considerably narrower than the IS spectra from the sameheight. (Reprinted from P. N. Collis and J. Röttger. J. Atmos. Terr. Phys. 52, 569, copyright1990, with permission from Elsevier Science.)

dreds of kilo-electron volts – generally greater than those which produce the visualaurora. As with the aurora, there is probably some measurable AA somewhere inthe auroral zone in any given period of 24 h. AA is essentially conjugate, occur-ring almost simultaneously (though not necessarily with the same intensity) inmagnetically conjugate regions.

The other significant D-region absorption event at high latitude is polar-capabsorption (PCA), which may produce higher overall values of absorption thandoes AA but occurs much less frequently, only several times a year on the long-term average. PCA events are caused by the precipitation of 1–1000-MeV protonsof solar origin into the polar D region. The occurrence and severity of PCAincreases from solar minimum to maximum, and there may be ten or a dozenevents in an active year. They produce a fairly uniform blanketing of the polar capdown to about 60° geomagnetic, and have been known to black out trans-polarHF propagation for 10 days at a time.

Both AA and PCA affect the lower frequencies more than they do the higherones because the absorption varies (to a first approximation) as f 2. At ELF andVLF, propagating in the waveguide mode, an increase in precipitation causes sig-nificant variation in the dimensions of the waveguide and thereby produces bothamplitude and phase changes in the received signals.


7.2 Auroral radio absorptionAgy, V. (1970) HF radar and auroral absorption. Radio Sci. 5, 1317.

Ansari, Z. A. (1965) A peculiar type of daytime absorption in the auroral zone. J.Geophys. Res. 70, 3117.

Appleton, E. V., Naismith, R., and Builder, G. (1933) Ionospheric investigations inhigh latitudes. Nature 132, 340.

Bailey, D. K. (1968) Some quantitative aspects of electron precipitation in and nearthe auroral zone. Rev. Geophys. 6, 289.

Berkey, F. T. (1968) Coordinated measurements of auroral absorption and luminosityusing the narrow beam technique. J. Geophys. Res. 73, 319.

Berkey, F. T., Driatskiy, V. M., Henriksen, K., Hultqvist, B., Jelly, D. H., Schuka, T. I.,Theander, A., and Yliniemi, J. (1974) A synoptic investigation of particle precipitationdynamics for 60 substorms in IQSY (1964–65) and IASY (1969). Planet. Space Sci.22, 255.

Bewersdorff, A., Kremser, G., Stadnes, J., Trefall, H., and Ullaland, S. (1968)Simultaneous balloon measurements of auroral X-rays during slowly varying iono-spheric absorption events. J. Atmos. Terr. Phys. 30, 591.

Collis, P. N., Hargreaves, J. K., and Korth, A. (1984) Auroral radio absorption as anindicator of magnetospheric electrons and of conditions in the disturbed auroral D-region. J. Atmos. Terr. Phys. 46, 21.


Collis, P. N., Hargreaves, J. K., and White, G. P. (1996) A localised co-rotating auroralabsorption event observed near noon using imaging riometer and EISCAT. Ann.Geophysicae 14, 1305.

Ecklund, W. L. and Hargreaves, J. K. (1968) Some measurements of auroral absorp-tion structure over distances of about 300 km and of absorption correlation betweenconjugate regions. J. Atmos. Terr. Phys. 30, 265.

Elkins, T. J. (1972) A Model of Auroral Substorm Absorption. Report AFCRL-72-0413.Air Force Cambridge Research Laboratories, Bedford, Massachusetts.

Foppiano, A. J. and Bradley, P. A. (1984) Day-to-day variability of riometer absorp-tion. J. Atmos. Terr. Phys. 46, 689.

Foppiano, A. J. and Bradley, P. A. (1985) Morphology of background auroral absorp-tion. J. Atmos. Terr. Phys. 47, 663.

Friedrich, M. and Torkar, K. M. (1983) High-latitude plasma densities and their rela-tion to riometer absorption. J. Atmos. Terr. Phys. 45, 127.

Friedrich, M. and Kirkwood, S. (2000) The D-region background at high latitudes.Adv. Space Res. 25, 15.

Hajkovicz, L. A. (1990) The dynamics of a steep onset in the conjugate auroral riome-ter absorption. Planet. Space Sci. 38, 127.

Hargreaves, J. K. (1966) On the variation of auroral radio absorption with geomag-netic activity. Planet. Space Sci. 14, 991.

Hargreaves, J. K. (1967)Auroral motions observed with riometers: movements betweenstations widely separated in longitude. J. Atmos. Terr. Phys. 29, 1159.

Hargreaves, J. K. (1968) Auroral motions observed with riometers: latitudinal move-ments and a median global pattern. J. Atmos. Terr. Phys. 30, 1461.

Hargreaves, J. K. (1969a) Auroral absorption of HF radio waves in the ionosphere: areview of results from the first decade of riometry. Proc. Inst. Elect. ElectronicsEngineers 57, 1348

Hargreaves, J. K. (1969b) Conjugate and closely-spaced observations of auroral radioabsorption – I. Seasonal and diurnal behaviour. Planet. Space Sci. 17, 1459.

Hargreaves, J. K. (1970) Conjugate and closely-spaced observations of auroral radioabsorption – IV. The movement of simple features. Planet. Space Sci. 18, 1691.

Hargreaves, J. K. (1974) Dynamics of auroral absorption in the midnight sector – themovement of absorption peaks in relation to the substorm onset. Planet. Space Sci.22, 1427.

Hargreaves, J. K. and Chivers, H. J. A. (1964) Fluctuations in ionospheric absorptionevents at conjugate stations. Nature 203, 963.

Hargreaves, J. K. and Sharp, R. D. (1965) Electron precipitation and ionospheric radioabsorption in the auroral zones. Planet. Space Sci. 13, 1171.

Hargreaves, J. K. and Cowley, F. C. (1967a) Studies of auroral radio absorption eventsat three magnetic latitudes. 1. Occurrence and statistical properties of the events.Planet. Space Sci. 15, 1571.

Hargreaves, J. K. and Cowley, F. C. (1967b) Studies of auroral radio absorption eventsat three magnetic latitudes. 2. Differences between conjugate regions. Planet. SpaceSci. 15, 1585.


Hargreaves, J. K. and Ecklund, W. L. (1968) Correlation of auroral radio absorptionbetween conjugate points. Radio Sci. 3, 698.

Hargreaves, J. K., Chivers, H. J. A., and Axford, W. I. (1975) The development of thesubstorm in auroral radio absorption. Planet. Space Sci. 23, 905.

Hargreaves, J. K. and Berry, M. G. (1976) The eastward movement of the structure ofauroral radio absorption events in the morning sector. Ann. Geophysicae 32, 401.

Hargreaves, J. K., Taylor, C. M., and Penman, J. M. (1982) Catalogue of Auroral RadioAbsorption During 1976–1979 at Abisko, Sweden. World Data Center A, USDepartment of Commerce, Boulder, Colorado.

Hargreaves, J. K., Feeney, M. T., Ranta, H. and Ranta, A. (1987) On the prediction ofauroral radio absorption on the equatorial side of the absorption zone. J. Atmos. Terr.Phys. 49, 259.

Hargreaves, J. K. and Devlin, T. (1990) Morning sector precipitation events observedby incoherent scatter radar. J. Atmos. Terr. Phys. 52, 193.

Hargreaves, J. K., Detrick, D. L., and Rosenberg, T. J. (1991) Space-time structure ofauroral radio absorption events observed with the imaging riometer at South Pole.Radio Sci. 26, 925.

Hargreaves, J. K., Browne, S., Ranta, H., Ranta, A. Rosenberg, T. J., and Detrick, D.L. (1997) A study of substorm-associated nightside spike events in auroral absorptionusing imaging riometers at South Pole and Kilpisjärvi. J. Atmos. Solar–TerrestrialPhys. 59, 853.

Hartz, T. R., Montbriand, L. E. and Vogan, E. L. (1963) A study of auroral absorp-tion at 30 Mc/s. Can. J. Phys. 41, 581.

Hartz, T. R. and Brice, N. M. (1967) The general pattern of auroral particle precipita-tion. Planet. Space Sci. 15, 301.

Holt, O., Landmark, B., and Lied, F. (1961) Analysis of riometer observationsobtained during polar radio blackouts. J. Atmos. Terr. Phys. 23, 229.

Jelly, D. H., Matthews, A. G., and Collins, C. (1961) Study of polar cap and auroralabsorption. J. Atmos. Terr. Phys. 23, 206.

Jelly, D. H., McDiarmid, I. B., and Burrows, J. R. (1964) Correlation between inten-sities of auroral absorption and precipitated electrons. Can. J. Phys. 42, 2411.

Jelly, D. H. (1970) On the morphology of auroral absorption during substorms. Can. J.Phys. 48, 335.

Kavadas, A. W. (1961) Absorption measurements near the auroral zone. J. Atmos.Terr. Phys. 23, 170.

Leinbach, H. and Basler, R. P. (1963) Ionospheric absorption of cosmic radio noise atmagnetically conjugate auroral zone stations. J. Geophys. Res. 68, 3375.

Little, C. G. and Leinbach, H. (1958) Some measurements of high-latitude ionosphericabsorption using extraterrestrial radio waves. Proc. IRE 46, 334.

Little, C. G., Schiffmacher, E. R., Chivers, H. J. A., and Sullivan, K. W. (1965) Cosmicnoise absorption events at geomagnetically conjugate stations. J. Geophys. Res. 70,639.

Nielsen, E. (1980) Dynamics and spatial scale of auroral absorption spikes associatedwith the substorm expansion phase. J. Geophys. Res. 85, 2092.


Parthasarathy, R. and Berkey, F. T. (1965) Auroral zone studies of sudden onset radiowave absorption events using multiple station and multiple frequency data. J. Geophys.Res. 70, 89.

Parthasarathy, R., Berkey, F. T., and Venkatesan, D. (1966)Auroral zone electron fluxand its relation to broadbeam radiowave absorption. Planet. Space Sci. 14, 65.

Penman, J. M., Hargreaves, J. K., and McIlwain, C. E. (1979) The relation between 10to 80 keV electron precipitation observed at geosynchronous orbit and auroral radioabsorption observed with riometers. Planet. Space Sci. 27, 445.

Pudovkin, M. I., Shumilov, O. I., and Zaitseva, S. A. (1968) Dynamics of the zone ofcorpuscular precipitations. Planet. Space Sci. 16, 881.

Ranta, H., Ranta, A., Collis, P. N., and Hargreaves, J. K. (1981) Development of theauroral absorption substorm: studies of the pre-onset phase and sharp onset using anextensive riometer network. Planet. Space Sci. 29, 1287.

Stauning, P. and Rosenberg, T. J. (1996) High-latitude daytime absorption spikeevents. J. Geophys. Res. 101, 2377.

7.3 The polar cap eventAkasofu, S.-I. and Chapman, S. (1972) Solar–Terrestrial Physics. Oxford UniversityPress, Oxford.

Bailey, D. K. (1959) Abnormal ionization in the lower ionosphere associated withcosmic-ray flux enhancements. Proc. IRE 47, 255.

Bakshi, P. and Barron, W. (1979) Prediction of solar proton spectral slope from radioburst data. J. Geophys. Res. 84, 131.

Castelli, J. P., Aarons, J., and Michael, G. A. (1967) Flux density measurements ofradio bursts of proton-producing flares and nonproton flares. J. Geophys. Res. 72,5491.

Chivers, H. J. A. and Hargreaves, J. K. (1965) Conjugate observations of solar protonevents: delayed ionospheric changes during twilight. Planet. Space Sci. 13, 583.

Collis, P. N. and Rietveld, M. T. (1990) Mesospheric observations with the EISCATUHF radar during polar cap absorption events: 1. Electron densities and negativeions. Ann. Geophys. 8, 809.

Gillmor, C. S. (1963) The day-to-night ratio of cosmic noise absorption during polarcap absorption events. J. Atmos. Terr. Phys. 25, 263.

Hargreaves, J. K., Ranta, H., Ranta, A., Turunen, E., and Turunen, T. (1987)Observation of the polar cap absorption event of February 1984 by the EISCAT inco-herent scatter radar. Planet. Space Sci. 35, 947.

Hargreaves, J. K., Shirochkov, A. V., and Farmer, A. D. (1993) The polar cap absorp-tion event of 19–21 March 1990: recombination coefficients, the twilight transition andthe midday recovery. J. Atmos. Terr. Phys. 55, 857.

Hultqvist, B. (1969) Polar cap absorption and ground level effects. Solar Flares andSpace Research (eds. C. de Jager and Z. Svestka), p. 215. North-Holland, Amsterdam.

Jackman, C. H., Fleming, E. L., and Vitt, F. M. (2000) Influence of extremely largeproton events in a changing stratosphere. J. Geophys. Res. 105, 11659.


Leinbach, H. (1967) Midday recoveries of polar cap absorption. J. Geophys. Res. 72,5473.

Obayashi, T. (1959) Entry of high energy particles into the polar ionosphere. Rep.Ionosphere Space Res. Japan 13, 201.

Ranta, H., Ranta, A., Yousef, S. M., Burns, J., and Stauning, P. (1993) D-region obser-vations of polar cap absorption events during the EISCAT operation in 1981–1989.J. Atmos. Terr. Phys. 55, 751.

Reagan, J. B. and Watt, T. M. (1976) Simultaneous satellite and radar studies of the D-region ionosphere during the intense solar particle events of August 1972. J. Geophys.Res. 81, 4579.

Reid, G. C. (1961) A study of the enhanced ionisation produced by solar protonsduring a polar cap absorption event. J. Geophys. Res. 66, 4071.

Reid, G. C. (1967) Ionospheric disturbances. In Physics of Geomagnetic Phenomena(eds. Matsushita and Campbell), p. 627. Academic Press, New York.

Reid, G. C. (1986) Solar energetic particles and their effects on the terrestrial environ-ment. In Physics of the Sun (ed. P. A. Sturrock), vol. 3, p. 251. Reidel, Dordrecht.

Reid, G. C. and Sauer, H. H. (1967) The influence of the geomagnetic tail on low-energy cosmic-ray cutoffs. J. Geophys. Res. 72, 197.

Sauer, H. H. (1968) Nonconjugate aspects of recent polar cap absorption events.J. Geophys. Res. 73, 3058.

Shea, M. A. and Smart, D. F. (1977) Significant solar proton events, 1955–1969. InSolar–Terrestrial Physics and Meterology: Working Document II, p. 119. SCOSTEP.

Shea, M. A. and Smart, D. F. (1979) Significant solar proton events, 1970–1972. InSolar–Terrestrial Physics and Meterology: Working Document III, p. 109. SCOSTEP.

Shea, M. A. and Smart, D. F. (1995) Solar proton fluxes as a function of the observa-tion location with respect to the parent solar-activity. Adv. Space Res. 17, 225.

Smart, D. F. and Shea, M. A. (1989) Solar proton events during the past three solarcycles. Spacecraft and Rockets 26, 403.

Smart, D. F. and Shea, M. A. (1995) The heliolongitudinal distribution of solar-flaresassociated with solar proton events. Adv. Space Res. 17, 113.

Uljev, V. A., Shirochkov, A. V., Moskvin, I. V., and Hargreaves, J. K. (1995) Middayrecovery of the polar cap absorption of March 19–21, 1990: a case study. J. Atmos.Terr. Phys. 57, 905.

Weeks, L. H., CuiKay, R. S., and Corbin, J. R. (1972) Ozone measurements in themesosphere during the solar proton event of 2 November 1969. J. Atmos. Sci. 29,1138.

7.4 Coherent scatter and the polar mesosphere summer echoCho, J. Y. N. and Kelley, M. C. (1993) Polar mesosphere summer radar echoes: obser-vations and current theories. Rev. Geophys. 31, 243.

Collis, P. N. and Röttger, J. (1990) Mesospheric studies using EISCAT UHF and VHFradars: a review of principles and experimental results. J. Atmos. Terr. Phys. 52, 569.

Czechowsky, P., Reid, I. M., Ruster, R., and Schmidt, S. (1989) VHF radar echoes


observed in the summer and winter polar mesosphere over Andøya, Norway.J. Geophys. Res. 94, 5199.

Ecklund, W. L. and Balsley, B. B. (1981) Long-term observations of the Arctic meso-sphere with the MST radar at Poker Flat, Alaska. J. Geophys. Res. 86, 7775.

Hoppe, U.-P., Hall, C., and Röttger, J. (1988) First observations of summer polar mes-ospheric back-scatter with a 224 MHz radar. Geophys. Res. Lett. 15, 28.

Kelley, M. C., Farley D. T., and Röttger, J. (1988) The effect of cluster ions on anoma-lous VHF back-scatter from the summer polar mesosphere. Geophys. Res. Lett. 14,1031.

Palmer, J. R., Rishbeth, H., Jones, G. O. L., and Williams, P. J. S. (1996) A statisticalstudy of polar mesosphere summer echoes observed by EISCAT. J. Atmos. Terr. Phys.58, 307.

Röttger, J. (1994) Polar mesosphere summer echoes: dynamics and aeronomy of themesosphere. Adv. Space Res. 14, 123.

Röttger, J., Rietveld, M. T., La Hoz, C., Hall, T., Kelley, M. C., and Swartz, W. E.(1990) Polar mesosphere summer echoes observed with the EISCAT 993-MHz radarand the CUPRI 46.4-MHz radar, their similarity to 224-MHz radar echoes, and theirrelation to turbulence and electron density profiles. Radio Sci. 25, 671.


Chapter 8

High-latitude radio propagation: part 1 – fundamentalsand experimental results

There cannot be a greater mistake than that of looking superciliouslyupon practical applications of science. The life and soul of science is itspractical application

Lord Kelvin

8.1 Introduction

Propagation of radio waves from ELF to UHF frequencies via the high latitudeionosphere is sometimes radically different from propagation at middle and lowlatitudes. This is primarily due to the fact that the magnetic field-lines at “cor-rected geomagnetic latitudes” greater than 60° allow solar and magnetosphericparticles and plasma to penetrate into the ionosphere. This results in the creationof many large-magnitude irregularities with scale sizes from meters to kilometers,most of which are aligned with the geomagnetic field in the auroral E and Fregions. There are also sun-aligned arcs plus patches and blobs of ionization in thepolar F region. Because of the extremely wide variation in ionospheric characteris-tics at high latitudes, this chapter contains many examples of actual propagationbehavior.

In contrast, it should also be mentioned that there is a wide spectrum of less-intense ionospheric irregularities in the mid-latitude ionosphere. Since mostantennas used for communication and ionospheric sounding up until the 1960shad rather large antenna half-power beamwidths (typically 50°50° in azimuthand elevation), these small irregularities were not observed. Starting in the early1960s, several very-high-resolution HF backscatter sounders were constructedand employed in ionospheric research (see descriptions of the systems and resultsby Croft, 1968, and Hunsucker, 1991, Ch. 4). These systems revealed a plethoraof echoes from irregularities, mostly of meter wavelengths. Hunsucker (1971),using a high-resolution HF sounder, found that irregularities of varying scale sizeand apparent motion were present in about 90% of the observations made duringalmost half a sunspot cycle in the mid-latitude ionosphere.

417

Starting in the late 1960s, several programs for prediction of HF ionosphericpropagation were developed for “main-frame” computers, followed in the mid-1970s by PC-based programs. These programs were intended to provide HF-communications-circuit planners with median values of maximum, minimum, andoptimum working frequencies as a function of the number of sunspots (or solarflux), time of day, season, path-length, and orientation. Further refinement ofthese programs made it possible to specify the type of antenna, transmitter power,receiver sensitivity, and receiver-location noise level. The actual prediction of“skywave” field strength has not turned out to be quantitatively accurate becauseof the difficulty of specifying mode structure, polarization loss, and non-deviative(and deviative) absorption loss (Hunsucker, 1992). Only two extant HF-propagation-prediction programs include high-latitude ionospheric effects, andthey are either qualitative or have not been adequately validated to inspire users’confidence.

The effects of the polar cap and the auroral-oval ionosphere on signals ofvarious frequencies differ substantially, and the morphology, phenomenology,and physics of these regions have been described in considerable detail in previouschapters of this book. (The fundamentals of the propagation of EM waves aredescribed in Chapter 3, and the radio techniques for studying the high-latitudeionosphere have been described in Chapter 4.) The most profound effects of thehigh-latitude ionosphere on radio-wave propagation occur during geomagneticstorms and substorms (see Chapter 6).

The emergence and proliferation of shortwave (SW) international broadcast-ing stations during the period 1930–1940 brought to the attention of some broad-casters the high unreliability of polar HF paths. Much of the history ofhigh-latitude radio-propagation research has been summarized in several booksand review papers (Rawer, 1976; Hunsucker, 1967; Hunsucker and Bates, 1969;Davies, 1990; Hunsucker, 1992). Other sources of research results on high-latitudeHF propagation are in the Proceedings of the Ionospheric Effects Symposium(IES), held every three years in Alexandria, Vancouver, USA, and in the books ofabstracts published at national meetings and general assemblies of the URSI.Research into high-latitude propagation began in earnest during the IGY andInternational Geophysical Cooperation (IGC) (1957–1959), which (fortuitously)coincided with the record-breaking sunspot maximum of cycle 19.

During the period from the end of the Second World War through about 1975,it was thought by some that the best way to avoid the sometimes disastrous effectsof auroral and polar ionospheric disturbances on high-latitude paths was simplyto avoid these paths most of the time. Starting about 1975 there was a renewal ofinterest in studying high-latitude ionospheric effects because of the deployment ofsome sophisticated ELF/VLF and VHF/UHF satellite navigation and “over-the-horizon (OTH)” HF radar systems. During the “Cold War” (c. 1948–1991), boththe USA and the USSR extensively deployed very sophisticated radio communi-

418 High-latitude propagation: 1

cations and navigation systems in the Arctic regions, so considerable research wascarried out in these areas of technology. Some of the research remains classified,but much was published in NATO and AGARD (Advisory Group for AerospaceResearch and Development) conference reports (Landmark, 1964; Lied, 1967;Folkestad, 1968; Deehr and Holtet, 1981; Soicher, 1985).

The use of rather sophisticated modulation techniques like frequency-shift-keying (FSK), coded-pulses, frequency-hopping, and spread-spectrum on HFpolar circuits has also prompted recent research on devising realistic atmosphericmodels, propagation-prediction techniques and rapid circuit sounding andswitching (Goodman, 1992).

The atmospheric density, temperature, composition, and dynamics fromground level up to ionospheric heights differ at high latitudes (sometimes drasti-cally) from the values for mid-latitude and equatorial regions (see Chapters 1, 2,5, 6, and 7). We will address the effects of these variations on specific frequencybands from ELF through UHF.

8.2 ELF and VLF propagation

Propagation in this part of the radio spectrum is best described and understoodby invoking the Earth–ionosphere waveguide mode (Watt, 1967; Wait, 1970;Davies, 1970; Davies, 1990, Ch. 10) or the wave-hop (Berry, 1964) mode. Theeffectiveness of the “waveguide” mode depends upon the long- and short-termvariations in conductivity of the Earth’s surface and the lower ionosphere (Dregion).

VLF propagation, in general, is characterized by relatively low path attenua-tion (2–3 dB per megameter, where 1 Mm1000 km), is relatively stable with time,and the phase delay during propagation follows a predictable diurnal pattern.Propagation distances from 5000 to 20000 km are realized; however, atmosphericnoise levels are high – thus decreasing the signal-to-noise ratio (SNR), and thesignal bandwidths are low (20–150 kHz). Large antennas and high power trans-mitters are required to achieve a usable SNR at long distances. Because of thelarge wavelengths, it is economically and physically difficult to erect adequateantennas, so practical antennas have radiation efficiencies of 10%–20% – thusrequiring high transmitter power.

The OMEGA VLF navigation system is deployed globally and has for manyyears been an important and much utilized navigation aid. OMEGA operates atthe low end of the VLF band (10–14 kHz) and is still used as a backup naviga-tional aid, even with the advent of the GPS satellite navigational system. (See Ch.10 of Davies, 1990 for further details of ELF–VLF–LF propagation.)

The variation of phase speed for a perfectly conducting Earth is shown inFigure 8.1, and the actual phase variation of signals from WWVL at f20 kHz,


on a 113-km path is shown in Figure 8.2. At high latitudes, some irregularities inthe lower ionosphere can influence VLF propagation (Wait, 1991).

The lowest frequencies used for communication purposes are the ELF band(3–300 Hz), which is used primarily for very-low-data-rate communication tosubmerged submarines. One operational transmitter is located at the US Navy’sWisconsin Test Facility and was described in the military literature as the“Project Sanguine/Seafarer/ELF.” The effective radiated power (ERP) is 0.25 Win the 40–50-Hz band and 0.5 W in the 70–80-Hz band. In practice, the ELFsystem is a “bell-ringer” that signals the submerged submarine to ascend to anappropriate depth to receive communication on VLF. For more details on ELFcommunication, see Bannister (1993) and Davies (1990, Ch. 10). At high lati-tudes, lower-ionospheric irregularities can effectively change the waveguide char-acteristics (Wait, 1970; Hunsucker, 1992). Fraser-Smith and Bannister (1998)recently measured ELF transmissions from a heretofore-unknown source, whichthey identified as a Russian ELF transmitter operating on 82 Hz, located on theKola Peninsula at 69° N, 33° E. This signal was received as far away as Dunedin,New Zealand, which was the antipodal point (D16.5 Mm) and at ArrivalHeights, Antarctica (D18 Mm). Figure 8.3 shows the average amplitude spec-trum of lower ELF radio noise at the Sondrestrom receiving site in Greenlandduring January 1990.


Figure 8.1. The variation of phase-speed with VLF frequency for a perfectly conductingEarth, sg$, r2105 for (a) mode number 1 and (mode number 2) (from Davies, 1990).


Figure 8.2. The diurnal variation of WWVL at 20 kHz over the 113-km path from FortCollins to Wiggins, CO (from Davies, 1990).

Theoretical and measured values of the Kola-Peninsula transmitter facility82-Hz field strength versus distance are shown in Figure 8.4. Note the predictedand measured increases at antipodal distances.

Another recent paper (Chrissan and Fraser-Smith, 1996) presents some newinformation on the noise-envelope amplitude-probability-distribution models ofradio noise at VLF/ELF frequencies. Three noise models are used for comparisonof data and the two which most closely describe the data are the “Hall” and the“-stable” models and the authors conclude that the -stable should be used inthe polar regions, except at the peak of the diurnal and seasonal storm cycle.

The effects of the high-latitude ionosphere on ELF signals during PCAs (SPEs)have been described using full-wave theory for the TEM mode with measurementsmade in the Gulf of Alaska. During the 23 November 1982 SPE event, a submarine-borne receiver measured an unusually severe reduction in signal, which was attrib-uted to lateral refraction bending the signal path away from the polar-cap boundaryand into the central cap – where the phase velocity of the TEM mode is slowest.Figure 8.5 shows the geometry of the ELF path from the Western Test Facility to the


Figure 8.3. The average amplitude spectrum of the lower ELF band. Note the RussianELF transmission at 82 Hz and the and Bannister power-line frequencies (50 and 60 Hz)and their harmonics (from Fraser-Smith, 1998).

Gulf of Alaska and Figure 8.6 illustrates the variation of the 76-Hz signal. ELF raytrajectories for weak, moderate, and strong SPEs on the path from the Western TestFacility to the Gulf of Alaska are shown in Figure 8.7.

SPEs also have a profound effect on VLF polar transmissions, and one of thefirst documentations of these events was presented by Bates (1962), who describedthe effects of a relatively weak SPE on the VLF signal from England to Alaska.The 16.0-kHz signal from the GBR VLF station in Rugby, England was moni-tored at College, Alaska during the SPE event of 10 November 1961. Twentyminutes after the solar flare believed responsible, the GBR signal shifted phase byapproximately 250° and the amplitude decreased by 20 dB over a 1-h period.During this event the diurnal variations of phase and amplitude increased in mag-nitude and changed markedly from normal patterns, and the effective height ofthe VLF waveguide over the polar cap dropped to about 5 km below the normal


Figure 8.4. Measured and theoretical values of the KPTF 82-Hz signal strength versusrange (0°). CO, Connecticut; KB, King’s Bay, Georgia; SS, Søndrestrømfjord; HA,Hawai; DU, Dunedin; and AH, Arrival Heights.

D-region height. Systems such as the US Navy’s OMEGA VLF network dependupon phase differences for their navigational positional accuracy, so polar iono-spheric events can cause serious errors.

Figure 8.8 is a map of the Rugby, England to College, Alaska VLF propaga-tion path, the cosmic-noise absorption from the College, Alaska riometer for 10November 1961 is shown in Figure 8.9, and the amplitude and corrected phase ofthe GBR transmissions are shown in Figure 8.10.

The results of a three-year study of VLF propagation (during sunspotminimum) monitored at College, Alaska have been reported by Bates and Albee(1965) and Albee and Bates (1965). During that period, 1846 optically detectedsolar flares were observed on sunlit paths, of which 66 produced phase anomalieson the NBA (non-polar) path. Table 8.1 lists the frequencies of the VLF stationsmonitored during this study and Figure 8.11 is a map showing the propagationpaths. It can be seen that only the paths from GBR and NAA can truly be calledhigh-latitude paths, but, during major SPEs, small portions of the other paths maybe affected by the boundary of the PCA, as was the ELF transmission noted pre-viously.

Some typical navigation-location errors measured during the SPE of 6–9March 1970 (3.8 dB maximum), are shown in Figure 8.12. Documentation on thebehavior of ELF/VLF signals on polar paths during major SPEs (30 MHz absorp-


Figure 8.5. A map showing the geometry of the ELF path from WTF to the Gulf ofAlaska (from Field et al., 1985).


Figure 8.6. The 76-Hz signal received in the Gulf of Alaska (from Field et al., 1985).

Figure 8.7. Ray trajectories for the three SPE strengths (r2 Mm) (from Field et al., 1985).

Fig

ure

8.8

.M

ap s

how

ing

the

16.0

-kH

z pr

opag

atio

n pa

th f

rom

Rug

by, U

K t

o C

olle

ge, A

lask

a (a

fter

Bat

es, 1

961)

.


Figure 8.9. Cosmic-noise absorption from the 27.6-MHz College riometer (after Bates,1961).

1.5

1.0

0.5

0

AT

TE

NU

AT

ION

(d

B)

UNIVERSAL TIME14 16 18 20 22 00 02 04 06

Figure 8.10. The amplitude and corrected phase of the GBR 16.0-kHz signal received atCollege, Alaska on 10 November 1961 (after Bates and Albee, 1966).

Table 8.1. A list of VLF stations monitored at College, Alaska from 1961 to 1964

Station Frequency (kHz) Location Period recorded

NBA 18.0 Balboa, Panama August 1961–Dec 1963GBR 16.0 Rugby, England October 1961–1964NAA Various Cutler, Maine November 1962–1964NPM 19.8 Hawaii April 1962–1964NPG Various Jim Creek April 1963–December 1963WWVL 20.0 Fort Collins, Colorado January 1964–December 1964

Fig

ure

8.1

1.

VL

F p

ropa

gati

on p

aths

to

Col

lege

, Ala

ska

duri

ng t

he 1

961–

1964

stu

dy (

afte

r B

ates

and

Alb

ee, 1

966)

.

tion 10 dB) is difficult to find, but they should produce profound phase andamplitude variations.

The DECCA navigational system, which utilizes frequencies from 70 to100 kHz, is designed for high accuracy over medium ranges and depends upon thegroundwave for its accuracy, so we are not very concerned about high-latitudeionospheric effects. Another hyperbolic radio navigation system is the LORAN-Cglobal network, operating on 100 kHz, which also depends upon the groundwavefor its accuracy. Some LORAN-C systems developed in the 1980s augmented thereceiver by skywave signals in addition to groundwave and there were some indi-cations that errors of up to 20 km occurred during geomagnetic disturbances(Hunsucker, 1992)

8.3 LF and MF propagation

The basic propagation modes for LF through MF (300 kHz to 3 MHz) aregroundwave at all hours, augmented by skywave modes at night. Groundwavepropagation covers ranges of 1 km to several hundreds of kilometers from thetransmitter, with extended ranges over sea water and erratic results over moun-tainous terrain. The discontinued LORAN-A navigation system was a hyperbolic


Figure 8.12. OMEGA location errors in nautical miles during the SPE of 6–9 March 1970in Norway (from Larsen, 1979).

2

1

0

1

2

3

4

5

6

7

8

9

HYPERBOLICNORWAY–HAWAII)

RANGING(NORWAY)

30µs = 3 nm HYPERBOLIC6 nm RANGING

CORRECTED

ACTUAL

0 6 12 18 24 6 12 18 24 6 12 18 24 6 12 18 24 6 12 18 24

6 MAR 7 MAR 8 MAR 9 MAR 10 MAR

1970

LO

CA

TIO

N E

RR

OR

(n

m)

line-of-position system, which was primarily groundwave propagation but alsowas sometimes affected by skywave. Another primarily groundwave navigationalsystem (which is currently being phased out) was the Non-Directional-Beacon(NDB) system operating in the 250–450-kHz band, but again there do not seemto be documented examples of high-latitude ionospheric effects . . .

In the MF band (300 kHz to 3 MHz) – the US standard AM broadcast bandis 550 kHz to 1570 kHz – station frequencies (channels) are assigned on a ground-wave and skywave non-interference basis for each 10 kHz channel. In the conti-nental USA one is rarely out of groundwave range of several nearby commercialbroadcast stations and at night-time additional stations from distances of 1000km and greater are heard. From 1.6 to 3.0 MHz various land and maritime nav-igational and fixed services operate using the skywave mode but reliable propaga-tion is difficult because of D-region absorption, which is particularly high atauroral and polar latitudes. As an example, most of the fixed-service communica-tion services in Alaska at these frequencies have been discontinued.

In the northern tier of the USA it was necessary to modify the FCC mid-latitudefrequency assignment procedure to allow for auroral E-layer anomalous-propagation modes. A five-and-a-half-year investigation (for one half of solarcycle 21) of skywave transmissions from clear-channel 50-kW standard broadcaststations in the USA, including Alaska, and Canada received at Fairbanks, Alaskawas reported by Hunsucker et al. (1988). Some results of this investigation are thefollowing.

The site was located at the Ace Lake Field Site of the Geophysical Institute ofthe University of Alaska-Fairbanks at geographic coordinates of 64° 52 N lati-tude, 147° 56 W longitude at a north geomagnetic latitude of 64° 45 and a dip of76° 54. The receiving/recording system was built around a commercial general-purpose receiver modified for analog automatic-gain-control output and thereceiver frequency was automatically stepped through 16 channels every 5 min bythe system programmer. Digital tape-cassette recordings of signal amplitude werecontinuously made on ten or more standard broadcast stations, then the data weretransferred to standard-format computer tape for analysis on a VAX 11/780-785computer. A noise source was also recorded continuously for regular system cal-ibration and occasional aural checks were made to insure that the proper identifi-cation of individual stations was achieved. The three different antennas usedduring this program were carefully calibrated against each other on standardbroadcast-band groundwave and skywave transmissions. Other details of theinstrumentation are given in Hunsucker et al. (1987; 1988).

Figure 8.13 is a plot of sunspot-cycle variation during the course of the AlaskaMF experiment, with the average monthly number of sunspots varying from 140to 20. The range of geomagnetic activity was from Kp0 to Kp9, including thelargest geomagnetic storm previously recorded at the College, Alaska observatory(8–9 February, 1986).


Because of the comprehensive nature of the Alaska MF data set (MF skywavesignal strengths on paths inside, tangential to, and transverse to the auroral ovalfor a wide range of numbers of sunspots), we will present some of the salientresults. Table 8.2 lists the Fairbanks MF channel assignments for 1985.

There was no such thing as a “typical daily variation” for any of the MFsignals received at Fairbanks during this experiment because of the pronouncedseasonal, sunspot-cycle, and ionospheric storm (auroral) effects. To illustrate theeffects of auroral disturbances on the daily variations in MF skywave signalstrength, Figures 8.19(a)–(c) show the variations in signal strength and auroral-oval locations for selected days near the Fall equinox of 1985 (see page 000).Specifically, Figure 8.14 shows typical variations in signal for quiet magnetic con-ditions. The equinoctial recovery from the summer low field strength is quiteapparent. Figure 8.15 is a plot of the auroral oval at 1000 UT on 4 September,1985, coinciding with the peak diurnal signal strength in Figure 8.14. Note thatthe auroral oval is well poleward of any of the propagation paths monitored atFairbanks.

The greater variability in signal associated with higher local magnetic activity(College Ak20) is shown in Figure 8.16 and the auroral oval for 1000 UT is pre-sented in Figure 8.17, showing its equatorward expansion south of Fairbanks. Itshould also be remembered that the AA region extends 1°–2° equatorward of the


Figure 8.13. Solar-cycle variation during the period of the Alaska MF experiment.

June1976

June1977

June1978

June1979

June1980

June1981

June1982

June1983

June1984

June1985

June1986

June1987

June1988

June1989

June1990

200

180

160

140

120

100

80

60

40

20

Mean of Cycles 8–20

Solar Cycle 21Beginning June 1976

Sm

oo

thed

Rz

Solar Cycle 22Beginning September 1986

LEGEND= Observed Smoothed= Predicted Smoothed

visual auroral oval. The variability of the MF signal strengths is most probablydue to the increase in AA and sporadic-E ionization associated with the auroraloval.

Figure 8.18 shows the MF signal behavior for a quite disturbed day (College,Ak46) with extreme signal variation. The auroral oval extends well equator-ward of Fairbanks and probably affects all paths monitored. The channel-3(Anchorage, Alaska) path lies entirely inside the auroral oval and its extreme var-iability is probably due to intense “patches”of sporadic-E ionization. Signals onchannels 4 and 5, Edmonton, Alberta (Canada) and Casper, Wyoming, respec-tively, are from paths passing obliquely through the auroral oval and show pro-found absorption effects. The KFAX, San Francisco path (channel 6) is roughlyperpendicular to the auroral oval and its ionospheric reflection points are mainlyequatorward of the oval, so it is affected less than are channels 3–5. Channels 11and 14 (McGrath and Kotzebue, both in Alaska) behave similarly to Anchoragebecause the paths lie entirely inside the auroral oval. Channel 13 was


Table 8.2. Fairbanks MF receiver channelassignments for 1985a (from Hunsucker, 1988)

Channel Frequency (kHz) Station

0 450 b

1 1000 Noise-diode calibrator2 450 b

3 750 KFQD, Anchorage4 1260 CFRN, Edmonton5 1030 KTWO, Casper6 1100 KFAX, San Francisco7 450 b

8 1260 CFRN, Edmonton9 450 b

10 450 b

11 870 KSKO, McGrath12 750 KFQD, Anchorage13c 1170 KJNP, North Pole, Alaska14 720 KOTZ, Kotzebue15 1510 KGA, Spokane

Notes:a The top-loaded vertical antenna (TLVA) was utilizedfor the entire year.b Channel programmed to a quiet frequency, not activeat this time.c Channel assignment changed to KJNP from CHUOttawa on 3 July 1985.


Figure 8.14. Variations in MF-signal strength for broadcast stations monitored inFairbanks for a quiet equinoctial day in 1986. Fairbanks local time (150° Western MeridianTime) is 10 h less than UT (after Hunsucker, 1988).

Figure 8.15. The location of the auroral oval at 1000 UT on 4 February 1986 Q0. (afterHunsucker, 1988).


Figure 8.16. Signal behavior during a moderately disturbed equinoctial day (Ak20), 8September 1986 (from Hunsucker, 1988).

Figure 8.17. The location of the auroral oval for a moderately disturbed day, 8 September1985, at 1000 UT, Q4 (from Hunsucker, 1988).

programmed to receive the groundwave signal from a local 50-kW station, KJNP,but, when the station was “off the air” (0830–1330 UT), signals from an unknownAM station were intermittently received.

The sunspot cycle also exerts profound effects on the MF skywave-signalstrengths measured at Fairbanks, depending, of course, on the frequencies, thepath-lengths, and the orientations relative to the auroral oval. Tables 8.3 and 8.4show the sunspot-cycle effects on four selected paths.

The seasonal behavior of MF skywave signals received at Fairbanks for 1985(sunspot-minimum year) is shown in Table 8.5, from which it may be seen that,except for one or two exceptions, the highest signal strengths occurred in thewinter and the lowest signal levels occurred in the summer, with intermediatevalues during the equinoxes.

Effects of the great geomagnetic storm of February 1986 on MF skywave recep-tion at Fairbanks were documented by Hunsucker et al. (1987) and, since this wasprobably the most systematic investigation, we will present some of the salienteffects. The magnetic storm of 8–9 February, 1986 was one of the largest for theprevious 40 years and especially dynamic at high latitudes. The College USGSObservatory measured an H-component maximum excursion of 6110 nT and thelocal and planetary K indices were 9 for several hours on 8 February, 1986. The


Figure 8.18. MF-signal behavior during a disturbed day (after Hunsucker, 1988).


Table 8.3. Characteristics of four selected MF propagation paths (fromHunsucker, 1988)

Power Frequency output

Call letters Location (kHz) (KW) Path-length and remarks

KSKO McGrath, 870 5 Short north–south Alaska auroral path.

KTWO Casper, 1030 50 D3553 km, long path. Wyoming One ionospheric

reflection point in theauroral oval duringmoderately disturbedconditions.

KFAX San Francisco, 1100 50 D 3464 km, long path. California One ionospheric

reflection point in theauroral oval duringdisturbed conditions.

KGA Spokane, 1510 50 D 2640 km. Similar toWashington the KTWO path.

Table 8.4. Sunspot-cycle effects – a comparison of changes in signal strength onfour paths from 1981 to 1985 (midwinter) (from Hunsucker, 1988)

1981 (Average 1985 (Averagerelative international relative international

sunspot number sunspot number Increase in signal 147) 12) strength 1981–1985

Signal Signal Signal Signal Signal Signal present maximum present maximum present maximum

Station (%)a (V)b (%)a (V)b,c (%)a (dB)b,c

KSKO 53 7 75 60 22 18.7KTWO 30 8 54 8 24 0KFAX 62 9 71 70 8 17.8KGA 50 7 46 8 4 1.2

a Percentage of operating period when signal was present.b All signal levels are referred to receiver input.c See the text for a discussion of the increase in signal.

most pronounced effects were on the 1984-km path from the 50-kW station,CERN, in Edmonton, Alberta at 1260 kHz. Figures 8.19(a), (b), and (c) are mapsshowing the Edmonton–Fairbanks propagation path in relation to the auroraloval for periods before, during, and after the storm and the amplitude of the signalis displayed just below each map. Relatively normal night-time skywave propaga-tion is seen in Figure 8.19(a), two days before the storm when the auroral oval waspoleward of the path. During the maximum phase of the storm – shown in Figure8.19(b) – there was almost complete absorption on the path. Three days after thestorm, the skywave signal had almost returned to its pre-storm level (Figure8.19(c)).

Some of the field-strength measurements collected at Fairbanks have beencompared with field strengths predicted by various methods and the full resultshave been published in FCC Rule Change Docket 20642. Table 8.5 shows someselected examples of comparisons between measured and modeled values.

Some conclusions of the Alaska MF study are as follows.

(1) The “high-end” commercial electronically scanned receiver, noise calibra-tion, and digital data-recording systems worked exceptionally well duringthe five and a half years of the experiment.

(2) An absolutely necessary requirement for a program of this sort is regularcareful aural monitoring in order to positively identify the transmitters.

(3) The selection of a “radiofrequency-interference-quiet” remote receivingsite in Alaska produced excellent high-SNR data.

(4) When the MF skywave propagation paths traversed the auroral oval therewere profound variations in signal as a function of frequency, geomagneticactivity, time of day, and season.


Table 8.5. Measured and predicted field strengths for 1987

Median field strength for 1987dB (1 V m1)

Method of prediction Path 1 Path 2

Measured 26.8 34.7FCC curve (Also used by region 2) 33.2 54.8Cairo curve 40.2 55.0CCIR method (Recommendation 435) 16.2 54.4Modified FCC method 27.7 44.1

Notes:Measured values are for the sixth hour after sunset at the mid-point of the path.Path 1 – San Francisco to Fairbanks, 3464 km, KFAX, 1100 KHz, 50 kW.Path 2 – Anchorage to Fairbanks, 431 km, KFQD, 750 kHz, 10 kW.

Fig

ure

8.1

9.

Beh

avio

r of

the

MF

sig

nal f

rom

Edm

onto

n, A

lber

ta a

nd F

airb

anks

, Ala

ska

befo

re, d

urin

g an

d af

ter

the

grea

t ge

omag

neti

c st

orm

of

8–9

Feb

ruar

y 19

86 (

afte

r H

unsu

cker

et

al.,

1987

).

(a)

(b)

(c)

(5) These results prompted the FCC to issue new engineering skywave curvesdescribing possible skywave interference between standard AM broadcast-ing stations in the northern tier of the USA, including Alaska, andCanada, thus making channel assignments more realistic.

8.4 HF propagation

The ITU HF band (3–30 MHz) is basically a skywave band day and night and isused for broadcasting, point-to-point, and surveillance (actually, the range of2–30 MHz is primarily propagated by skywave). At mid-latitudes the average char-acteristics of HF propagation are reasonably predictable, except during geomag-netic storms. Fortunately, there are several books describing basic HFpropagation (Maslin, 1987; McNamara, 1991; Davies, 1990, Ch. 6; Goodman,1992) for those wanting more detailed accounts.

8.4.1 Tests carried out between Alaska and Scandinavia onfixed frequencies

Serious and methodical investigations of the behavior of trans-polar HF propa-gation on paths between Scandinavia and Alaska were initiated in the mid-1950s.Although most of the early CW transmissions were degraded by SW interference,subsequent transmissions utilized pulses, which were much more resistant to theSW interference. Up until about 1969, the results of most of the trans-polar HFpropagation experiments were published in institutional reports, not in the “openliterature,” and, because of the importance of these data, we will present selectedextracts from these experiments starting in 1956. It was fortuitous that the cali-brated pulsed HF trans-polar transmissions began just before the maximum ofsunspot cycle 19 – the highest maximum on record! The following results areextracted from a report by the Geophysical Institute of the University of Alaska(Owren et al. 1959) and represent HF propagation conditions near the maximumof sunspot cycle 19.

Early in 1956 (sunspot number (SSN) 50) the Norwegian ResearchEstablishment (NDRE) and the UAF Geophysical Institute agreed to cooperatein a program of test transmissions across the north polar region in order to inves-tigate the propagation conditions. The first propagation test was made using a3-kW CW transmission and a FSK teletype signal on 3.3 and 7.7 MHz fromFairbanks, Alaska and a 5-min h1 transmission of a 5-kW CW signal on 5.9MHz from Harstad in northern Norway. In addition, the receiver stations inAlaska were to monitor the 100-kW broadcast transmissions on 629 kHz fromVigra in southern Norway. Receiver stations were set up at College and Barrow inAlaska and at Harstad, Norway, as well as on west Spitzbergen, Svalbard. As the


test progressed, modifications to the original plan had to be made. The 3.3-MHztransmission from Fairbanks had to be canceled because of interference withother services. The Norwegian receiver stations were unable to pick up the 7.7-MHz transmission and on 12 July this was replaced by a pulsed transmission on12.3 MHz beamed from College to northern Norway. This signal was immediatelypicked up and identified by the Spitzbergen station, illustrating the advantage ofpulsed transmissions.

The College receiver station was unable to identify the 5.9-MHz transmissionfrom Harstad, but Barrow succeeded after coming into operation on 13 July. Thesignal was never received well, even at Barrow. Completely negative results wereobtained regarding the Vigra MF transmissions, both at Barrow and College

A supplementary program for monitoring Norwegian and Russian MF andHF broadcast transmitters in the frequency range 0.5–22 MHz was put into effectat College on 6 July and at Barrow on 13 July. Good results were obtained for theNorwegian SW transmissions at 17.825 MHz from Frederickstad in southernNorway.

The July 1956 test showed clearly the superiority of pulsed signals over FSKand CW types of transmission and further indicated that future tests should beconcentrated on frequencies in the HF band. Several other monitoring tests werecarried out during 1956 and 1957, with rather inconclusive results, but the fourthand fifth tests in January and February 1958 (SSN200.9) proved to be more suc-cessful. During this part of the IGY a three-frequency HF backscatter sounderoperating on 12, 18, and 30 MHz was located at College, Alaska (Peterson et al.,1959). This backscatter sounder had three three-element Yagi antennas mountedon a single rotating mast with transmitter pulse outputs of 4 kW (the antennarotated at 1 RPM). Another pulse transmitter at College also operated on 6 MHzusing a halfwave dipole antenna. The Geophysical Observatory at Kiruna,Sweden participated in the January–February 1958 tests, with encouragingresults.

Specifically, it was found that the 12-MHz signals could be picked up even whenthe antenna was rotating, in fact, the pulse emission from College could bereceived throughout the rotation cycle. Later it was found that the 18-MHz pulsedtransmission could similarly be received over half the rotation cycle. The 30-MHzsignals were also found to be detectable at Kiruna, but intermittently rather thanregularly. The Kiruna Geophysical Observatory thereafter set up a program ofcontinuous monitoring of the College pulsed transmissions on 12, 18, and 30MHz starting in May 1958. The College transmissions on 12, 18, 24, and 30 MHzwere recorded at Kiruna utilizing a rhombic antenna connected to a communica-tion receiver modified for pulse reception. The receiver output was displayed onan oscilloscope and recorded photographically.

We will include many examples of HF signal behavior over paths of variouslengths, at various frequencies with various orientations with the auroral oval and


polar-cap ionosphere over a wide range of geophysical activity in order to fullyillustrate the extreme variability of high-latitude HF propagation.

Analysis of simultaneous College HF backscatter and the signal received inKiruna revealed that the three-hop mode (not the two-hop mode) was predomi-nant on the 5300-km path illustrated in Figure 8.20.

HF trans-polar propagation data for the maximum of sunspotcycle 19

In total 672 h of simultaneous recordings of received signal strengths at Kirunaand groundscatter echoes observed at College for the month of December 1958were analyzed. Approximately 30% of the 672 h of data was lost due to SW inter-ference at the frequencies being used and the usual equipment failures. It washoped that groundscatter observed on the HF propagation paths would be a goodindicator of forward-propagation conditions, so groundscatter observed fromCollege appearing within 30° of the Kiruna azimuth in the 1000–1900-km rangewas interpreted as the first hop of a three-hop mode. Similarly, groundscatter inthis direction in the 2000–3000-km range was considered as the first hop of a two-hop mode.


Figure 8.20. The College–Kiruna propagation path (D5300 km) (after Owren et al.,1959).


Figure 8.21. A comparison of the College, Alaska signal received at Kiruna and Collegegroundscatter for 18 MHz on 4 December 1958 (after Owren et al., 1959).

Figure 8.22. The average signal strength at Kiruna, Sweden for the month of December1958 (from Owren et al., 1958).

Periods of high signal strength at Kiruna were sometimes observed during theinterval 07–18 UT when there were no groundscatter echoes in the direction ofpropagation. This could indicate that the echoes were present but were below thesensitivity threshold of the receiver or that a one-hop Pedersen propagation modewas operative. The histogram for 18 MHz in Figure 8.21 illustrates this conditionduring the interval 12–15 UT (for SSN180.5). When the College transmissionswere “readable” at Kiruna (the photographic records of the signals were scaled inarbitrary units from zero to three and a “readable” signal is defined as one ofstrength &0.5), groundscatter echoes from the polar region indicated the relativeoccurrence of the following propagation modes (Table 8.6).

As a result of this and other groundscatter–signal-strength comparisons, it wasconcluded that groundscatter was not a very good indicator for the propagationof HF signals at high latitudes.

The histogram in Figure 8.22 shows the average signal strengths at Kiruna forthe month of December 1958 for 12 and 18 MHz. The pronounced dip at 1500UT in the 12- and 18-MHz histograms occurs during the period of maximuminterference at both Kiruna and College. The diurnal maximum of D-regionabsorption in the region north of College also occurs during this interval.

Favorable circumstances made the month of August 1959 (SSN151.3) par-ticularly suitable for a detailed study of the effects of solar-particle precipitationand radiation on high-latitude HF propagation. First, the solar events occurredafter a quiet period with an unusual distinctiveness and included both low- andhigh-energy particle precipitation. Secondly, comprehensive geophysical observa-tions obtained during the IGY were available, including radiation measurementsmade by Explorer VI, absorption measurements from an extended chain of sta-tions, and good coverage of the arctic by ionosonde. Thirdly, a network of arcticand subarctic experimental HF circuits was in operation through the joint effortsof the University of Alaska Geophysical Institute, the Kiruna Geophysical


Table 8.6. The relative occurrence of propagationmodes and groundscatter

Percentage oftime during which

Propagationmode occurred

mode 12 MHz 18 MHz

Three-hop 65 61Two-hop 11 15No indication 24 24(polar groundscatter absent)

Observatory, Sweden, and the Radioscience Laboratory of Stanford University.The entire program was sponsored by the Electronics Research Directorate of theUS Air Force Cambridge Research Laboratories, Massachusetts. Historicallythen, this was probably the first extensive, coordinated, well-instrumented HFpropagation experiment at high latitudes during a disturbed period.

The experimental circuits utilized the Stanford IGY backscatter sounders for12, 18, and 30 MHz located at College, Alaska, Thule, Greenland, and Stanford,California. The receiving stations were in operation at Kiruna, Sweden on thedistant side of the arctic region and at Boston, College, and Stanford on theNorth-American continent. We consider here only the 12- and 18-MHz pulsetransmissions from College, Alaska – since the College receiver station for theThule and Stanford transmissions was not operational in August 1959. TheCollege sounder transmitted 1-ms pulses at 18.75 pulses s1 with a peak power of4–5 kW, using the rotating Yagi-antenna system.

The College–Kiruna great-circle path is a trans-polar circuit of length 5300 kmpassing within a few degrees of the north geographic pole and essentially insidethe auroral zone – considering the low radiation angle (about 10°) and the iono-spheric reflection points. The College–Boston great circle path is 5300 km longand crosses the auroral zone tangentially. The College–Stanford path is 3500 kmlong and lies outside the auroral zone under normal conditions. Figure 8.23 is amap showing the propagation paths and supporting ionospheric observations.

The first 13 days of August 1959 were characterized by low solar activity andmagnetically quiet conditions, in particular the days of 11–14 August. On 14August and again on 18 August there occurred major solar flares in an activeregion that crossed the solar central meridian on the 16th. Many lesser flares wereobserved in this region during its passage over the Sun’s disk. The two major flareswere both followed by geomagnetic storms that together account for the 5 days ofthe month designated as magnetically disturbed. The second flare was also accom-panied by cosmic-ray emission, causing a weak (1-dB) PCA.

The first major flare, of importance 2, occurred on 14 August at 0040 UT andwas accompanied by a sudden cosmic-noise absorption and a gradual SW fadeout(SWF). A severe sudden-commencement magnetic storm, which must be assumedto have been due to the low-energy particle emission associated with the 2 flare,started on 16 August at 0404 UT and lasted about 40 h, until the evening of the17th. Riometer observations indicated that emissions of energetic particlesoccurred between geomagnetic latitudes of 52°and 62° with peak intensity around57° or 58°. Thus, the initial impact of the auroral particles was in subauroral lat-itudes and the severe magnetic storm typically caused a southward shift andexpansion of the zone of activity. Following the initial phase of the magneticstorm, AA was consistently greater at College than it was at King Salmon, Alaskaduring the remainder of the storm. This confirmed the findings of Basler, namelythat AA peaks a few degrees equatorward of the visual auroral oval, which findingis of considerable importance to studies of auroral HF propagation.


The second major flare, of importance 3, started on 18 August at 1015 UT andwas accompanied by a SWF as well as by solar-noise emission, as is evident fromthe Thule riometer record. The Thule measurements show that a weak PCA eventstarted at about 1200 UT. The PCA reached a maximum of 1.5 dB at 27.5 MHzon 19 August at about 2000 UT, and the recovery was completed by 21 August at1200 UT. A sudden-commencement magnetic storm, of moderately severe inten-sity, started on 20 August at 0412 UT and lasted into the forenoon of the 24th.The solar–terrestrial events of August 1959 and the radio propagation behavioron several trans-polar paths are shown in Figure 8.24.

In general, it is seen that peaks in AA were the most important factor explain-ing decreasing signal strength on most of the HF paths. It should be emphasizedthat this was a very small PCA. The behavior of HF propagation on the three


Figure 8.23. A map of propagation paths showing the auroral zone based on Vestine’s iso-chasms (after Owren et al., 1963).

Fig

ure

8.2

4.

Sola

r–te

rres

tria

l con

diti

ons

and

tran

spol

ar H

F r

adio

pro

paga

tion

for

near

-sun

spot

-m

axim

um c

ondi

tion

s on

15

and

18 A

ugus

t 19

59 (

from

Ow

ren

et a

l., 1

963)

.

(b)

circuits shown in Figure 8.25 during the magnetic storm of 16–17 August 1959 isillustrative of near-sunspot-maximum conditions.

8.4.2 Tests involving transmission between Alaska and thecontinental USA

The College–Stanford circuit (basically a mid-latitude path,D3500 km)

During the undisturbed periods of August, the 12-MHz transmission was receivedwith consistently high signal strength at all hours of the day at Stanford. Thebehavior of the circuit is illustrated in Figure 8.26, a contour plot of signal outage


Figure 8.25. A map showing three great-circle HF-propagation paths and the normal andexpanded auroral zones (after Owren et al., 1963).

in which the cross-hatched area indicates when the signal strength was less than 6dB above 1 V.

The plot shows that normally there were no diurnal outage periods. Thegroundscatter observed simultaneously at College indicated a two-hop F-layermode. This implies that the propagation path traverses the D layer at about geo-magnetic latitudes of 64°, 57°, 55°, and 45° N. A sudden circuit black-out startedat the time of onset of auroral absorption at King Salmon, Alaska (geomagneticlatitude 57.4°). There were some temporary recoveries during the storm, whichappear to be reasonably well related to the decreases in absorption at King Salmonif some allowance for longitude difference were made. The final recovery tookplace on 18 August at 0900 UT. There were no 18-MHz data due to interferenceduring this period.

The College–Boston circuit (tangential to the auroral oval,D5300 km)

The 12-MHz circuit had a diurnal black-out of duration about 5 h during theundisturbed period 7–15 August. The ionospheric data indicated that the signaloutage was controled by AA of the outgoing signal near College. Consistently, themajor black-out during the storm period started on 16 August at 1000 UT, 4 hafter the College–Stanford 12-MHz blackout, as the ionospheric disturbancesspread to the normal auroral-zone regions.


Figure 8.26. A contour plot of College-to-Stanford (11.634-MHz) signal outage in August1959 (from Owren et al., 1963).

The 18-MHz signal at Boston was generally sub-marginal except during thevery quiet pre-storm period 13–15 August. The signal-outage contour plot(Figure 8.27) and the College absorption-contour plot (Figure 8.28) show a strik-ing similarity during the pre-storm period.

It is probable that this circuit was controled more by AA near Churchill,Canada than by MUF factors. The circuit suffered a complete black-out duringthe 16–17 August storm, as might have been expected.

8.4.3 Other trans-polar HF experiments on fixedfrequencies

The College–Kiruna circuit (trans-polar, D5300 km)

Both the 12- and the 18-MHz signals were received at Kiruna most of the timeduring 1–12 August, with very little outage during 13–15 August. This great-circlepath passes over west Spitzbergen and the ionosonde observations atLongyearbyen show that, during 1–15 August, the 12-MHz signal was nearly theoptimum trans-polar traffic frequency, whereas the arctic ionosphere could not atany time support a conventional, multihop propagation mode at 18 MHz.


Figure 8.27. A contour plot of College-to-Boston (17.900-MHz) signal outage in August1959 (after Owren et al., 1963).

24

20

16

12

08

04

001 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

EA

ST

ER

N S

TAN

FO

RD

TIM

E

AUGUST

During the 16–17 August storm the 12-MHz signal weakened somewhat butremained essentially receivable at Kiruna. The 18-MHz signal blacked out early,at 2240 UT on 15 August as the increasing magnetic activity reached a Kp of 5,and remained essentially out until midday on the 21st.

The previous winter (the solar-cycle-19 maximum of 1958–1959) exhibitedunusually favorable propagation conditions on the propagation path betweenCollege and Kiruna. The HF pulse reception at Kiruna showed, as a rule, threedifferent propagation modes on 18 MHz, with intervals of 3 and 6–7 ms, respec-tively, which could hardly be explained as alternate modes on the great-circle path.Thus, there was some indication that non-great-circle propagation modes might beavailable at certain times – which might arise from sharp horizontal gradients inelectron density from the polar cap equatorward though the auroral-oval iono-sphere.

In order to test this hypothesis, a so-called “pinwheel” experiment wasemployed at Kiruna, featuring a specially designed rotating three-element Yagiantenna for 18 MHz. The antenna was moved step-wise from 60° NEE over geo-graphic north to 60° NWW and back, stopping in each indicated position for 1min (the horizontal beamwidth was estimated to be 60°). The backscatter sounder


Figure 8.28. A contour plot of absorption at College. The crosshatched sections denoteabsorption values exceeding 1 dB and the solid black areas indicate PCA events (fromOwren et al., 1963).

in Alaska transmitted with a higher pulse-repetition frequency (PRF) during theminute the transmitter was pointing towards Kiruna, so a direction mark wasthereby registered on the reception records. This experiment was performedduring the winter of 1961 (near the sunspot minimum), when only one propaga-tion mode was operative. Results of this experiment indicated that the signal fromthe NE was sometimes stronger than the signal from the NW. The opposite shouldbe expected for the great-circle path.

College–Kjeller and Thule–Kjeller propagation-path analysis(SSN38.3–80.2)

From January 1961 through June 1962 backscatter transmissions from College,Alaska and from Thule, Greenland were monitored at Kjeller (near Oslo),Norway. A limited amount of data was also obtained at receiving stations atIsfjord on Spitzbergen island. The locations of the transmitting and receiving sta-tions are shown in Figure 8.25. The Thule–Isfjord path is entirely within theauroral zone, all other paths traverse the oval at approximately right angles. Thegreat-circle distances for the various paths are

College–Kjeller, 6000 km;

Thule–Kjeller, 3350 km;

College–Isfjord, 4050 km; and

Thule–Isfjord, 1250 km.

Of particular interest with these trans-polar circuits is the orientation of the ray-paths relative to the D-region absorption regions in the auroral oval. The verticalplane geometry, assuming a symmetrical mode structure and an F-region reflec-tion height of 300 km, is shown in Figures 8.29–8.31, with the hatched areasdepicting the approximate position of the auroral oval and the shaded areasdenoting maximum absorption.

Under quiet conditions the three- and four-hop College–Kjeller transmissionswere found to be vulnerable to AA at the transmitter end of the circuit, whereasthe auroral–oval absorption region at the receiver end should leave all convention-ally propagated signals virtually unaffected. Of course, during disturbed condi-tions the auroral oval expands considerably and could seriously influence circuitsotherwise not exposed to AA. On the College–Kjeller path, the most likely modesare those of one, two or three hops; the two-hop mode is, however, strongly dis-criminated against by the radiation pattern of the antenna. Furthermore, the con-ventional propagation mode for 18 MHz from College during the wintersunspot-minimum conditions is highly improbable, so unconventional modes aremost likely.

The gross behavior of these circuits is displayed on the following selected his-togram plots. The first type gives, on a daily basis, the total number of hours with


Fig

ure

8.2

9.

Col

lege

–Kje

ller

idea

lized

mod

e ge

omet

ry (

from

Ow

ren

et a

l., 1

963)

.

Fig

ure

8.3

0.

Kje

ller–

Col

lege

idea

lized

mod

e ge

omet

ry (

from

Ow

ren

et a

l., 1

963)

.

Fig

ure

8.3

1.

Kje

ller–

Thu

le id

ealiz

ed m

ode

geom

etry

(fr

om O

wre

n et

al.,

196

3).

signals present, hours with blocking interference, and outages. The heights of theblack and dotted columns measure periods of reception and interference, respec-tively. Transmitter or receiver off periods are represented by empty columns witha lower-case e inserted. Whenever the e sign appears above a black or dottedcolumn, this implies for that particular day an outage period corresponding to thedistance from the boundary of the upper column to the top line (the 24-h line).Magnetically quiet and disturbed days are denoted by the capital letters Q and D.Filled triangles below the bottom line serve to indicate times for the occurrence ofsudden-commencement magnetic storms. Figures 8.32 and 8.33 represent condi-tions for summer (July–August 1961) for SSN50 and winter (October–December, 1961) on the cis-polar Thule–Kjeller circuit for 12 and 18 MHz.

Similarly, Figures 8.34 and 8.35 illustrate the behavior of the trans-polarCollege–Kjeller HF circuit for SSN50 in summer (July–September 1961) andwinter (October–December 1961) for 12 and 18 MHz.

The seasonal variation of signals on these two circuits was studied by plottingsignal strengths on selected quiet days, using riometer and K indices as indicatorsof disturbance. Each curve represents average values for the days chosen. If pos-sible, 8–10 days were picked out for each of the months selected for displayingcharacteristic seasonal quiet-day trends. In some cases, interference and outagestended to seriously constrain the amount of data available: therefore the curvesshown are not equally reliable for defining quantitatively the seasonal propertiespertaining to the transmission in case. It should also be noted that these data wereobtained for relatively low sunspot activity (SSN50). These seasonal statisticsare for the 18-MHz circuit and the College 12- and 18-MHz circuit. Figures8.36–8.38 show the seasonal variations on these circuits.

From April 1961 until June 1962 (SSN38.3–64.3) the Geophysical Institutemonitored HF pulse transmissions from the 12-, 18-, and 30-MHz backscattersounders located at Thule, Greenland (D2900 km). The equipment parameterswere given by Peterson et al. (1959). Because of severe SW interference, the12-MHz transmissions were 80%–90% unusable and the 30-MHz signals fromThule were blanked out by the College 30-MHz backscatter sounder, so only the18-MHz Thule data were usable. The Thule–College path crosses the auroral ovalat near normal incidence only once (compared with the trans-polar paths) and avertical ionosonde at Resolute, Canada provided data near the midpoint of thepath. Figure 8.39 shows the vertical-plane geometry of this path.

The seasonal variation is illustrated by plots of the hourly average values of the12- and 18-MHz Thule pulsed-signal strength for winter, summer, and equinoxshown in Figure 8.40.

The signal strengths shown in Figure 8.40 are scaled in arbitrary units and arebased on 2–4-week periods centered on the dates of the winter and summer sol-stices and the autumnal equinox of 1961. The highest signal levels are in the winterperiod and the lowest for the summer solstice, with the equinoctial values falling


in between – which is similar to the case of mid-latitude HF propagation. Theaverage signal strength of the Thule 18-MHz pulsed transmissions as a functionof season is shown in Table 8.7.

The high average winter values are probably due to low absorption and highcritical frequencies. The high occurrence of auroral-E ionization during the nightand normal F-layer propagation modes during the day qualitatively explain therelatively high average signal strength during the equinoctial period. The greatestvariation in signal amplitude occurred during the equinoctial period and the leastvariation during the summer/winter periods.

The diurnal variation of the average 18 MHz signal strength (in dB above 1 V)is illustrated in Figure 8.41 for a geomagnetically quiet day (11 June, 1961) forthree polar paths. The solid line below the signal plot indicates that very stronginterference was present at College. The open lines denote intervals when thevertical ionosonde located at Resolute, Canada indicated that the MUF (3000) F2was less than 18 MHz. During these periods, communication with the Thule–College circuit should have been impossible, but actually quite high signal levelswere recorded at College. This illustrates once again that predictions based on ver-tical-incidence data are usually very unreliable for polar propagation paths at fre-quencies near 18 MHz.

The reliability of these 18-MHz cis-polar and trans-polar circuits is quite wellillustrated in Figure 8.41 It should be remembered, however, that this was a geo-magnetically quiet day about three years after solar maximum. In general, it wasfound that the Thule signal was present practically 24 h per day, except during dis-turbed periods. This illustrates the importance of auroral-E ionization in support-ing 18-MHz propagation during periods when F-region ionization is low in thepolar ionosphere.

Hourly average signal levels for the months of June 1961 (SSN55.8) and June1962 (SSN38.3) are shown in Figure 8.42 for UT morning and evening periods.The period 07–21 UT is not presented because many data were lost due to exces-sive interference on this circuit. Because of the paucity of data, it was not possibleto draw definite conclusions about sunspot-cycle effects on this propagation path

8.4.4 College–Kiruna absorption studies at fixed frequencies

Fixed-frequency pulse transmissions from the IGY sounder at College, Alaskamonitored in Kiruna, Sweden were compared with absorption measurementsfrom a vertical riometer at Thule, Greenland (geomagnetic latitude 88° N) and anoblique-incidence riometer located at College. The oblique riometer at Collegeutilized a three-element Yagi antenna 0.5 above ground, directed towards a geo-graphic bearing of 015°. Simultaneous data on absorption and signal strengthwere obtained for two PCA events, a strong event in July 1959 (SSN155.8) anda weak event in May 1960 (SSN117.0).


Fig

ure

8.3

2.

Thu

le–K

jelle

r 18

-MH

z (a

) an

d 12

-MH

z (b

) pr

opag

atio

n co

ndit

ions

for

sum

mer

196

1 (f

rom

Ow

ren

et a

l., 1

963)

.

24 20 16 12 8 4

(a)

HOURS

24 20 16 12 8

(b)

HOURS

DQ

QQ

QQ

DD

DD

DD

DQ

DD

QQ

QQ

DD

QQ

QQ

QD

D5

1015

2025

305

1015

2025

305

1015

2025

30

sign

als

pres

ent

nois

y pe

riod

equi

pmen

t fai

lure

e

OC

TOB

ER

NO

VE

MB

ER

DE

CE

MB

ER

510

1520

2530

510

1520

2530

510

1520

2530

6

sudd

en c

omm

ence

men

t

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

e

e

ee

e

e

ee

e

e

e

e

e

e

ee

e

e

Fig

ure

8.3

3.

Thu

le–K

jelle

r 18

-MH

z (a

) an

d 12

-MH

z (b

) pr

opag

atio

n co

ndit

ions

for

win

ter

1961

(fr

om O

wre

n et

al.,

196

3).

20 16 12 8 4

(a)

HOURS

20 16 12 8 4

(b)

HOURS

DQ

QD

DD

DQ

QQ

DD

DQ

QD

DD

D5

1015

2025

305

1015

2025

305

1015

2025

30

JULY

AU

GU

ST

SE

PT

EM

BE

R

DD

QQ

QQ

QD

QQ

Q

e

sign

als

pres

ent

nois

y pe

riod

equi

pmen

t fai

lure

sudd

en c

omm

ence

men

te

510

1520

2530

510

1520

2530

510

1520

2530

ee

ee

ee

ee

e

e

e

e

ee

e

e

e

e

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

e

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

e

e

e

e

e

e

e

ee

ee

ee

ee

ee

e

e

e

e

Fig

ure

8.3

4.

Col

lege

–Kje

ller

18-M

Hz

(a)

and

12-M

Hz

(b)

prop

agat

ion

cond

itio

ns fo

r la

te s

umm

er 1

961

(fro

m O

wre

n et

al.,

196

3).

510

1520

2530

510

1520

2530

510

1520

2530

sign

als

pres

ent

nois

y pe

riod

equi

pmen

t fai

lure

sudd

en c

omm

ence

men

te

20 16 12 8 4

(a)

HOURS 24 16 12 8 4

(b)

HOURS

OC

TOB

ER

NO

VE

MB

ER

DE

CE

MB

ER

e

24 20

510

1520

2530

510

1520

2530

510

1520

2530

DQ

QQ

QQ

QD

DD

DD

DQ

QD

DD

QD

DQ

DD

QQ

QQ

Q

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

ee

e

ee

e

ee

ee

ee

ee

ee

ee

ee

ee

ee

e

e

e

e

ee

e

e

e

ee

ee

e

e

e

e

e

e

e

e

e

e

ee

e

Fig

ure

8.3

5.

Col

lege

–Kje

ller

18-M

Hz

(a)

and

12-M

Hz

(b)

prop

agat

ion

cond

itio

ns fo

r fa

ll-w

inte

r 19

61 (

from

Ow

ren

et a

l., 1

963)

.


Figure 8.36. Seasonal signal-strength variation on the College–Kjeller 18-MHz propaga-tion path (from Owren et al., 1961).

SIG

NA

L S

TR

EN

GT

H (

dB

)

60

50

40

30

20

10

0

JUNE 1961OCT. 1961JAN.–FEB. 1962MAR. 1962

00 03 06 09 12 15 18 21 24

HOURS, UT

Figure 8.37. Seasonal signal-strength variation on the Thule–Kjeller 18-MHz propagationpath (from Owren et al., 1963).

SIG

NA

L S

TR

EN

GT

H (

dB

)

60

50

40

30

20

10

0

JAN.–FEB. 1961MAR.–APR. 1961JUNE 1961OCT. 1961

00 03 06 09 12 15 18 21 24

HOURS, UT


Figure 8.38. Seasonal signal-strength variation on the College–Kjeller12-MHz propagationpath (from Owren et al., 1963).

SIG

NA

L S

TR

EN

GT

H (

dB

)

60

50

40

30

20

10

0

JAN. 1961MAR.–APR. 1961JUNE 1961OCT.–NOV. 1961

00 03 06 09 12 15 18 21 24

HOURS, UT

Table 8.7. The average signal strength of the Thule18-MHz pulsed transmissions

TimeSignal strength (dB above 1 V)

UT 150° WMT Summer Winter Equinox

00 1400 35 49 4308 2200 31 38 3717 0700 41 42 3920 1000 27 49 40

Fig

ure

8.3

9.

Mos

t pr

obab

le F

-lay

er m

odes

for

the

Thu

le–C

olle

ge p

ropa

gati

on p

aths

(fr

om O

wre

n et

al.,

196

3).

Fig

ure

8.4

0.

Seas

onal

var

iati

ons

of t

he 1

2- a

nd 1

8-M

Hz

(MC

) T

hule

sig

nals

rec

eive

d at

Col

lege

, Ala

ska

in 1

961.

The

ord

inat

e sh

ows

hour

ly a

vera

ge v

alue

s of

sig

nal s

tren

gth.

The

hor

izon

tal b

ars

labe

led

QR

M d

enot

e pe

riod

s of

high

loss

of

data

due

to

seve

re in

terf

eren

ce (

from

Ow

ren

et a

l., 1

963)

.

Fig

ure

8.4

1.

Qui

et-d

ay v

aria

tion

s of

18-

MH

z si

gnal

str

engt

h fo

r th

ree

prop

agat

ion

path

s (S

SN

55.8

for

June

196

1) (

from

Ow

ren

et a

l., 1

963)

.

The strong PCA event (SSN155.8)

In the period 1937 UT 9 July 1959 through 2115 UT 16 July 1959, four solar flaresof importance 2 to 3 were observed. Absorption events associated with theseflares are listed in Table 8.8.

During this period data on the signal strength of pulse transmissions originat-ing at College and received at Kiruna were available. The system parameters aregiven in Table 8.9.

The length of the transpolar path is 5300 km. Figure 8.43 shows the details ofthe PCAs and HF “black-outs” (no signal received). The times of occurrence of theflares are indicated by the letter F, whereas the symbol SC denotes the start of asudden-commencement geomagnetic storm. The periods of total black-outs of the12- and 18-MHz College–Kiruna pulsed circuit are shown at the top of Figure 8.43.The 12-MHz black-out lasted almost three days longer than the 18-MHz black-out,which qualitatively illustrates the frequency dependence of signal attenuation in theD region. The sharp flattening off of the absorption on 11 July is mostly due toabsorption exceeding the useful dynamic range of the riometer. It should be empha-sized that this was a particularly complex event, with low-energy cosmic rays bom-barding the polar cap almost continuously for about 14 days.


Figure 8.42. The solar-cycle variation of the Thule 18-MHz signal. Hourly average signallevels are plotted on the ordinate (from Owren et al., 1963).

The weak PCA event of 13 May 1960 (SSN117.0)

The PCA event of 13 May 1960 was a small, typical event with maximum absorp-tion values of 4.5 and 3.5 dB on the vertical riometers at Thule and College,respectively. Two solar flares of importances 2 and 3 occurred sometime before0522 UT on 13 May.

Simultaneous absorption data from the College oblique riometer and the18-MHz signal strength on the College–Kiruna propagation path are shown inFigure 8.44. The first absorption peak at approximately 0600 UT corresponds toa dip in the 18-MHz signal from 3 to 1 on the arbitrary scale. The path was com-pletely blacked out from 1045 to 2110 UT, corresponding to another absorptionpeak of approximately the same amplitude (6 dB). This illustrates the relativelypoor peak-to-peak correlation of attenuation and absorption of the 18-MHzsignal for all except strong PCA events. It should be emphasized that the obliqueriometer measures the absorption encountered by the signal on only the last“hop” of the propagation mode and not the attenuation on the other hops. Duringa strong PCA event the region of absorption includes all of the College–Kirunapropagation path; consequently one might expect better peak-to-peak correlationfor the strong events.


Table 8.8. Absorption events

Maximum Date Starting time absorption at

(UT) Duration (h) 27.6 MHz (dB)

10 July 0700 (Thule) 90 (College) 2014 July 0700 51 23.716 July 2250 34 21.2

Table 8.9. System parameters

Power Pulse-repetitionFrequency output frequency Pulse (MHz) (kW peak) (pulses s1) length (s)

11.634 5.0 18.75 120017.900 5.0 18.75 1200

Fig

ure

8.4

3.

Eff

ects

of

larg

e P

CA

s of

9–2

3 Ju

ly 1

959

on 1

2- a

nd 1

8-M

Hz

tran

spol

ar t

rans

mis

sion

s (f

rom

Ow

ren

et a

l., 1

961)

.


Figure 8.44. Weak PCA effects on the College–Kiruna 18-MHz circuit, measured using theCollege oblique riometer (from Owren et al., 1963).


Figure 8.45. Effects of a strong AA event on 11 September 1961 on the 18-MHzThule–College path (from Owren et al., 1963).

8.4.5 Effects of auroral-zone-absorption events on HFpropagation

Simultaneous absorption and signal-attenuation data for the Thule–College18-MHz path for a very strong auroral-zone-absorption (AA) event on 11September 1961 (SSN52.3) are shown in Figure 8.45. Continuous data wereavailable except during the periods 0330–0730 and 0830–1100 UT, when stronginterference made identification of the Thule pulsed signal doubtful. A completeblack-out of the 18-MHz signal occurred between 1140–2400 UT and lasted untilthe absorption level returned to approximately 1.5 dB at 0730 UT on 12September. This was the strongest AA event recorded during this investigationand should not be regarded as a typical event. It was the only AA event studiedwhich produced a black-out on this path.

8.4.6 Sweep-frequency experiments

Forward oblique sounding investigations near sunspot minimum

During 1963 and 1964 the GI/UAF operated a combination HF step-frequencybackscatter and synchronized forward sounding system (Davies, 1990) at Collegeutilizing commercial pulse sounders and antennas. Figure 8.46 shows the fivepropagation paths studied in the course of this experiment and some of the perti-nent parameters of the system are listed in Table 8.10. The data described in thissection were obtained during the period November 1963 (SSN23.8) to February1964 (SSN17.8) and represent winter, sunspot minimum conditions.

Thule–College path

The Thule–College great-circle path is 2900 km long and the most probable prop-agation modes are one-, two-, and three-hop F-modes; two-, three-, and four-hop


Table 8.10. Parameters of the forward sounding system

Power output 30 kW (rated), 15 kW (measured)

Frequency range 4–64 MHz

Short-pulse mode PRF 50 pulses s1, pulse length 100 s, bandwidth 16 kHz, fourpulses per channel

Long-pulse mode PRF 20 pulses s1, pulse length 1000 s, bandwidth 4 kHz, twopulses per channel

Antennas Granger Model 726-4/64 log periodic vertical monopole LPAsdirected at true azimuths of 015°, 105°, 210°, 270°, and 325°

Fig

ure

8.4

6.

Gre

at-c

ircl

e H

F-p

ropa

gati

on p

aths

stu

died

dur

ing

1963

and

196

4, a

long

wit

h an

app

roxi

mat

e au

rora

l ova

l for

Kp

4 (f

rom

Bat

es a

nd H

unsu

cker

, 196

4).

modes or combination E–F modes. Figure 8.47 shows typical winter long- andshort-pulse records from Thule. The upper long-pulse record displays a commonwinter auroral-E with a LOF of 5 MHz and a MOF of 17 MHz. The short-pulserecord below illustrates another common mode with an auroral-E LOF of4 MHz and a MOF of 20 MHz. Very spread-F modes are present between 5 and10 MHz.

Auroral-E modes

The relative occurrence of auroral-E modes on the Thule–College circuit is shownin Figure 8.48 (lower plot). The histogram peaks around 2000–2400 UT (10–14,150° WMT and has a minimum around 1100–1400 UT (01–04, 150° WMT). Thehistogram gives the fraction of time that auroral-E is present on this path, andshows that the dominant winter mode for this path is, in fact, supported byauroral-E.

The histogram in Figure 8.49 shows the diurnal variation of the averageMOF for each hour and the upper plot gives the highest MOF observed duringeach hour for the period 27 November 1963 to 12 February 1964. A maximum


Figure 8.47. Winter,sunspot-minimum modestructure on theThule–College HF prop-agation path. The long-pulse record is shownabove and the short-pulse record is shownbelow (after Bates andHunsucker, 1964).

is indicated between 0730 and 1230 UT (2130–0230, 150° WMT) in the averageMOF curve for Thule, which corresponds to the diurnal auroral peak atCollege.

Other winter modes

In addition to the predominant auroral-E modes during the winter on theThule–College path, various other modes are observed. Figure 8.50 (lower record)shows the typical auroral-E along with what appears to be a very spread-F mode


Figure 8.48. The lower plot shows the diurnal variation of the auroral-E maximumobserved frequency MOF for the same period (after Bates and Hunsucker, 1964).

at lower frequencies. Short-pulse (100 s) records of the Thule signal taken in lateFebruary and March (daytime) show the normally expected F modes (the upperrecord in Figure 8.51) with a subsequent decrease in occurrence of the auroral-Emode. That this is to be expected is discussed in following sections.

Off-path modes

One of the most interesting high-latitude HF modes is the “Off-path” or “non-great-circle” (NGC) mode. Two examples of these modes on the Thule–College


Figure 8.49. The diurnal variation of the auroral-E maximum observed frequency (MOF)(after Bates and Hunsucker, 1964).

path are shown in Figure 8.52, with the direct path (probably auroral-E) at theproper time delay, plus several distinct NGC modes. The time delay and lack ofretardation of the mode structure do not allow a “multiple-hop” interpretation,so we postulate a NGC mode.

The Andøya–College path (D5000 km)

The most prominent winter-night mode on the Andøya–College trans-polar pathappears to be at least partially supported by auroral-E, in that the signal exhibitsthe constant-range discrete-signal characteristics. Occasionally some retardationis present at the upper frequency end of the trace, but in most cases the signalappears similar to that shown in Figure 8.47. The relative occurrence of auroral-Eon the Andøya–College path has two diurnal peaks, as shown in Figure 8.48


Figure 8.50. Typical winter records from Thule, Greenland. The long-pulse record is shownabove and the short-pulse record is shown below (after Bates and Hunsucker, 1964).

(upper trace), one peak is centered around 0400 UT (1800 local time). Auroral-EMOFs as high as 32 MHz and an average MOF of 18 MHz are observed on thiscircuit, as shown in Figure 8.49 (upper plot). There are two diurnal peaks inaverage MOF on the Andøya circuit as opposed to the single maximum on theThule path. Average MOF maxima occur at the times 0600–0900 and 1900–2100UT (2000–2300 and 2000–2300, 150° WMT).

8.4.7 Other results from HF high-latitude studies fromc. 1956–1969

Most of these results are taken from a survey paper by Hunsucker and Bates(1969).


Figure 8.51. Normal F-modes from Thule (above) and Andøya (below). (after Bates andHunsucker, 1964).

Auroral-E ionization effects

It appears that, during the winter months, high-latitude HF propagation is pre-dominantly supported by auroral-E (AE) ionization, even during moderatesunspot activity (SSN35). The importance of AE in high-latitude HF propaga-tion during winter night-time conditions was reported by Hunsucker and Stark(1959). Results obtained on trans-polar paths monitoring HF pulsed transmis-sions and fixed-frequency backscatter soundings to the north revealed that AEactivity peaks during the period 1800–0600 150° WMT.

Folkestad (1963, personal communication) reported that signal strengths of


Figure 8.52. Direct and off-path modes from Thule long-pulse records. In the lower iono-gram, the maximum off-path delay is 11 ms (after Bates and Hunsucker, 1964).

18-MHz pulsed transmissions on the trans-polar College to Kjeller path duringJanuary and February 1961 peaked during 2100–0600 150° WMT, further illus-trating the role of AE in trans-polar propagation during the winter night. The highpercentage occurrence of AE of maximum frequency 5 MHz in the polar regionsduring the winter night was also emphasized by Leighton et al. (1962) using IGYresults. See Figures 8.50–8.52.

In an early study of the relationship between visual aurora and vertical iono-sonde observations, Heppner et al. (1952) found a high correlation betweencertain zenithal auroral forms and values of fEs (the AE cutoff frequency). In par-ticular, rayed bands at the zenith gave the highest correlation with fEs, whereascomplete absorption was indicated 100% of the time when pulsating auroral formswere overhead. Another study of the relation between visual aurora and vertical-ionosonde fEs data was performed at an auroral-zone station by Hunsucker andOwren (1962). Using all-sky-camera photos, they found that the motion of anauroral arc or band from a low elevation angle to a position near the zenith wasaccompanied by an increase in the value of fEs by a factor of two or greater. Withdiscrete auroral forms near the zenith, values of fEs from 8 to 11 MHz werecommon, with a maximum value of 13 MHz (also see Hunsucker, 1965).

The results of this investigation are in good agreement with the foregoing find-ings concerning the occurrence and behavior of auroral-oval E-region ionizationduring winter-night sunspot-minimum conditions. The MOF peak on theThule–College circuit coincides with the period of maximum auroral activity nearthe College end of the circuit (2130–0230, 150° WMT). AE propagation on theAndøya–College path is a much more complicated phenomenon, displayingseveral diurnal peaks in activity. This is to be expected, since the 5000-km propa-gation path traverses the auroral oval twice and hence exhibits sunrise/sunseteffects twice, compared with once on the Thule–College path.

The transition from the winter “night modes” (AE propagation) to the F-propagated “day modes” for the polar paths investigated takes place at about themiddle of February. As the reflection points on the Thule–College and Andøya–College paths become sunlit, the normal multihop F modes propagate – as shownin the records in Figure 8.51.

NGC modes

Very strong evidence of HF/NGC propagation modes associated with the auroraloval was presented by Egan and Peterson (1962). Monitoring of the 12- and18-MHz pulsed signals from Thule and College at Stanford revealed very strongdelayed modes with time delays of up to 12 ms between the direct mode and the“sidescatter” modes. Ortner and Owren (1961) also presented evidence for theexistence of such modes on the 18-MHz trans-polar path between College andKiruna, Sweden. Additional evidence for the existence of NGC modes was givenby Hunsucker (1964a; 1964b) for a synchronized step-frequency circuit betweenCollege and Oya, Norway and for the 18-MHz College–Thule circuit.


Bates et al. (1966) presented a detailed study on the relationship of the aurorato NGC HF propagation on HF forward-sounding records received at Collegefrom various sites during 1963 and 1964. No direction-finding equipment wasavailable, so a statistical analysis was performed in order to determine the type ofsidescatter involved. The number of occurrences during several periods wasmaximum at night. The excess propagation time on the Palo Alto to College pathvaried inversely with magnetic activity. A comparison of simultaneous Collegebackscatter and Palo Alto to College off-path data showed that the locus of off-path sidescatter extended north of the ionospheric backscattering belts. Theseresults were interpreted as showing that the deviated modes were produced bysidescatter from the auroral belt as shown for the Palo Alto-College path in Figure8.53.


Figure 8.53. A schematic representation of the requirement for overlap between the ellipti-cal locus of possible off-path sidescatter points on the Palo Alto-to-College path and thescattering belt, as determined from the College backscatter data (from Bates et al., 1966).

The HF forward-sounding experiments in the early 1960s emphasized (amongother things) the importance of the AE mode for winter-night near-sunspot-minimum conditions on polar paths. During the period 27 November 1963–12February 1964, the average percentage occurrence of AE on the Thule–Collegeand Andøya–College circuits was over 50%. AE MOFs as high as 46 MHz withtypical values of 18 MHz were observed on the Thule–College path and MOFs ashigh as 32 MHz with typical values of 16 MHz were observed on theAndøya–College path. This suggests that long-haul HF traffic might be routedover polar paths during winter-night near-sunspot-minimum periods when mid-latitude MOFs are quite low. The importance of NGC modes in carrying theMOF was also illustrated in these early-sixties studies during the midwinterperiod.

Bates and Albee (1966) also pointed out the importance of the F1-layer effectson long-distance, high-latitude (and even some sub-polar) HF circuits. During the1964 sunspot-minimum period the F2 critical frequency at College was not appre-ciably greater than that of the F1 layer. This condition resulted in considerablymodified conditions, and frequently the F1 layer carried the maximum propagat-ing frequency.

The terminology of Bates and Albee (1966) is illustrated by an example, the 4F2mode. In this case there are four reflections from the F2 layer, with ground reflec-tion in between and the end-points are the ground. The 4F2 mode is also calledthe fourth-order F2-layer forward-propagation mode. In general, the first numbergives the mode order and the rest of the symbols denote the ionospheric layerinvolved. Propagation modes involving successive reflections both from the Elayer and from the F layer are termed combination E–F modes.

Figures 8.54(a) and (b), recorded on the 3500-km Palo Alto to College path,illustrate normal F-region oblique ionograms. The mode structure is well defined,the low ray traces showing a slight retardation at the low-frequency end, the highrays are relatively short, and the magnetoionic splitting can be seen. Figures8.54(c) and (d) illustrate ionograms that are representative of the summer 1964and 1965 records obtained on all paths to College. The downward curving portionof each record is composed of several discrete traces produced by signals in thefirst four F layer modes. Traces such as those in Figures 8.54(c) and (d) are termed“long-tailed” traces.

Figures 8.55–8.58 show long-tailed traces on the signals recorded at Collegefrom Thule (2900 km), Palo Alto (3500 km), Fort Monmouth (5200 km), andOkinawa (7600 km) for the morning of 18 July 1964. This was one of the fewinstances when the long-tailed traces were observed on the four paths more or lesssimultaneously.

A noteworthy feature is the sudden appearance and disappearance of the long-tailed traces. Long-tailed traces were never as outstandingly developed on theThule to College signal as they were on the other paths; this is undoubtedly due


Fig

ure

8.5

4.

For

war

d ob

lique

iono

gram

s re

cord

ed o

n th

e P

alo

Alt

o-to

-Col

lege

pat

h: (

a) a

nd (

b) w

ere

reco

rded

dur

ing

Oct

ober

(SSN

19

.7),

and

(c)

and

(d)

dur

ing

July

196

5 (S

SN

15.5

). T

ime

mar

ks a

re 1

.0 m

s ap

art.

One

mic

rose

cond

pul

se p

er c

hann

elw

as t

rans

mit

ted

per

1000

fre

quen

cy c

hann

els

betw

een

4 an

d 24

MH

z (f

rom

Bat

es a

nd A

lbee

, 196

6).

(a)

(b)

(c)

(d)

Fig

ure

8.5

5.

A lo

ng-t

aile

d tr

ace

sequ

ence

rec

orde

d on

18

July

196

4 (S

SN

10.3

) on

the

Thu

le-t

o-C

olle

ge p

ath.

Tim

es a

re U

T.T

he p

ulse

wid

th is

1 m

s (a

fter

Bat

es a

nd A

lbee

, 199

6).

to the decrease of the F1-layer critical frequency with latitude relative to that ofthe F2 layer.

Figure 8.59 contains an oblique ionogram that was derived from the verticalionogram in Figure 8.59. For simplicity only E and F modes are shown; combi-nation E–F modes are ignored. Signals from the E layer produce the constant timetraces shown in Figure 8.59. The downward-curving portion of the obliqueionogram is primarily composed of two lines, which correspond to the F1 and F2critical frequencies. Each down-curving line is approximately the vertical-to-oblique transformation of the critical frequency.

Figure 8.59 shows that the gap between the lines was produced by the relativecloseness of the E and F1 critical frequencies, while the great decrease in traveltime with increasing frequency was produced by the nearness of the F1 and F2critical frequencies. The complete signal-trace structure for the first four modes is


Figure 8.56. A long-tailed trace sequence recorded on 18 July 1964 on the Palo Alto-to-College path. Times are UT. The pulse width used was 1 ms (after Bates and Albee, 1966).

Fig

ure

8.5

7.

A lo

ng-t

aile

d tr

ace

sequ

ence

rec

orde

d on

18

July

196

4 on

the

For

t M

onm

outh

-to-

Col

lege

pat

h. T

imes

are

UT.

Pul

sew

idth

1 m

s (a

fter

Bat

es a

nd A

lbee

, 196

6).

Fig

ure

8.5

8.

A lo

ng-t

aile

d tr

ace

sequ

ence

rec

orde

d on

18

July

196

4 on

the

Oki

naw

a-to

-Col

lege

pat

h. T

imes

are

in U

T. P

ulse

wid

th1

ms

(aft

er B

ates

and

Alb

ee, 1

966)

.

contained in Figure 8.61; no consideration was given to possible shielding effectsof lower layers on the very-oblique modes. The F1 layer controls the maximumfrequency for the path in the calculated example; experimentally the F1 layerappeared to carry the MOF during much of the 1964 summer daytime on the PaloAlto and Thule to College paths. Tveten (1961) found that the F1 layer frequentlycarried the MOF on the Barrow (Alaska) to Boulder (Colorado) path, andMaliphant (1969) noted the importance of F1 propagation on the long trans-Atlantic Slough to Ottawa path.

Possible ducted modes

In the previous section it was shown that the long-tailed traces were produced bysignals in the first three or four F2-layer modes. Figure 8.62 illustrates traces,however, that cannot be explained in that manner; these traces will be referred toas “delayed-long-tailed” (DLT) traces.

We will first assume that these DLT traces were produced by higher-orderF2-mode signals. Rough estimates of the travel time, and hence reflection heights,can be made by assuming that the first-arriving signals propagated via theminimum-order E or F1 modes possible for the paths in question. The recordsshown in Figures 8.60(a), (b), and (c) will be analyzed in this fashion; theminimum time delays between the first-arriving and the DLT signals were approx-


Figure 8.59. The forward ionogram for a 3500-km path derived from the vertical-incidenceionogram in the inset (from Bates and Albee, 1966).


Figure 8.60. Records showing highly delayed signals. Traces in (a) and (b) were obtained onthe Palo Alto-to-College path with four 100-s pulses per channel, per 100 frequency chan-nels between 4 and 24 MHz. Records (c) and (d) were obtained using two 1.0-ms pulses perchannel on the Fort Monmouth- and Palo Alto-to-College paths, respectively (from Batesand Albee, 1966).

Table 8.11. Virtual-reflection heights for each hopmode

Virtual-reflection height (km)

Palo Alto Fort Monmouth

Mode 1.75 ms 2.3 ms 3.0 ms

1F2 8502F2 500 560 7003F2 350 390 5254F2 250 300 400

(a)

(b) (d)

(c)

imately 1.75, 2.3, and 3.0 ms, respectively. Applying the Martyn and Breit–Tuvetheorems (and accepting the small errors due to the spherical geometry), thesedata correspond to virtual reflection heights for each hop mode (Table 8.11)

In each case F2 vertical-incidence heights near 500 km were observed atCollege, so that the most probable modes would be the third and second, andfourth- and third-order F2 modes in that order for the Palo Alto and the FortMonmouth to College paths. This leads to the conclusion that some, if not all, ofthe lowest-possible F2-mode signals were absent if the DLT signals were in realityhigher-mode signals. The horizon-cutoff height for the one-hop mode on the FortMonmouth to College path was 600 km, so 1F2-mode signals would not beexpected (although they might be possible).

The absence of the lowest-possible F2 modes can be explained qualitatively byconsidering the rate of change of the secant of the angle of incidence with respectto the angle of incidence at the transmitter as a function of the reflection height.When such a computation is performed, it is evident that the secant of the angleof incidence increases more rapidly for F1-layer heights than it does for F2-layerheights. The secant factor is proportional to the maximum frequency reflected bythe layer; hence, if the F1 and F2 critical frequencies differed by only 10%, theF1 layer would prevent all but relatively high-angle rays from penetrating to theF2 layer at frequencies capable of undergoing F2 reflection. A relatively thick,dense F1 layer can therefore act as a shield for oblique F2 propagation to preventthe low-angle F2 modes from propagating.

It may be constructive to consider the possibility that the DLT traces were notproduced by a normal Earth–ionosphere hop-mode signal, but that the signal wasducted in the ionosphere. One type of ducted mode, usually termed an elevated ortilted mode, has been proposed to explain long-distance backscatter echoes andtrans-equatorial propagation . Tilted modes do not appear to be the explanationin this case because the 3500-km Palo Alto path, for example, is so short that anexcessive tilt would be required, and, furthermore, the required upward tilt to thesouth is in the wrong direction to that observed. An examination of the monthlymedian ionospheric heights and critical frequencies observed at various Alaskan,Canadian, and US sites during the summer of 1964 when the DLT signals weremost likely to be observed indicated that the virtual height of the F2 layerdecreased and the F1 and F2 critical frequencies increased to the south (CRPL Fseries, part A), thus producing a strong downward tilt to the south from College.

Actual ducting within the ionosphere is, however, another matter. For the caseat hand, ducting between the F1- and F2-layer maxima seems the most probableexplanation for the single long-tailed traces. Such ducting could occur only ifseveral relatively special ionospheric conditions applied. An electron-densityvalley must exist in order to provide the necessary velocity minimum aroundwhich the guided wave propagates. The wave could enter the duct at the beginningof a valley, or where a strong-enough horizontal gradient occurs in the F1 layer


to allow penetration at one point but not at a point further along the path. A tiltedF2 layer might not be necessary, but a tilt would help by gradually changing theangle of incidence of the propagating wave.

The proposed model is speculative and might not be necessary to explain theobserved records. The high-order-mode model, though, will not satisfactorilyexplain the existence of the single DLT trace on a record, whereas ducting will,because only one ducted mode would generally be expected. A further observa-tional point in favor of the idea of a ducted mode is the extreme variability of thesingle long-tailed trace. Within the span of several soundings (20 min apart) thetrace appeared and disappeared. The high-ray trace was well defined and exhib-ited extreme retardation; this behavior is not a characteristic of normal Earth–ionosphere hop-mode signals. These observations are not readily explainable bythe idea of a higher-order mode.

A summary of summer 1964 data

The F1 layer considerably modified HF propagation conditions on high latitudepaths during daytime in the summer of 1964. This period was characterized by thickF1 and F2 layers in the 4- and 5-MHz ranges, respectively. Conditions on the PaloAlto to College path were relatively predictable, in that frequencies in the 10–18-MHz range propagated during most of the day with no great variation in flight time.On the Fort Monmouth to College path, however, the minimum-flight-time traceswere in roughly the same frequency range as those on the Palo Alto paths, but theyoccasionally disappeared when the lower-frequency DLT traces appeared. Thismay have been partly an equipmental effect because the antennas used had rela-tively poor radiation characteristics at low elevation angles, but the records clearlyindicate that there was an appreciable drop in signal strength. Whether these signalswould have been received with better antennas is unknown.

In any case, the records clearly show that signals propagated in relatively high-angle F2 modes. Antennas for paths in the 5000-km range that discriminateagainst such high-angle radiation may cause communication outages, while nonewould be noted with less-directive arrays. Thus, less-directive arrays have theirplace, alongside very directive antennas in communications-antenna applicationsat high latitudes.

8.4.8 Doppler and fading effects on HF high-latitudepropagation paths

A survey of polar and auroral-region effects on HF propagation (Hunsucker andBates, 1969) lists some of the results given in Chapter 8, along with results fromother investigations. Two important results in the survey paper are the Dopplerspreading and fading of HF signals which traverse the auroral oval. Lomax (1967)presented an example of typical power spectra observed at the Palo Alto receiv-ing site for transmissions from Thule, Greenland and Fort Monmouth, New


Jersey, as displayed in Figure 8.61. Some more recent data on Doppler shifts andspreading will be presented in Chapter 9. Anyone who has monitored HF trans-missions that have traversed the auroral ionosphere will probably have encoun-tered “auroral flutter,” which results from the reception of multiple signalcomponents from auroral ionospheric irregularities. Koch and Petrie (1962)studied fading characteristics on a long path and found that fading rates higherthan 20 Hz were present for a small percentage of the time on 10, 15, and 20 MHz.These fading rates exhibited only a minor diurnal trend, with the maximum occur-ring during morning hours. A study of the fading correlation bandwidth and


Figure 8.61. Typical power-spectra transmissions from Thule (a) and Fort Monmouth (b)monitored at Palo Alto (from Lomax, 1967).

short-term frequency stability on the same path was performed by Auterman(1962). He found that the mean fading correlation bandwidth was 4.3 kHz, thevalue exceeded 90% of the time was 1.0 kHz, and the bandwidth generally wassmaller during periods of high geomagnetic activity.

One of the best-instrumented medium-range, high-latitude HF forward-


Figure 8.62. A map showing the Sodankylä–Lindau HF path in relation to the auroralzone, Arctic and other instrumented stations (after Rose, 1967).

sounding circuits was the 1965-km path between Sodankylä, Finland and Lindau,Germany described by Moller (1964) and Rose (1964). Stanford ResearchInstitute provided English translations of these important reports in 1964 and1967, respectively. Figure 8.62 is a map showing the Sodankylä–Lindau propaga-tion path, the location of the Arctic circle, the auroral “zone,” and the location ofsupporting instrumentation at Kemi, Luleå, Uppsala, Kiruna, and Lycksele. Avertical section through the Lindau–Sodankylä path is shown in Figure 8.63, witha possible mode structure indicated.

It is obvious that the Sodankylä end of the path was the most affected bythe auroral ionosphere and the geometry of the Sodankylä end of the 2F-layermode is shown in Figure 8.64. Table 8.12 lists the parameters of the HF pulse-transmission/reception system used on the Sodankylä–Lindau circuit.

The system at Sodankylä was also operated as a backscatter sounder atselected intervals. Simultaneous vertical soundings from the Uppsala ionosonde(located near the mid-point of the path) and forward transmission on the


Figure 8.63. A vertical section through the Lindau–Sodankylä HF propagation path (afterMoller, 1964).

Figure 8.64. A partial cross-section through the Sodankylä–Lindau path, showing aquarter of the 2F mode (after Rose, 1967).

Sodankylä–Lindau HF circuit for a summer evening in 1958 (SSN187) areshown in Figure 8.65 along with calculations of the vertical and obliquely derivedcritical frequencies. Figure 8.66 shows similar plots for a winter early morning.

It should be noted that the Lycksele ionosonde was located approximately500 km from the Sodankylä end of the circuit and thus indicated the presence ofAE ionization – which is reflected in the complex mode structure of Figure 8.68.This is further illustrated in Figure 8.67, in which the Lycksele ionogram clearlyshows strong AE ionization.

Many examples of the seasonal and diurnal behavior on this HF path duringthe maximum of solar cycle 19 are shown in Section C of Moller’s (1964) reportand representative forward ionograms are shown in Figures 8.68–8.72. The effects


Table 8.12. HF-system parameters

Transmitter output power 50 kWPulse duration 100 sPulse-repetition frequency 50 HzFrequency range 1.4–22.6 MHzSweep duration 8 minAntennas: three rhombic antennas each for the transmitter and the receiverNominal gain of each antenna 10 dBReceiver bandwidth 16 kHz

Figure 8.65. Oblique and vertical ionograms for the Sodankylä–Lindau HF circuit for asummer evening (30 June 1958) during sunspot cycle 19 (SSN87; Kp4) (after Moller,1964).


Figure 8.66. An oblique ionogram for the Sodankylä–Lindau HF circuit for a winter earlymorning (13 November 1958) near sunspot maximum (SSN181; Kp3). The ionogramsare from Uppsala and Lycksele (after Moller, 1964).

Figure 8.67. Effects of auroral-E ionization on mode structure on the Sodankylä–LindauHF circuit at local midnight, midwinter (8 November 1958), sunspot-cycle maximum (afterMoller, 1964).

Fig

ure

8.6

8.

Nor

mal

sum

mer

day

tim

e So

dank

ylä–

Lin

dau

iono

gram

s fo

r 14

Jun

e 19

58 (

reco

rded

onc

e pe

r ho

ur f

rom

000

0 to

120

0M

EZ

) (a

fter

Mol

ler,

1964

).


Figure 8.69. Normal fall daytime Sodankylä–Lindau ionograms for 17–18 September 1958(recorded once per hour from 1900 to 1600 MEZ) (after Moller, 1964).

Fig

ure

8.7

0.

Sim

ulta

neou

s ve

rtic

al a

nd o

bliq

ue io

nogr

ams

show

ing

stro

ng s

prea

d-F

eff

ects

nea

r m

idni

ght

duri

ng s

unsp

ot m

axim

um, 7

–8 N

ovem

ber

1958

(af

ter

Mol

ler,

1964

).


Figure 8.71. Effects of dense auroral-E ionization on 4 September 1958 on theSodankylä–Lindau HF circuit (after Moller, 1964).

Figure 8.72. Some effects of a moderate geomagnetic disturbance on 7 October 1958 on theSodankylä–Lindau HF circuit (after Moller, 1964).

of strong spread-F on the mode structure of the Sodankylä–Lindau HF circuit aregraphically illustrated in Figure 8.70, effects of AE on the oblique circuit areshown in Figure 8.71 and moderate geomagnetic effects in Figure 8.72.

Another analysis of data from the 4–64 MHz forward sounding path fromAndøya, Norway to College, Alaska for the year 1964 (D5000 km; sunspotminimum) was conducted by the SRI and presented by Bartholomew (1969).Figures 8.73, 8.74, and 8.75 display the propagation spectra for summer, equi-noxes, and winter, respectively.

As pointed out by Bartholomew, the width of the frequency spectrum thatoccurred 50% of the time was fairly constant for all seasons. The median MOFand LOF propagating at least 50% of the time present relatively small diurnalvariations in any season. The median MOF and LOF increased by about 3 MHzas the season changed from winter to summer. This behavior is fairly typical ofhigh-latitude propagation, since solar illumination in this region exhibits moreseasonal than diurnal variation.

The median LOF on disturbed days was generally higher than that on quietdays, but the median MOF on disturbed days was not consistently different fromthe quiet-day median MOF. Periods of increased LOF at least qualitatively arerelated to periods of increased auroral absorption – especially during equinoctialand winter months. It is also interesting to note that, during equinoctial and


Figure 8.73. The Andøya–College HF average monthly propagation spectrum for summer1964 (D denotes disturbed and Q denotes quiet periods) (after Bartholomew, 1969).


Figure 8.74. The Andøya–College HF propagation spectrum for 1964 equinoxes (afterBartholomew, 1969).

Figure 8.75. Andøya–College HH propagation for Winter 1964. (after Bartholomew, 1969).

summer periods, there was significant propagation on frequencies greater than20 MHz even during this sunspot minimum period. This, again, illustrates theimportance of AE ionization in supporting propagation on high-latitude HFpaths. Multipath propagation was a quite significant factor on theAndøya–College path and the seasonal dependences of multipath and propaga-tion outages are displayed in Figures 8.76, 8.77, and 8.78 (for summer, equinoxes,and winter 1964, respectively). These figures also display the percentages of actual


Figure 8.76. Andøya–College HF multipath and propagation outage for summer 1964(after Bartholomew, 1969).

Figure 8.77. Andøya–College HF multipath and propagation outage for 1964 equinoxes(after Bartholomew, 1969).

propagation-outage time. Outage was highest at night, especially in winter, anddisturbed days generally had greater outage than did quiet days.

Short-pulse data were used to determine the Andøya–College mode structurefor January through March 1964. Although the short-pulse data were more sparsethan the long-pulse data (with less reliable statistics implied), it is instructive tonote the complex mode structure during the month of March 1964 shown inFigure 8.79. Some of the morphology of off-path (NGC) propagation on theAndøya–College HF path is displayed in Figure 8.80, showing seasonal, quiet/disturbed conditions and diurnal effects.

A good example of how actual LUF and MUF values differ from predictedvalues on this 5000-km trans-polar path is shown in Figures 8.81, 8.82, 8.83, and8.84 for January, April, June, and September 1964, respectively. The predictionprogram utilized was the predecessor to the IONCAP program. The MUF usedhere is the highest frequency expected to propagate at least 50% of the time, whilethe LUF, used is the lowest usable frequency with 50% reliability. The FOT isdefined as the optimum traffic frequency – an estimate of the frequency that willpropagate at least 90% of the time. The wide divergence between predicted andobserved values is apparent.

The Norwegian Defense Research Establishment (NDRE) also conducted ananalysis of data on paths from Andøya to College and to Fort Monmouth, NewJersey in 1964 as shown on the map in Figure 8.85.

Plots of the diurnal and seasonal behavior of MOFs on the Andøya–Collegeand Andøya–Ft Monmouth trans-polar paths for winter (January 1964) andspring (March 1964) are shown in Figure 8.86. Plots for May and July 1964 areshown in Figure 8.87.


Figure 8.78. Andøya–College HF multipath and propagation outage for winter 1964 (afterBartholomew, 1969).


Figure 8.79. The Andøya–College HF mode configuration for March 1964; (a) quiet daysand (b) disturbed days (Bartholomew, 1969).

(a)

(b)


Figure 8.80. Diurnal and seasonal behavior of Andøya–College HF NGC propagation forquiet and disturbed conditions (after Bartholomew, 1969).

Figure 8.81. Predicted MUF, LUF, and percentage occurrence of observed signal forJanuary 1964 on Andøya–College HF circuit (after Bartholomew, 1969).


Figure 8.82. Predicted MUF, LUF, and percentage occurrence of observed signal for April1964 on Andøya–College HF circuit (after Bartholomew, 1969).

Figure 8.83. Predicted MUF, LUF, and percentage occurrence of observedsignal for June1964 on Andøya–College HF circuit (after Bartholomew, 1969).


Figure 8.84. Predicted MUF, LUF, and percentage occurrence of observed signal forSeptember 1964 on Andøya–College circuit (after Bartholomew, 1969).

Figure 8.85. Location of trans-polar paths investigated by the NDRE (from Folkestad,1968).


Figure 8.86. Plots of observed MOF distribution (shaded area), median frequencies(broken line), MUFs predicted from vertical ionosonde data (solid line), and periods withpredicted screening by the E-layer (the heavy solid line near the bottom of the MOF plot)for College–Andøya (right-hand plots) and Fort Monmouth Andøya (left-hand plots). Atthe bottom of the figure, the vertical lines represent the number of detectable signals as apercentage of the total number of readings. Data from January 1964 are shown in (a) anddata from March 1964 are shown in (b).


Figure 8.87. Plots in the same format as Figures 8.86(a) and (b) for May and July 1964.

As pointed out by Folkestad, (1) the observed median and maximum values ofthe MOFs were substantially above the predicted MUFs most of the time; (2) forthe spring and summer months the predicted MUFs were about 5 MHz below thecorresponding observed medians; and (3) during the early morning hours duringthe winter, the transmissions on the Andøya–College circuit (approximatelynormal to the auroral oval) were more reliable than were those on the Andøya–Fort Monmouth path (tangential to the auroral oval). This is qualitativelyexplainable by invoking the greater amount of time the second path spends in the


Figure 8.88. Circuit behavior during disturbed periods in April 1964 for the Andøya trans-missions received in College and Fort Monmouth, along with riometer absorption valuesfrom Longyearbyen, Tromsø, and Andøya.

AA regions. Examples of circuit behavior during selected disturbed periods inApril 1964 are shown in Figure 8.88.

Another early HF high-latitude propagation experiment was conducted inApril–June 1960 (SSN113–120) on the Barrow, Alaska to Boulder, Coloradopath (D4495 km) by the US National Bureau of Standards (NBS) and reportedby Tveten (1961). The experiment utilized two modified C-3 ionosondes in a syn-chronized sweep-frequency sounder system. The ionosondes transmitted 100-spulses at a PRF of 25 pulse s1 with an output power of 10 kW using terminatedhorizontal V antennas approximately 400 ft long on each leg with the apex 70 fthigh. The records were taken at the rate of one 7.5-min sweep every hour andended at 7.5 min past the hour. Figure 8.89 is a map of the Boulder–Barrow pathwith an estimated auroral oval for low Kp. It is obvious that only the northern halfof the path will be affected by auroral phenomena.


Figure 8.89. The Boulder–Barrow HF propagation path with the auroral oval for low Kp(from Tveten, 1962).

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Typical oblique ionograms from the Barrow–Boulder path for summer andequinoctial periods are shown in Figures 8.90(a) and (b), respectively. The missinglow-angle ray on the 1F2 (one-hop F2-layer mode) in the ionogram in Figure8.90(b) is probably due to shielding by the AE layer on the northern end of the path.

The NBS/CRPL “two-control-point” method for computing the 4000-kmMUF was employed to compare produced values with values observed on thispath and the results are shown in Figures 8.91 and 8.92 for April and June 1960,showing the large discrepancies typical of this type of path.

Some limited data on a very long path from McMurdo (Antarctica) to Thule(Greenland) for sunspot-maximum conditions were presented by Gerson (1964).This path was 18730 km long and was possibly affected both by the northern andby the southern auroral ovals. Frequencies of 13 and 17 MHz and output powersof 0.5–1.0 kW into delta antennas were used. Figure 8.93 is a plot of periods whenthe McMurdo transmissions were received at Thule during the period 15–17 May1958 (SSN191). A minor SWF was observed on 17 May.

Results from a well-instrumented and documented high-latitude HF-propagation experiment were reported by Jull (1964) shortly after the maximumof solar cycle 19 (1960 and 1961). Five propagation paths in the polar, auroral,and subauroral regions were studied using synchronized oblique-soundingsystems and a network of six vertical 30 MHz riometers. Figure 8.94 shows the


Figure 8.91. A comparison of observed and predicted MUFs for the Boulder–Barrow pathfor April 1960 (from Tveten, 1962).

location of the HF sounding paths and the riometers and the path characteristicsare tabulated in Table 8.13.

Except for PCAs, the attenuation of HF signal on these circuits is due to AA,and the relative occurrence of absorption at the six riometer stations from July1959 to June 1961 is shown in Figure 8.95 and the percentages of the AA timeoccurred for various values of Kp are shown in Figure 8.96. Although they wereobtained some 38 years ago, these two figures remain quite useful for estimatingeffects of AA on HF circuits. The statistical distributions of AA are described indetail in Section 7.2.


Figure 8.92. The same as Figure 8.91, but for June 1960 (from Tveten, 1962)

Figure 8.93. Periods of reception of 13- and 17-MHz transmissions from McMurdo toThule during 15–19 May 1958 (from Gerson, 1964).

Table 8.14 gives estimates of attenuation on three paths due to AA extrapolatedfrom the riometer data. The absorption is calculated from the 30-MHz riometervertical-absorption values using the inverse frequency-squared relation for theone-hop F-layer mode.

The behavior of the change in the lowest-usable frequency (LUF) – which isclosely related to absorption – on four of these paths during an intense PCA isshown in Figure 8.97.


Figure 8.94. A map showing the location of five HF forward-sounding circuits and sup-porting riometers in relation to an idealized polar-cap absorption area for the CanadianDRTE propagation experiment in 1960–1961 (after Jull, 1964).

Table 8.13. Riometers and path characteristics

Relevant riometer

HF forward-sounding Path-length stations

circuits (km) North South

N–S subauroral: OT–Ch 1900 Ch VDN–S transauroral: OT–RB 3400 CH CJN–S innerauroral: Ch–RB 1830 RB ChTrans-Atlantic: OT–HA 5640Ground–air: HAL–A/C 0–2520

Notes:OT, Ottawa; Ch, Coral Harbour; RB, Resolute Bay; HA, The Hague; HAL, Halifax.


Figure 8.96. The time-percentage occurrence of auroral absorption as a function of Kp for1959–1961 (after T. R. Hartz, L. E. Montbriand and E. L. Vogan. A study of auroralabsorption at 30 Mc/s. Can. J. Phys., 41, 581 (1963).)

Figure 8.95. The percentage of time (solid line) and percentage of half-hour periods(dashed line) for which auroral absorption equaled or exceeded 1.0 dB, as functions of geo-magnetic latitude. Locations of the stations are indicated by the two-letter abbreviations onthe abscissa (after T. R. Hartz, L. E. Montbriand and E. L. Vogan. A study of auroralabsorption at 30 Mc/s. Can. J. Phys., 41, 581 (1963).)


Table 8.14. Extrapolation of 1-dB cosmic-noise absorption to attenuation of10 MHz one-hop F-layer transmissions

Absorption on the Absorption on the Absorption on north north side of the path south side of the and south sides of

Circuit only (dB) path only (dB) the path (dB)

OT–Ch 29 25 54OT–RB 46 43 89Ch–RB 32 29 61

Note:OT, Ottawa; Ch, Coral Harbour; RB, Resolute Bay.

Figure 8.97. July 1961 PCA effects on four HF circuits: (a) for the 30-MHz riometer atResolute Bay; and (b) LUFs.

A unique part of Jull’s (1962) HF high-latitude-propagation experiment wasthe monitoring of 3–23-MHz transmissions from Halifax, Nova Scotia by an air-craft flying in the subauroral and auroral regions during disturbed periods. In par-ticular, one ground–air trial was flown on 15–16 December 1960 (SSN83)during a minor geomagnetic storm (Kp4). The flight plan is shown in Figure8.98 and the observed MUFs and LUFs as functions of time and location areshown in Figure 8.99.

During the flight it was found that taking soundings every 5 min provided suffi-cient information to select proper operating frequencies for ground–air commu-nication. It was further found from this study that selection of communicationsfrequencies once every hour instead of selecting frequency changes on the basis ofsounding data would have resulted in the aircraft being out of contact with theground station for 30% of the flight period. Some conclusions of Jull et al. (1964)


Figure 8.98. The flightplan for ground–airtrials of 15–16 December1960 (from Jull, 1962)


Figure 8.99. The MUF and LUF observed during the ground–air trials of 15–16 December1960: (a) the outgoing leg of the flight; and (b) the incoming leg of the flight (from Jull,1964).

(a)

(b)

were that, during PCA events of low or moderate intensity, the optimum trafficrouting is via AE in the oval and the optimum routing is through intermediaterelay stations.

The characteristics of 25.5-MHz one-hop propagation on a 950-km Alaskanpath over a 14-month period shortly after the maximum of solar cycle 22 were pre-sented as a function of Kp by Hunsucker et al. (1996). The location of the E-regionreflection point was within the auroral oval for 3Kp5 and the specific behav-ior of the signal was related to auroral-oval phenomena such as substorms, geo-magnetic storms, and the Harang discontinuity. The location of the auroralelectrojet with respect to the mid-point of the path was also found to be of con-siderable importance. A map showing the Wales to Fairbanks, Alaska path in rela-tion to the equatorward edge of the auroral oval as a function of Kp is shown inFigure 8.100 and an example of the typical behavior of signal amplitude is shownin Figure 8.101.

It is reasonable to assume that the AE mode (defined on p. 480) is uncontami-nated by F-layer propagation, because, during the period of maximum occurrence


Figure 8.100. A map showing the Wales-to-Fairbanks, Alaska propagation path in relationto the equatorward edge of the auroral oval as a function of Kp.

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of AE (2100–0400 LT) – especially September through March – data from anionosonde and an incoherent-scatter radar located near Fairbanks have shownthat there is not enough F-region ionization present to support an F-layer mode;also the antenna takeoff angles, path distance, and operating frequency tend toexclude the possibility of an F-mode. An AE “burst” was defined as a signalreceived for 2 min or more. Burst duration, date/time of start of the burst, signalstrength in decibels, and Earth-current amplitude and direction data were scaledfrom strip-charts, and Ap and Kp values were added and tabulated on spreadsheets.Figures 8.102, 8.103, and 8.104 show the diurnal and seasonal behaviors forwinter, spring, and summer of 1992, respectively and Figure 8.105 shows theoccurrence of AE as a function of Kp. The seasonal characteristics of the AEbursts are illustrated in Table 8.15.

A schematic representation of the Harang discontinuity showing the eastwardand westward electrojets is shown in Figure 8.106 and the responses of the AEsignal and Earth current are shown in Figure 8.107. The high-latitude currentsystems are discussed in Sections 2.5.3 and 6.4.4.

From analysis of the 14 months of data obtained during 1991–1992, it wasfound that the AE signal was very “bursty” in character, with bursts lasting from1 min to over 3 h, with an average duration of 11 min and an average signal ampli-tude 20–30 dB above the detection threshold of 115 dB m for the receiver. Outof 1445 observations, 981 events (68%) lasted 10 min, 234 (16%) had durationsbetween 11 and 20 min, 90 (6%) had durations between 21 and 30 min, and 11 haddurations greater than 90 min. One of these “long” events occurred in the Fall,and the rest occurred in the Spring or Summer.

Although the signal characteristics are quite poorly correlated to Kp, they arequalitatively correlated to the local magnetic indices and to Earth-current data atthe receiving site in Fairbanks. The behavior of the Wales AE signal on 25.5 MHzreceived at Fairbanks very closely resembled the occurrence statistics of the visualaurora and VHF/UHF auroral-radar results. It is believed that this is the firstquantitative demonstration of the “forward-propagation behavior” of a one-hop“auroral-E” path near the upper end of the HF band and it was suggested that itmight be possible to utilize the “auroral-E-burst” mode for data transmissionand/or communication over path-lengths on the order of 1000 km inside and par-allel to the auroral oval or to enhance the meteor-burst-communication (MBC)mode at high latitudes.

For 2 weeks during the period of observations of the Wales–Fairbanks AEexperiment, Wagner et al. (1995) conducted a “HF-channel-probe” experiment ona 1294-km path between Sondrestrom, Geenland and Keflavík, Iceland. Thechannel probe measured delay, Doppler, and amplitude characteristics on thispath. The equipment parameters, paths and other characteristics of these twoexperiments are compared in Table 8.16.

Results from the Wagner et al. (1995) HF-channel-probe observations are sum-marized as: (1) strong, specularly reflected ionospheric returns characteristic of a



Figure 8.102. The occurrence of AE for Winter 1991–1992 (from Hunsucker et al., 1996).

Figure 8.103. The occurrence of AE for Spring 1992 (from Hunsucker et al., 1996).


Figure 8.105. The occurrence of AE as a function of Kp (from Hunsucker et al., 1996).

Figure 8.104. The occurrence of AE for Summer 1992 (from Hunsucker et al., 1996).


Table 8.15. Seasonal characteristics of AE signals recorded in Fairbanks, Alaska(from Hunsucker et al., 1996)

No. ofevents of Longest

Average Average duration durationduration amplitude exceeding observed No. of

Season (min) (dB m) 60 min (min) events

Autumn(August–October 1991) 9.9 17.4 7 120 403

Winter(November–December1991, January 1992) 8.3 19.0 2 84 383

Spring(February–April 1992) 8.6 18.4 1 65 272

Summer(May–July 1992) 21.0 19.2 21 192 388

Figure 8.106. A schematic representation of the Harang discontinuity, illustrating thedirection of flow of equivalent current in the electrojet.

SUN

12h

18h 6h

50°

60°

70°

Harang Discontinuity

Wes

twar

d Elec

trojet

Eastward Electrojet

quiescent daytime ionospheric channel during magnetically quiet conditions; (2)strong specular multipath signals reflected from horizontal gradients of electrondensity – which are regularly encountered at night; (3) weak scatter returns, alsopersistent at night; and (4) maximum Doppler shifts of 16 Hz at 7.5 MHz nearmidnight (E layer ) and a maximum Doppler shift of 22 Hz at 14.5 MHz nearmidnight. They infer that the drift speed of irregularities is 1200 m s1 parallelto the great-circle propagation path for the midday “disturbed” ionosphere. Itappears that most of the observations discussed were for subauroral ionosphereconditions.

Warrington et al. (1997) have analyzed data on two paths: one within the polarcap (Clyde River on Baffin Island to Alert, Canadian NWT, 1345 km) and ClydeRiver to Prudhoe Bay, Alaska, 2955 km). Measurements of several parameters,including Doppler spreading, were made during July and August 1989 and duringJanuary and February 1989 (near the maximum of sunspot cycle 22).


Figure 8.107. An example of simultaneous recording of Earth-current measured atFairbanks, Alaska and Wales AE signal amplitude recorded at Fairbanks, on 9 October1991 (Kp7; Ap101) (from Hunsucker et al., 1996).

Transmissions were made using 2-min sequences once per hour on each of 14frequencies from 3 to 23 MHz; each sequence included a 30-s period of CW trans-mission during which the Doppler spectrum of the received signal was measured.The Doppler spreading was quantified in terms of the area under the normalizedsignal-amplitude spectrum, minus the area estimated to be due to noise – theresulting area was multiplied by 20 and the product, referred to as the Dopplerspread index (DSI), employed as a measure of the Doppler spread.

Within the polar cap (Alert to Clyde River) the DSI varied between 10 and30 Hz in the summer and between 30 to 75 Hz during the winter, whereas thelong path (Prudhoe–Clyde River) had a mean DSI of 60 Hz in the summer and 90Hz during winter months. Specific details of the Doppler spread as a function offrequency, path, and magnetic activity are presented in their paper. It was not pos-sible to accurately specify the time when the principal ionospheric reflectionpoints were actually within the auroral oval.

8.5 VHF/UHF and microwave propagation

The international frequency-band delineations (LF/MF/HF/VHF, etc.) are some-what arbitrary but still useful. Instead of defining the bands in decades, it wouldbe better to define the bands in terms of their propagation behaviors in terrestrialatmospheric regions.

Propagation in the VHF through UHF bands (30 MHz to 3 GHz) and micro-waves (1–10 GHz) is either LOS in the troposphere, with typical path lengths of30–50 km, or via satellite–Earth links. Until the advent of satellite communications


Table 8.16. A comparison of pertinent parameters for the Wales–Fairbanks andthe Sondrestrom–Keflavík HF experiments

Wales–Fairbanks HF Sondrestom–Keflavík HF experiment experiment

Path location In auroral oval for 5Kp3 Subauroral, except for Kp5

Mid-point of path 66° CGL 72° CGLPath length 950 km 1294 kmDominant mode Auroral-E E and some FFrequency 25.545 MHz 3–11 MHz

Transmission mode CW Complex pulse simultaneouswith Chirpsounder

Transmitter power 100 W CW 170 W pulse

Duration of experiment 14 months 2 weeks (13 March–2 April (July 1991–September 1992) 1992)

in the 1960s, the “backbone” of the USA’s trans-continental communicationssystem was the Bell microwave relay system.

More rarely used modes are the beyond-line-of-sight modes, ionospheric scatter(ionoscatter) using frequencies from 30 to 150 MHz over path-lengths of1000–2000 km, and troposcatter using frequencies from 200 MHz up to 19 GHzon path-lengths from 300 to 600 km. Since the forward-scattered energy for ionos-catter and troposcatter links is extremely weak (compared with that for LOSpaths), scatter systems must utilize high transmitter power, very large antennaapertures, high-gain receiver front-ends and multiple diversity.

These systems provided very high circuit reliability (99.9%) for high-securitycommunications, but were very costly to install and maintain. The “White Alice”system supplied the communications between the “Dewline” (Distant EarlyWarning) radar system in the northern USA and Canada from the late 1950s untilsatellite communication came into use.

Another scatter mode that is still in use is meteor scatter (from meteor ioniza-tion trails in the E region) using frequencies from 40 to 150 MHz, because suchsystems offer very secure and survivable communications. It is a signal burstsystem with typical bandwidths of 100 kHz, Doppler spread of 5 Hz, and an infor-mation duty cycle of 5% (see Davies, 1990; Section 13.4; and Weitzen, 1988).

Millimeter waves ( f 1.50–13 GHz) have also been used for terrestrial LOScommunication links, but such use is limited by rather severe atmospheric absorp-tion and high-latitude effects are not very well documented.

Advances (in the last 20 years or so) in our understanding of the phenomenaof VHF terrestrial propagation at high latitudes stem primarily from studies of theeffects of AE ionization on MBC, and studies of trans-ionospheric propagationin the development of morphological models of scintillation effects. The lattersubject will be covered in Section 9.2.2 and we will list salient effects of the formersubject herewith.

Meteor scatter was developed as a relatively inexpensive, high-data-rate, securecommunication system primarily for the military, using frequencies typically inthe 40–104-MHz region. Cannon et al. (1985) described results from a MBC prop-agation experiment involving transmission between Bodo, Norway and Wick,Scotland at 40 and 70 MHz. It was found that excess D-region ionization pro-duced a rotation of polarization, which caused some deterioration of normalsystem performance and they also concluded that frequencies close to 40 MHzmay be too low for use at high latitudes.

In another investigation, Ostergaard et al. (1985) reported the advantages ofusing adaptive techniques to improve system performance on a 1200-km path innorthern Greenland and qualitatively described some of the effects of AE ioniza-tion, irregularities, and D-region absorption on MBC systems. The applicabilityof adaptive antenna beam steering to the prediction model for MBC systems,including high-latitude effects, was reported by Akram and Cannon (1994).

Specifically, the prediction models gave good results during the winter and


equinoctial months but poor agreement during the summer on the Sondrestrom–Narsarsuaq, Geenland path. Cannon et al. (1996) found that, on the Greenlandpaths at 35 and 45 MHz, MBC is sustained by E-region ionization and at 65 and85 MHz the path is dominated by meteor-scatter modes.

Although it is not due to the high-latitude ionosphere, VHF/UHF propagationin the Arctic and Antarctic regions frequently displays anomalous behavior.Kennedy and Rupar (1994) describe the Arctic Unattended PropagationExperiment (ARUPEX) on the north slope of Alaska, which operates on a path-length of 50.9 km at 142.875 and 420.5 MHz. Many instances of VHF/UHFducting and diffraction anomalies were observed.

8.6 Summary

A considerable amount of useful data on the phenomena of HF polar and auroralpropagation was obtained during the period c. 1956–1997 and presented in thischapter, providing at least some qualitative indications for ameliorating the mostdeleterious effects. An extreme range of solar activity occurred during this period,from the maximum of solar cycle 19 in March 1958 (SSN201.3) to a minimumvalue of SSN9.6 in October 1964, providing “worst-case scenarios” for high-latitude HF propagation.

At ELF/VLF frequencies, the most profound effects are probably those asso-ciated with polar-cap absorption (PCA) – also known as solar-proton events(SPEs). Refer to Section 7.3. These events result in a lowering of the reflectionheight of the D region, which changes the dimensions of the Earth–ionospherewaveguide, which then produces changes in the phase and amplitude of theELF/VLF signals. There seems to be no firm quantitative evidence that thesechanges produce serious effects on ELF transmissions received in submerged sub-marines, but there is some evidence that polar effects on VLF transmissions mightdegrade the accuracy of certain navigational systems.

An investigation in Alaska and the northern tier of the continental USA hasrevealed that E-region ionization in the auroral oval called auroral-E (AE) canstrongly influence MF skywave propagation at night. A 5-year monitoringprogram revealed the diurnal, seasonal, and sunspot-cycle behavior of MF sky-waves at high latitudes and resulted in the US Federal CommunicationsCommission issuing new skywave curves describing possible skywave interferencebetween standard AM broadcasting stations in the northern tier of the USA,Alaska and Canada, thus making channel assignments more realisitc.

Since skywave propagation dominates at high frequencies, the polar andauroral-oval ionosphere profoundly affect HF propagation at high latitudes, and,during the period 1956–1969, there were many studies of the behavior of HF polarand auroral circuits. These studies revealed that the primary disturbance param-eters were PCAs, AA events, AE ionization and F1-layer effects. The behavior of

8.6 Summary 531

HF propagation at high latitudes, then, is determined by the location of the prop-agation path in relation to the intersections of the path with the auroral and polarD region and the E-region and F-region reflection points. Because of the complex-ity of HF high-latitude mode structure and ionosopheric intersections, it is notpossible to do accurate three-dimensional ray-tracing unless one has an accuratethree-dimensional realtime description of the irregular structure of the high-latitude ionosphere – thus making it very difficult to devise reliable HF-propagation-prediction programs.

Other important propagation phenomena on high-latitude HF paths (com-pared with mid-latitude paths) are increases in Doppler shift and spread, fadingand non-great-circle (NGC) propagation. At certain times and on certain paths,the maximum operating frequency (MOF) may be carried by AE ionization,F1-layer effects, the NGC propagation mode, or possibly ducted modes.

Studies during this period also revealed that VHF frequencies as high as 32MHz on trans-polar paths and 46 MHz on a path from Thule, Greenland toCollege, Alaska were possible during periods of sunspot maximum. The effects ofthe polar and auroral ionosphere on trans-ionospheric signals will be described inthe next chapter.


Section 8.1Croft, T. A. (1968) Skywave backscatter: a means for observing our environment atgreat distance. Rev. Geophys. Space Phys. 10, 73–155.

Davies, K. (1990) Ionospheric Radio. Peter Peregrinus Press, on behalf of the Instituteof Electrical Engineers, London.

Deehr, C. S. and Holtet J. A. (eds.) (1981) Exploration of the polar upper atmosphere.Proc. NATO Advanced Study Institute held at Lillehammer, Norway; 5–16 May 1980.Reidel, Dordrecht.

Folkestad, K. (1968) Ionospheric Radio Communications. Plenum Press, New York.

Goodman, J. (1992) HF Communication – Science and Technology. Van NostrandReinhold, New York.

Hunsucker, R. D. (1967) HF propagation at high latitudes, QST Mag. February, 16–19and 132.

Hunsucker, R. D. (1971) Characteristic signatures of the midlatitude ionosphereobserved with a narrow-beam HF backscatter sounder. Radio Sci. 6535–548.

Hunsucker, R. D. (1991) Radio Techniques for Probing the Terrestrial Ionosphere.Springer-Verlag, Heidelberg.

Hunsucker, R. D. (1992) Auroral and polar-cap ionospheric effects on radio propaga-tion. IEEE Trans. Antennas Propagation 40, 818–828.

Hunsucker, R. D. and Bates, H. F. (1969) Survey of polar and auroral region effects onHF propagation. Radio Sci. 4 347–365.


Landmark, R. (ed.) (1964) Arctic Communications. Published on behalf ofNATA/AGARD; Pergamon Press, New York.

Lied, F. (1967) Arctic Communications, with Emphasis on Polar Problems.AGARDograph 104; Technivision; Maidenhead.

Rawer, K. (1976) Manual on Ionospheric Absorption Measurements. World DataCenter A Solar–Terrestrial Physics, Boulder, Colorado.

Soicher, H. (ed.) (1985) Propagation effects on military systems in the high-latituderegion. In Proc. AGARD Conference, CP-382.

Section 8.2Albee, P. R. and Bates, H. F. (1965) VLF observations at College, Alaska of variousD-region disturbance phenomena. Planet. Space Sci. 13, 175–206.

Bannister, P. (1993) ELF propagation highlights. In AGARD Conference Proc. 529, pp.2-1–2-15.

Bates, H. F. (1961) An HF Sweep-frequency Study of the Arctic Ionosphere.Geophysical Institute,University of Alaska, College, Alaska.

Bates, H. F. (1962) VLF effects from the Nov. 10, 1961 polar-cap absorption event, J.Geophys. Res., 67, 2745–2751.

Bates, H. F and Albee, P. R. (1965) General VLF phase variations observed at College,Alaska. J. Geophys. Res. 70, 2187–2208.

Berry, L. A. (1964) Wave hop theory of long distance propagation of low-frequencyradio waves. Radio Sci. D 68, 12.

Chrissan, D. A. and Fraser-Smith, A. C. (1996) Seasonal variations of globally meas-ured ELF/VLF radio noise. Radio Sci. 31, 1141–1152.

Davies, K. (ed.) (1970) Phase and frequency instabilities in electromagnetic wave prop-agation. AGARD Conference Proc. 33. Technivision Services, Slough.

Fraser-Smith, A. C. and Bannister, P. R. (1998) Reception of ELF signals at antipodaldistances. Radio Sci. 33, 83–88.

Wait, J. R. (1970) Electromagnetic Waves in Stratified Media. Pergamon Press, Oxford.

Wait, J. R. (1991) EM scattering from a vertical column of ionization in the earth–ionosphere waveguide. IEEE Trans. Antennas Propagation 39, 1051–1054.

Watt, A. D. (1967) VLF Radio Engineering. Pergamon Press, Oxford.

Weitzen, J. A. (1988) Meteor scatter propagation. IEEE Trans. Antennas Propagation37, 1813.

Section 8.3Hunsucker, R. D., Delana, B. S., and Wang, J. C. H. (1987) Effects of the February1986 magnetic storm on medium frequency skywave signal received at Fairbanks,Alaska. Proc. IES87, 197–204.

Hunsucker, R. D. and Delana, B. S. (1988) High Latitude Field-strength Measurementsof Standard Broadcast Band Skywave Transmissions Monitored at Fairbanks, Alaska.Geophysical Institute, University of Alaska, Fairbanks, Alaska.


Section 8.4Auterman, J. L. (1962) Fading correlation bandwidth and short-term frequency stabil-ity measurements on a high-frequency transauroral path. NBS Tech. Note 165.

Bartholomew, R. R. (1966) Results of a High-latitude HF Backscatter Study. StanfordResearch Institute, Menlo Park, California.

Bates, H. F. and Hunsucker, R. D. (1964) HF/VHF Auroral and Polar Zone ForwardSounding. Geophysical Institute, University of Alaska, Fairbanks, Alaska.

Bates, H. F and Albee, P. R. (1966) On the Strong Influence of the F1 Layer on Mediumto High Latitude HF Propagation. Geophysical Institute, University of Alaska,Fairbanks, Alaska.

Bates, H. F., Albee, P. R., and Hunsucker, R. D. (1966) On the relationship of theaurora to non-great-circle HF propagation. J. Geophys. Res. 71, 1413–1420.

Bates, H. F. and Hunsucker, R. D. (1974) Quiet and disturbed electron density profilesin the auroral zone ionosphere. Radio Sci. 9, 455–467.

Egan, R. D. and Peterson, A. M. (1962) Backscatter observations of sporadic-E. InIonospheric Sporadic-E (ed. E. K. Smith), p. 9.

Gerson, N. C. (1964) Polar communications. In Arctic Communications (ed. B.Landmark), p. 83. Pergamon Press, Oxford.

Goodman, J. M. (1992) HF Communication – Science and Technology. Van NostrandReinhold, New York.

Hartz, T. R., Montbriand, L. E., and Vogan, E. L. (1963) Can. J. Phys. 41, 581.

Heppner, J. P., Byrne, E. C., and Belon, A. E. (1952) The association of absorptionand Es ionization with aurora at high latitudes. J.Geophys. Res. 57, 121–134.

Hunsucker, R. D. and Stark, R. (1959) Oblique fixed-frequency soundings. In FinalReport on Contract No. AF 19(604)–1859 (ed. L. Owren).

Hunsucker, R. D. and Owren, L. (1962) Auroral sporadic-E ionization. J. Res. NBSRadio Propagation D 66, 581–592.

Hunsucker, R. D. (1964a) Auroral absorption effects on a transpolar synchronizedstep-frequency circuit. Proc. IEEE, 52, March.

Hunsucker, R. D. (1964b) Auroral-zone absorption effects on an HF arctic propaga-tion path. Radio Sci. D 68, 717–721.

Hunsucker, R. D. (1965) On the determination of the electron density within discreteauroral forms in the E-region. J. Geophys. Res. 70, 3791–3792.

Hunsucker, R. D., Rose, R. B., Adler, R., and Lott, G. K. (1996) Auroral-E modeoblique HF propagation and its dependence on auroral oval position. IEEE Trans.Antennas Propagation 44, 383–388.

Jelly, D. H. (1963) J. Geophys. Res. 68, 1705.

Jull, G. W. (1964) HF propagation in the Arctic. In Arctic Communications (ed. B.Landmark), pp. 157–176. Pergamon Press, Oxford.

Koch, J. W. and Petrie, L. E. (1962) Fading characteristics observed on a high fre-quency auroral radio path. J. Res. NBS Radio Propagation D 66, 159–166.

Leighton, H. I., Shapley, A. H., and Smith, E. K. (1962) The occurrence of sporadic-Eduring the IGY. In Ionospheric Sporadic-E (ed. S. Matsushita and E. K. Smith), p. 166.MacMillan, London.


Lomax, J. B. (1967) High-frequency Propagation Dispersion. Stanford ResearchInstitute, Menlo Park, California.

McNamara, L. (1991) The Ionosphere: Communications, Surveillance and DirectionFinding. Krieger Publishing Co., Malabar, Florida.

Maslin, N. (1987) HF Communications – A System Approach. Plenum Press, New York.

Moller, H. G. (1964) Backscatter observations at Lindau-Hartz with variable fre-quency directed to the auroral zone. In Arctic Communications (ed. B. Landmark), pp.177–188.

Ortner, L. and Owren, L. (1961) Multipath Propagation on Transarctic HF Circuits.Kiruna Geophysical Observatory, Kiruna.

Ostergaard, J. C., Rasmussen, J. E., Sowa, M. J., McQuinn, J. M., and Kossey, P. A.(1985) Characteristics of high-latitude meteor scatter propagation parameters over the45–104 Mhz band. In Proc. AGARD (NATO) Conference.

Owren, L., et al. (1959) Arctic Propagation Studies at Tropospheric And IonosphericModes of Propagation. Geophysical Institute, University of Alaska, College, Alaska.

Owren, L. (1961) Influence of solar particle radiations on Arctic HF propagation, pre-sented at the AGARD Ionospheric Research Communication meeting, Naples, 15–20May.

Owren, L., Ortner, J., Folkestad, K., and Hunsucker, R. D. (1963) Arctic Propagationat Ionospheric Modes of Propagation. Geophysical Institute, University of Alaska,College, Alaska.

Peterson, A. M., Egan, R. D., and Pratt, D. S. (1959) The IGY three-frequency back-scatter sounder. Proc. IRE 47, 300–314.

Rose, G. (1964) Field strength measurements over a 2000 km subauroral path(Sodankylä–Lindau) compared with the absorption observed at the terminals. InArctic Communications (ed. B. Landmark). Pergamon Press, Oxford.

Tveten, L. H. (1961) Ionospheric motions observed with high-frequency backscattersounders. J. Res. NBS D 65, 115–127.

Tveten, L. H. (1961) Long-distance one-hop F1 propagation through the auroral zone.J. Geophys. Res. 66, 1683–1684.

Warrington, E. M., Dhanda, B. S. and Jones, T. B. (1997) Observations of Dopplerspreading and FSK signaling errors on HF signals propagating over a high-latitudepath. Proc. IEE, 6th International Conference on HF Radio Systems and Techniques,pp. 119–123.

Weitzen, J. A., Cannon, P. S., Ostergaard, J. C., and Rasmussen, J. E. (1993) High-latitude seasonal variation of meteoric and nonmeteoric oblique propagation at a fre-quency of 45 MHz. Radio Sci. 28, 213–222.

Section 8.5Akrun and Cannon, P. S. (1994) A meteor scatter communication system datathroughput model. IEE HF Radio Systems and Techniques Conference, University ofYork, Vol. 392, pp. 343–347.

Cannon, P. S., Dickson, A. H., and Armstrong, M. H. (1985) Meteor scatter commu-nication at high latitudes. In Proc. AGARD (NATO) Conference.


Cannon, P. S., Weitzen, J. A., and Ostergaard, J. (1996) The relative impact of meteorscatter and other long distance high latitude propagation modes on VHF communica-tion systems. Radio Sci. 31.

Ostergaard, J. C., Rasmussen, J. E., Sowa, M. J., McQuinn, J. M., and Kossey, P. A.(1985) Characteristics of high-latitude meteor scatter propagation parameters over the45–104 Mhz band. In Proc. AGARD (NATO) Conference.

Weitzen, J. A. (1988) Meteor scatter propagation. IEEE Trans. Antennas Propagation37, 1813.


Chapter 9

High-latitude radio propagation: part 2 – modeling, predictions, and mitigation of problems

There are no such things as applied sciences, only applications ofscience.

Louis Pasteur

9.1 Introduction

In Chapter 8 we reviewed the progress of our understanding of high-latituderadio propagation starting about 1956 when it was deemed to be a problemworth investigating, and continuing through the IGY, IGC, and IQSY interna-tional study periods until the present time. In the last 20 years we have made con-siderable progress in our level of understanding of the phenomena both ofauroral and of polar radio propagation and there has been a “sea-change” incommunications and computer technology. This forward leap in technologyincludes the availability of powerful, inexpensive computers and predic-tion/modeling/ray-tracing software, sophisticated modulation schemes,advanced antenna theory and practice, electronic-circuit VLSI, advancedground-based and satellite-borne geophysical sensors, and active-circuit sound-ing systems (see Chapter 4). This chapter will concentrate on experimentalresults obtained starting in the late 1980s, ionospheric modeling, ray-tracing,prediction techniques, mitigation techniques and the impact of space-weatherdata on ionospheric propagation.

The morphology of auroral-E (AE) propagation in the 25–30-MHz frequencyrange on 1000–2000-km paths tangential and normal to the auroral oval hasbeen documented by Hunsucker et al. (1996) and Nishino et al. (1999).

537

9.2 Ionospheric ray-tracing, modeling, and prediction ofpropagation

9.2.1 Ionospheric ray-tracing

In order to make useful predictions by applying ionospheric ray-tracing programsat high latitudes, one must have an accurate model of electron-density profiles ata sufficient number of points along the propagation path. Since most of thesophisticated ionospheric models available produce basically climatology (not“weather”) outputs on a fairly sparse data grid, they are at present not adequateto define the high-latitude ionosphere for the level of ray-tracing needed for pre-diction purposes. Additionally, none of these models includes D-region absorp-tion in the polar or auroral ionosphere (see Sections 7.2 and 7.3), which is afirst-order effect on HF propagation. Not all radio-propagation-prediction pro-grams utilize a ray-tracing algorithm; some use a “virtual-geometry” techniquewhereas others base their predictions on a data base of actual forward-soundingcircuits. The ray-tracing and virtual-geometry algorithms, of course, are verydependent on accurate ionospheric models and most of the data-based algorithmsare data-sparse for high-latitude regions.

Examples of the type of ray trace obtained in the auroral ionosphere are givenin Figures 9.1–9.3 The Jones–Stephenson (1975) three-dimensional ray-tracingprogram was used with a model based on parabolic fits to the Fairbanks vertical-incidence E- and F-region parameters to produce backscatter ray traces at threedifferent azimuths. These ray traces illustrate the complex three-dimensionalstructure of the auroral ionosphere.

9.2.2 Current high-latitude models

Criteria for deciding the applicability of the numerous ionospheric models to ade-quately describe the high-latitude ionosphere include prediction of the ionizationprofile from the lower D-region up to the upper F-region (500 km), polar plasmaconvection and the behavior of ionospheric currents, latitudinal coverage from55° to 90° CGL, a sufficiently dense grid of observations (cell dimension nolarger than 100 km), and realtime “space-weather” data input. Of the 16 iono-spheric models listed in the STEP Handbook by Schunk (1996), 11 include somehigh-latitude ionospheric parameters. Table 9.1 lists the high-latitude models fromSchunk (1996) plus the International Reference Ionosphere (IRI) model and theParameterized Realtime Ionospheric Specifications Model (PRISM).

Three other earlier ionospheric models have been used rather extensively inHF-propagation-prediction programs (Bent et al., 1975; Chiu, 1975; Rush et al.1984) and, although they are “global models,” they are seriously lacking in effec-tive high-latitude data.

Apropos the scintillation models (Section 5.3.3), Aarons et al. (1995) emphasized


Fig

ure

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set

of e

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for

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mou

w a

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ecan

(s

ee h

ttp:

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om/n

wra

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imat

olog

ical

mod

elV

HF

thr

ough

L b

and

(198

4) a

nd S

ecan

et

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1997

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intp

red)

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r P

hase

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ntill

atio

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mpi

rica

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se f

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olar

reg

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the importance of the variation of solar activity on F-layer irregularities at theequatorward edge of the auroral region over a solar cycle. Their data indicate thathigh-latitude F-layer irregularities occur less often in the auroral region and areof lower intensity during periods of low solar flux. Data from Goose Bay,Labrador, observing from 67° to 70° CGL, indicate that the occurrence of scintil-lation at 250 MHz during a year of low solar flux (1985) is enormously reducedcompared with the occurrence for the same magnetic conditions during a year ofhigh solar flux (1980).

Since the “bottom line” in the usefulness of communication and navigationsystem is the signal-to-noise ratio (SNR – see Section 3.2), it is obvious that radio-noise models are also required. Since 1988, CCIR Report 322-3 has been theaccepted global model of radio noise, but discrepancies have been noted and it hasbeen found that this model, specifically, does not yield very accurate data at highlatitudes, according to Sailors (1995).

Warber and Field (1995) have also provided a long-wave transverse-electric-transverse-magnetic noise-prediction model for the range of 10 Hz to 60 kHz.This model predicts the global distribution of r.m.s. noise, standard deviation,voltage deviation, and amplitude probability distribution for both polarizations.

Another investigation characterizing radio noise at geographic latitudes of30°–50° in the North Pacific using the US Navy HF Relocatable Over TheHorizon Radar (ROTHR) at Amchitka, Alaska was reported by McNeal (1995).The area probed was the subauroral ionosphere near sunspot maximum using fre-quencies from 5 to 28 MHz, and no attempt to characterize the degree of geomag-netic disturbance was made. McNeal found that, on the basis of a relatively smallsample, the ROTHR noise data were within 2.5 dB of the predictions of CCIRReport 322 for that latitudinal region. Also, vertical ionograms, oblique backscat-ter soundings, radar ground-backscatter amplitudes, and noise levels were com-pared with model predictions and the authors state that “the differences betweenthe model and median soundings are small enough to have negligible effect on pre-dictions.”

9.2.3 Validation of ionospheric models

In the last decade more effort has been devoted to attempts to verify and validatethe various ionospheric models, especially through PRIMO (Problems Related toIonospheric Modeling and Observations) workshops, the CEDAR (Coupling,Energetics and Dynamics of Atmospheric Regions) program, and other efforts, asreported by Schunk (1996), Anderson et al. (1998), and Szuszczewicz et al. (1998)

Anderson et al. (1998) compared five of the physical models listed in Table 9.1(TIGCM, TDIM, FLIP, GTIM, and CTIM) with each other and with dataobtained at the Millstone Hill ISR for four geophysical cases, thus this was basi-cally a mid-latitude evaluation. According to this study, the five models displayeddiurnal variations that, in general, agreed with measurements, but each one of the

9.2 Ray-tracing, modeling, and prediction 545

five models exhibited “a clear deficiency” in at least one of the four geophysicalcases that was not common to the other models. In a related study, Szuszczewiczet al. (1998) compared f0F2 and hmF2 outputs of four models (IRI, TIEGCM, FLIP,and CTIP) at mid-latitudes during magnetically quiet conditions (0Kp3) andfound accuracies “generally better than 5%.” Since both of these studies wererestricted to mid-latitudes, we cannot draw conclusions about comparisonsbetween modeled and real data at high latitudes. Doherty et al. (1999), Decker etal. (1999), Bishop et al. (1999), Bilitza (1999), Bust and Coco (1999), and Gangulyet al. (1999) have investigated the validity of the PIM, the PRISM, the IRI and theGPS/NNSS ionospheric models and data bases, and found that moderate successis achieved for predictions at middle latitudes. There have been few validations ofthese models at high latitudes.

9.2.4 The performance of ELF–HF predictions at highlatitudes

It should be re-emphasized that all of the radio-propagation-prediction programsare principally climatological models that produce median-value predictions; there-fore, they cannot be expected to produce weather-type results. (In spite of thiscaveat, however, some HF communicators persist in attempting to use thesemodels for “weather-type” propagation forecasting.) The programs should, cor-rectly used, produce the type of predictive data that will allow a mid-latitude-radio-circuit planner to design radio-communication or navigation-link behavioras a function of time, season, sunspot cycle, and equipment parameters, as well asspecify “worst-case scenarios.”

At high latitudes, we must conclude that the extant predictive systems are inad-equate – even for MUF and LUF predictions. Proponents of these programs areunderstandably reluctant to adequately test their programs at high latitudes, and,for whatever reasons, funding agencies also seem to be rather hesitant to ade-quately validate the programs. Part of this may be due to the de-emphasis of theuse of HF communications in the USA because of the predominance of commu-nication satellites, cables, and LOS UHF links, and the advent of certain adaptiveHF propagation techniques (see Section 9.6.4).

Validation of ELF/VLF prediction

A representative software package for predicting and assessing long-wave propa-gation is described by Ferguson and Snyder (1989). They describe a collection ofprograms developed by the US Navy Labs in San Diego, California for theEarth–ionosphere waveguide mode for VLF through LF (10 kHz through100 kHz) that predict signal strength and SNR on individual propagation pathsover wide geographic areas. The model includes some high-latitude phenomenaand is available in VAX/VMS language (Ferguson and Snyder, 1986). Ferguson(1995) also described a validation campaign for the Long Wave Propagation


Capability (LWPC) prediction program, using measured in-flight signal levelsfrom various transmitters from 10 to 60 kHz from 0° to 80° CGL. The averageabsolute differences between LWPC and measurements parametric in frequencyand distance interval are shown in Figures 9.4 (daytime) and 9.5 (night-time).

Validation of HF prediction

Compared with the ELF/MF portion of the spectrum, the HF (2–30 MHz)band has a plethora of prediction programs available, as described by Goodman(1992, Ch. 5) and by Sailors and Rose (1993) – the latter report also addresses theprediction of skywave signal strengths. Thirteen of the extant programs are listedin Section 3.3.5 of Goodman’s book, but only two of these programs (AMBCOMand ICEPAC) include some high-latitude ionospheric effects. The widely usedmid-latitude HF propagation program IONCAP was modified to include AE ion-ization plus polar and AA effects by Hunsucker (1971) to make predictions for USCoast Guard communications to aid search-and-rescue missions in the north


Figure 9.4. The daytime average absolute difference between LWPC and measurementsparametric in frequency and distance interval (from Ferguson, 1995).

Pacific, but there is no information on the reliability of these predictions. Davé(1990) illustrated the importance of mode-plot diagrams derived from the ray-tracing component of the AMBCOM program for determining the optimumpaths and ground ranges at high latitudes.

There is very little documentation in the literature concerning validation of pre-dictions of these programs with good-quality high-latitude ionospheric data,although several such comparisons are currently in progress. One candidate forproviding high-latitude HF propagation predictions is the PRISM/VOACAPprogram listed in Table 9.1, and other candidates include driving the ICEPAC orAMBCOM prediction programs with PRISM or with the PIM data base. Thebest HF high-latitude propagation-prediction program should ideally include arealistic quantitative first-principles model, a data base that accurately portraysthe polar and auroral D-, E-, and F-region parameters, a realistic radio-noise database, accurate equipmental and antenna parameters, an analytic ray-tracingprogram, and near-realtime “space weather” data inputs. These requirements are


Figure 9.5. The night-time difference in average absolute absorption as in Figure 9.4 (fromFerguson, 1995).

especially true for programs that purport to produce field strength predictions. Thereport by Sailors and Rose (1993) compares seven HF-propagation predictionprograms (three empirical programs – PROPHET, FTZ, and FTZ4, and fouranalytic programs – HFTDA, IONCAP, ASAPS, and AMBCOM) in terms oftheir abilities to predict signal strength. Of these programs, only AMBCOM hadan analytic ray-tracing routine and included high-latitude data.

One of the attempts at validation of a high-latitude HF propagation programusing real data was presented by Thrane et al. (1994) using the ICEPAC programfor predicting performance on two propagation paths within Norway. The geom-etries of the two paths in relation to the auroral zone are shown in Figure 9.6.

The results of this investigation indicated “that ICEPAC represents animprovement over IONCAP as far as the structure of the E and F regions is con-cerned . . . but that transmission losses are not properly included.” They also con-cluded that (1) the prediction code reproduced the main features of the observeddiurnal variation of channel reliability, but significantly overestimated both thereliabilities and MUFs (the discrepancies are particularly pronounced for magnet-ically disturbed conditions and for the short path within the auroral oval) and (2)


Figure 9.6. Locations oftransmitters and receiv-ers for short and longpaths (after Thrane etal., 1994).

the ICED electron density profiles used in ICEPAC depend upon the location ofthe control points of the paths relative to the auroral oval, and therefore changewith the level of geomagnetic disturbance.

Another attempt to validate certain HF-propagation-prediction programs waspublished in four reports based on master’s theses at the US Naval PostgraduateSchool at Monterey, California (Gikas, 1990; Tsolekas, 1990; Wilson, 1991;Burtch, 1991) analyzing PROPHET 4.0, IONCAP-PC 2.5, AMBCOM, andICEPAC, respectively. HF SNR data obtained during the trans-polar 1988–1989NONCENTRIC HF propagation experiment (Rogers et al. 1997) was used to testthe prediction models. The long path between Clyde River, Canada and Leicester,UK is shown with an auroral oval for Kp of 5 in Figure 9.7. Some of the SNRresults from these four studies are summarized in Table 9.2.

It is quite interesting that there were significant differences between the averageerror and/or standard deviation of error as a function of frequency for differentprediction programs and data obtained during the 1989 winter campaign, asshown in Figures 9.8–9.10.


Figure 9.7. The HF propagation path, between Clyde River, Canada and Leicester, UK(after Gikas, 1990).


Table 9.2. Selected SNR errors for four HF propagation programs

HF-propagation-prediction program Propagation path Results Reference

Advanced PROPHET Clyde River–Leicester 70% of Gikas (1990)4.0 predictions were

between 20 dB and 20 dB error

IONCAP-PC 2.5 Clyde River–Leicester Predicts error with Tsolekas (1990)an error less than 10dB, with significant errors during disturbed periods

AMBCOM Clyde River to three Average error was Wilson (1991)polar receiver sites typically distributed

between 20 and20 dB,absolutevalue of averageerror 7–11 dB

ICEPAC Clyde River to four Absolute errors Burtch 1991polar receiver sites from 0.3 to 26.4 dB

Figure 9.8. The average error for Site D, Winter 1989, of ICEPAC predictions from meas-ured values (after Burtch, 1991).


Figure 9.9. The average for Site D, winter 1989 of AMBCOM predictions from measuredvalues: total average error 13.5 dB, standard deviation 28.9 dB and total number of samples2919 (after Wilson, 1991).

Figure 9.10. The standard deviation of IONCAP-PC 2.5 prediction errors versus frequencyfor the Winter 1989 campaign (after Tsolekas, 1990).

9.2.5 Recent validation of selected ionospheric predictionmodels using HF propagation data

A study by Hunsucker (1999) illustrates the use of practical data on HF signalreception, along with space-weather parameters (such as the solar 10.7-cm radioflux, Kp, etc) to validate several HF propagation prediction (hereafter referred toas HFP) programs and one ionospheric model. All of the HFP programs weredesigned with the intent of providing information for planning HF circuits, not forshort-term forecasting.

The propagation data consisted of HF signal amplitudes obtained on auroral,subauroral, and mid-latitude propagation paths during July through December1993 on 5.6, 11.0, and 16.8 MHz at 6-min intervals. These data were obtainedduring the PENEX (Polar, Equatorial and Near-Equatorial Experiment) spon-sored by the US Navy. Some results on the equatorial parts of the experiment havebeen published by Smith (1998) and the “polar” data are used in the present anal-ysis. Only limited space-weather data were available in 1993 and not all of theavailable HFP software is structured to utilize space weather data as input, but theconcept is valid for future validation efforts.

A description of the PENEX

The PENEX program utilized a HF transmitter located at Cape Prince of Wales,Alaska and receivers at Fairbanks, Alaska, Seattle, Washington and Rock Springs,Pennsylvania, as shown in the map of Figure 9.11 The transmitting antennas foreach frequency were halfwave dipoles, one half wavelength above ground, and thereceiving antennas were HF log-periodic antennas (LPAs) at a height of 20 m.The elevation radiation patterns were modeled using the NEC analysis program(Burke, 1981) and no sharp nulls were found for the dominant modes. The fundingfor this project did not permit ray-tracing analysis of specific propagation modes.

The basic modulation scheme selected was direct-sequence–spread-spectrum(DS–SS), in which a digital code sequence modulates the carrier at a much higherrate than the information and produces a (sinx/x)2 power envelope. One of the“Gold Code” pseudo-random noise sequences was selected, producing a signalbandwidth of 40 kHz (Rose, 1993; Omura et al. 1985). This DS–SS technique pro-duced good HF signal levels, high rejection of interference, multipath rejection,and high-resolution range measurement over the planned paths using only 100 Wof transmitter power. (On the basis of preliminary measurements, this DS–SSsystem produced an estimated 40 dB gain over a “conventional” system such assingle-sideband transmission (Rose, 1993)). Transmissions were also made usingcontinuous-wave (CW) Morse code for station identification and frequency-shift-keying (FSK) for “housekeeping” data. These modulation schemes were usedsequentially on 5.604, 11.004, and 16.804 MHz from July through December1993. Only the 4 KB spread-spectrum sequence was analyzed, since it wasobserved to produce a higher SNR than did the other sequences, with reasonable


processing time. These three frequencies represent a reasonable sampling oftypical HF frequencies used during this part of the solar cycle. The Fairbanks andSeattle receiving stations employed receivers that recorded the DS–SS transmis-sions and the Rock Springs receiver received only the CW and FSK transmissions– resulting in quite low signal levels being received at Rock Springs. Therefore, theprincipal analysis effort of PENEX concentrated on the DS–SS data received inFairbanks and Seattle.

Approximately 900 h of signal-amplitude data for the three frequencies – rep-resenting diurnal, seasonal, and geomagnetic activity – were used as the presentdata base. Several days were also available from the Rock Springs station on a“hear–no– hear” basis. Table 9.3 describes the salient features of the four HFPprograms used in this investigation. Extensive discussions of the properties ofHFP software are given by Goodman (1992, Ch. 5), Davies (1990, Ch. 12), andSailors and Rose (1993). An example of the propagation predictions generated byone of the HFP programs that we used (VOACAP – see Lane, 1993) is shown inFigure 9.12 for the auroral oval path (Wales to Fairbanks) for a quiet day inNovember 1993. The horizontal lines below the prediction plot indicate the inter-vals when VOACAP predicted that propagation should occur on that frequency.


Figure 9.11. A map of the PENEX.

Ta

ble

9.3

.C

hara

cter

isti

cs o

fH

F-p

ropa

gati

on-p

redi

ctio

n pr

ogra

ms

used

in t

he a

naly

sis

Cha

ract

eris

tics

of

the

mod

elIn

puts

Out

puts

Rem

arks

and

ref

eren

ces

ASA

PS-

4 (A

dvan

ced

Vir

tual

geo

met

ry,

Tra

nsm

itte

r an

d re

ceiv

erA

LF,

BU

F, E

MU

F,

AL

F

abso

rpti

on-

Stan

d-A

lone

Pre

dict

ion

Aus

tral

ian

iono

sond

e co

ordi

nate

s, d

ate,

SSN

, M

UF,

OW

F, t

akeo

fflim

ited

fre

quen

cy,

Syst

em)

data

plu

s C

CIR

mod

els

sola

r fl

ux o

r T

-ind

ex,

angl

eB

UF

be

st u

sabl

e of

iono

sphe

ric

para

met

ers

syst

em p

aram

eter

s,

freq

uenc

y, E

MU

F

the

and

nois

e (a

n em

piri

cal

usab

le f

requ

enci

esE

-lay

er m

axim

um

mod

el)

freq

uenc

y,M

UF

m

axim

umus

able

fre

quen

cy v

ia t

heF

reg

ion,

OW

F

opti

mum

wor

king

fre

quen

cy;

Car

uana

(19

93)

ICE

PAC

(Io

nosp

heri

cV

irtu

al g

eom

etry

, bas

ed

Tra

nsm

itte

r an

d re

ceiv

er

MU

F, L

UF,

med

ian

fiel

d Q

eff

ecti

ve Q

inde

x;

Con

duct

ivit

y an

don

the

ana

lyti

cal

coor

dina

tes,

dat

e, S

SN o

r st

reng

th (

dB

)St

ewar

t (1

990,

pri

vate

E

lect

rody

nam

ics,

ION

CA

P p

rogr

am, w

ith

sola

r fl

ux, s

yste

m

com

mun

icat

ion)

Pre

dict

ion,

etc

.)ad

ded

high

-lat

itud

e,pa

ram

eter

s, f

requ

enci

es,

CC

IR a

nd U

RSI

dat

aQ

eff

base

sT

ab

le 9

.3.

(con

t.)

Cha

ract

eris

tics

of

the

mod

elIn

puts

Out

puts

Rem

arks

and

ref

eren

ces

PR

OP

HE

T 4

.3.2

Vir

tual

geo

met

ry, d

ata

Tra

nsm

itte

r an

d re

ceiv

er

LU

F, M

UF,

and

OW

F,

OW

F

opti

mum

ba

se o

f ob

lique

HF

co

ordi

nate

s, d

ate,

SSN

or

med

ian

fiel

d st

reng

thw

orki

ng f

requ

ency

;R

ose

soun

der

data

, has

an

sola

r fl

ux, K

p(a

n in

dex

(198

1)au

rora

l ova

l and

a r

ay-

of s

olar

-pro

ton

and

x-ra

y tr

acin

g m

odul

efl

ux)

VO

AC

AP

(V

oice

-of-

Vir

tual

geo

met

ry, b

ased

T

rans

mit

ter

and

rece

iver

M

UF,

med

ian

Fie

ld

Lan

e (1

996)

Am

eric

a ve

rsio

n of

on

the

IO

NC

AP

pro

gram

co

ordi

nate

s, d

ate,

SSN

stre

ngth

ION

CA

P)

(in

a us

er-f

rien

dly

shel

l),

CC

IR a

nd U

RSI

dat

am

odel

s

Not

es:

CC

IR, t

he I

nter

nati

onal

Rad

io C

onsu

ltat

ive

Com

mit

tee

of t

he I

nter

nati

onal

Tel

ecom

mun

icat

ion

Uni

on; S

SN, s

unsp

ot n

umbe

r; U

RSI

,In

tern

atio

nal U

nion

of

Rad

io S

cien

ce.

Fig

ure

9.1

2.

An

exam

ple

of V

OA

CA

P p

redi

ctio

n an

d P

EN

EX

mea

sure

men

ts.

Specific results of the PENEX

Over 900 h of PENEX signal reception at Fairbanks, Seattle, and Rock Springsfrom the period July through December 1993 were analyzed and discussed byHunsucker (1999), and examples of the signal amplitudes received in Fairbanksare given in Figures 9.13 and 9.14. The abscissae in Figures 9.13 and 9.14 areequivalent to the received signal amplitude in the DS–SS system. The horizontalbroken line near the bottom of the plot represents 10% of the peak value and isapproximately equivalent to the required signal level for HF single-sideband com-munications or short-wave (SW) broadcasting, on the basis of limited compari-sons of propagated signals.

We define the “correct prediction percentage” for each HFP for a 24-h periodas “the number of hours for which the program predicted that propagation willoccur on that frequency and path, compared with the number of hours that thepropagation actually occurred, plus the number of hours that the program pre-dicted no propagation on the path at that frequency. This is compared with thenumber of hours that no propagation occurred, all expressed as percentages. Webelieve that the “correct-propagation percentages” thus defined are at least semi-quantitative and should be valid and understandable both to the ionospheric-research community and to the HF-propagation/communications community.Tables 9.4 and 9.5 give the results of the comparison between predicted and


Figure 9.13. The PENEX signal amplitude for 27 September 1993, f11.0 MHz.

observed HF reception on the auroral-oval circuit and the subauroral (usuallymid-latitude) paths, respectively.

The unique feature of this HF dataset is that data are plotted approximatelyevery 6 min, whereas most comparisons between predictions and HF data usehourly average HF values. Thus, the present data present more HF “fine-structure” behavior. It has been known for some time that some variations in HFsignal are produced by the gravity-wave-induced traveling ionospheric distur-bances (TIDs). As noted by Hunsucker (1982), medium-scale TIDs typically haveperiods from 12–50 min (also see Section 1.6). If hourly values of HF data hadbeen used in comparison with the monthly hourly medians used by the predic-tions, the “correct-prediction percentages” would probably have been higher.

The Wales–Rock Springs, Pennsylvania path

McDowell et al. (1993) described the equipment and results of a “hear–no-hear”program monitoring the PENEX transmissions from Wales, Alaska at the RockSprings, Pennsylvania HF receiving site (latitude40.8° N, longitude 77.9° W).Because no complete PENEX receiver was available for this site, only the FSK andCW signals were recorded.


Figure 9.14. The PENEX signal amplitude for 24 August 1993, f11.0 MHz.

This was an interesting multihop 5925-km path, which, under quiet geomag-netic conditions (Kp3), can be considered to be a mid-latitude path.Measurements were made only during August and September 1993, and there wasusable data for 70% of the time.

As geomagnetic activity increases, however, this increasingly becomes anauroral path, with the entire path lying within the auroral oval when Kp8(magnetic-storm conditions) and the first 70% of the path lying within the oval forKp5. Specific examples of PENEX signal reception are given in Figure 9.15. Thestatistical auroral oval (Feldstein and Galperin, 1985) was utilized for this com-parison.

The 5.6-MHz signal, K indices, and predictions of the ASAPS-4 program (seeTable 9.4) during the geomagnetic storm of 13 September 1993 are shown as func-tions of UT in Figure 9.15(a). On this day, the entire path lay in the auroral ovalfrom 0300 to 1030 UT, the first 70% of the path was in the oval from 1030


Table 9.4. Percentages of correct predictions on Wales-to-Fairbanks HF circuit

Equinox (September) Winter (November)

Quiet Disturbed Quiet Disturbed

Frequency (MHz) A I P V A I P V A I P V A I P V Remarks

5.6 36 58 18 47 30 42 33 33 47 53 29 76 51 40 33 40 60 42% for5.6 MHz

11.0 15 12 40 12 15 18 10 18 70 90 77 58 35 40 15 30 35% for11.0MHz

16.8 77 77 67 77 33 33 21 33 48 48 98 48 20 30 20 30 47% for16.8MHz

Program average 43 49 42 45 45 26 31 21 55 64 68 61 32 34 25 40

Notes:A ASAPS-4; I ICEPAC; P PROPHET; V VOACAP.(1) There were no “summer” data for this path.(2) The average corrrect predictions for 5.6, 11.0, and 16.8 MHz for all seasons andlevels of disturbance are 42%, 35%, and 47%, respectively, for this auroral-oval path.(3) On “quiet” days the four programs predicted approximately equal percentages forequinox and winter.(4) On “disturbed” days the percentage of “corrrect” predictions decreased by a factorof two from the quiet-day predictions.(5) There is no significant advantage of one program over another on this path, exceptthat, on quiet fall days, all programs gave high percentages on 16.8 MHz.(6) For all programs, seasons, and frequencies, the aggregate correctly predictedpercentage is 44%.

Ta

ble

9.5

.P

erce

ntag

es o

fco

rrec

t pr

edic

tion

s on

Wal

es-t

o-S

eatt

le H

F C

ircu

it

Sum

mer

Fal

lW

inte

r

Qui

etD

istu

rbed

Qui

etD

istu

rbed

Qui

etD

istu

rbed

Fre

quen

cy (

MH

z)A

IP

VA

IP

VA

IP

VA

IP

VA

IP

VA

IP

V

5.6

2957

7878

3923

1584

4428

2090

5055

2750

5033

2542

4093

650

11.0

4570

5590

4355

5017

1353

7373

3049

6331

3040

4030

2040

4040

16.8

3550

3525

5340

5326

5040

8040

4937

8749

4040

3040

4067

3367

Pro

gram

ave

rage

3659

6464

4539

3942

3640

4167

4347

5943

4037

3237

3766

2652

Not

es:

A

ASA

PS-

4; I

IC

EPA

C; P

P

RO

PH

ET

V.4

; V

VO

AC

AP.

(1)

The

ave

rage

per

cent

ages

of

corr

ect

pred

icti

ons

for

5.6,

11.

0, a

nd 1

6.8

MH

z fo

r al

l sea

sons

and

leve

ls o

f di

stur

banc

e ar

e 45

%, 4

5%, a

nd 4

6%,

resp

ecti

vely

, for

thi

s m

id-l

atit

ude

path

.(2

) O

n qu

iet

days

, it

appe

ars

that

the

four

pro

gram

s av

erag

ed s

omew

hat

high

er in

the

sum

mer

tha

n th

ey d

id in

fal

l and

win

ter

(56%

, 46%

, and

45%

, res

pect

ivel

y).

(3)

On

dist

urbe

d da

ys, t

here

is n

o si

gnifi

cant

diff

eren

ce a

mon

g th

e ac

cura

cies

of

pred

icti

on fo

r su

mm

er, f

all,

and

win

ter

(41%

, 48%

, and

45%

,re

spec

tive

ly).

(4)

The

re is

no

sign

ifica

nt d

iffer

ence

bet

wee

n ac

cura

cies

of

pred

icti

on fo

r qu

iet

days

(46

%)

and

dist

urbe

d da

ys (

45%

).(5

) F

or a

ll pr

ogra

ms,

sea

sons

, and

fre

quen

cies

, the

agg

rega

te c

orre

ctly

pre

dict

ed p

erce

ntag

e is

45%

.

Fig

ure

9.1

5.

PE

NE

X s

igna

l am

plit

udes

on

(a)

5.60

4, (

b) 1

1.00

0, a

nd (

c) 1

6.80

4 M

Hz

on a

dis

turb

ed d

ay, 1

3 Se

ptem

ber

1993

;SS

N

11 a

nd 1

0.7-

cm fl

ux

80.

(b)

Fig

ure

9.1

5.

(con

t.)

(c)

to 1700 UT and the path was tangential to the equatorward edge of the oval at2300 UT, so this nominally mid-latitude path became an auroral path this day.ASAPS predicted only 33% propagation for the day, with moderately accuratesignal-strength predictions from 0400 to 1100 UT.

On 11.0 MHz (Figure 9.15(b)) ASAPS-4 did a good qualitative propagationprediction, but was almost anticorrelated in the signal-strength predictions.Figure 9.15(c) illustrates that the 16.8-MHz signal was barely detectable and thequalitative and quantitative predictions produced by ASAPS-4 were poor(50%).

Space-weather data applied to the PENEX

Some of the limited (1993) available space-weather data were utilized in this inves-tigation of a small sample of the PENEX data and it was found that thereappeared to be no connection between the measured signal strengths and the AE,the cross-polar-cap potential, and the local (Fairbanks) K index on the Wales-to-Seattle path. (One might expect the first hop of this path to be influenced by theauroral ionosphere). However, the peak values of Ap (the linear planetary mag-netic index) coincided with the signal-amplitude peaks on all three frequencies.The IMF Bz southward turnings do not seem to be closely related to variations inthe signal strength, as might be expected, since there is a time delay between thesouthward turning and the ionospheric response. There seems to be some relationbetween the GOES X-ray flux and the first peaks of signal strength on this sub-auroral circuit, which is probably related to increases in F-region ionization. Thereare not many investigations reported in the refereed literature on the relationshipof specific geophysical indices or other space-weather data to HF signal ampli-tudes (see discussions in Davies, 1965; Mather, et al., 1972; and Milan et al., 1998).

Auroral ovals and DMSP images applied to the PENEX

PENEX data were analyzed during the “National Space Weather Event” of 3–11November 1993 covering a large geomagnetic storm. During the disturbed day(4 November) predicted auroral ovals obtained from the PROPHET (see Rose,1982) software and based upon the Feldstein and Galperin (1985) ovals werecompared with DMSP optical-line-scanner auroral images (Figure 9.16).Figure 9.16 also shows the portions of the Wales–Fairbanks propagation path(straight line) which lay within the oval. It is seen that the auroral forms fromDMSP lie well within the predicted ovals and in the propagation path. The dis-crete auroral form shown in Figure 9.16 lay over the mid-point of theWales–Fairbanks path, which has been shown to be very closely related to highelectron density in the E region (Hunsucker and Owren, 1962; Hunsucker, 1965;Hunsucker et al., 1996).

An ionogram from the Fairbanks ionosonde recorded near the time when theauroral forms were in the propagation path which exhibits values of f0Es and fEs



Figure 9.16. “Space-weather data (a DMSP image in relation to the Wales–Fairbankspropagation path).

Figure 9.17. The HF propagation “metric” on 4 November 1993. The amplitude of thesignal is 16.8 MHz.

of 7.4 and 8.2 MHz, respectively, is shown in Figure 9.18, and Figure 9.19 displaysthe 16.8-MHz signal amplitude at Fairbanks on this disturbed day. As may be seenin Figure 9.17, the PENEX 16.8-MHz amplitude peak occurred when an auroralform lay over the mid-point and AE ionization was very intense, which is consis-tent with many previous examples given by Hunsucker. This is one illustration ofthe relation between the ground-based (ionosonde) and satellite (DMSP) space-seather data and the HF propagation metric.

We have given an example of how HF-propagation data can provide direct validation of space-weather effects upon actual operating systems. Analysis ofover 900 h of data on three frequencies on auroral, subauroral, and mid-latitudepaths was utilized for validation of four HF-propagation-prediction programs.The aggregate correct prediction from these programs was only 45% for a widevariation of geomagnetic activity in 1993. This should serve as another caution toHF communicators not to use HFP programs for forecasting propagation.

Limited space-weather data were available as inputs to prediction programsand models and were qualitatively useful in interpreting anomalies in HF propa-gation, but it was not possible to determine quantitative relationships betweenspecific indices and variations in signal for this small sample. Another feature of


Figure 9.18. “Space-weather data” (Fairbanks ionosonde).

this investigation is that amplitudes of HF signals, which respond to observedvariations in signal on high-latitude circuits, were recorded every 6 min.

An auroral oval from the PROPHET prediction program was in very goodagreement with a DMSP visual auroral image, with an ionogram obtained atFairbanks, Alaska, and with the received amplitude of the 16.8-MHz signal on theWales–Fairbanks path for the disturbed day of 4 November 1993.

It is hoped to continue this type of investigation utilizing the now plentifulspace-weather data with new HF propagation data and with several of the avail-able large ionosphere models. The PENEX research was supported by the USNaval Security Group Command, the Office of Naval Research, and the NavalPostgraduate School-Monterey, and one must acknowledge the data-analysiscontributions made at the Applied Research Laboratories of Pennsylvania StateUniversity and the antenna pattern analyses performed by J. K. Breakall.

9.3 Predictions of VHF/UHF propagation

As illustrated in Section 8.5, there are times (during sunspot-maximumperiods) when VHF signal propagation (up to 46 MHz) has been observed on2000–5000-km polar paths, but, as yet, there is no reliable method of predictingthese openings. Qualitatively, one can, however, expect these modes to occurduring geomagnetically disturbed periods in years with high numbers of sunspotswhen intense AE is present on part of the path.

9.4 Recent efforts at validation of ionospheric models

The Space Environment Corporation of Logan, Utah has under development an“Assimilating Ionosphere Model” (AIM) (Schunk and Sojka, 1999, private com-munication), and this program has been used to generate a database for 4 and 12November 1993 for comparison with the PENEX data. Days 308 and 316 of 1993(4 and 12 November) have been analyzed by displaying NmF2 over the region ofinterest at hourly intervals of UT. The approximate great-circle paths from Walesto Seattle and Fairbanks are shown as two sloping lines in the bottom-left-handpanel of Figure 9.19, which shows the plot for day 316 – corresponding to the geo-magnetically quiet day.

Each panel represents the F-region peak density (NmF2) color coded from 1011

to 1012 electrons m3 . The coordinates are mixed – the longitude is in geographiccoordinates, while the latitude axis is in geomagnetic coordinates. The importantdusk transition occurs from 0200 to 0600 UT and involves a relatively gradualdecrease with UT. In contrast, the dawn–sunrise region has sharp density gra-dients in UT (see from 1700 to 1900 UT). The night-time region has densitiesdropping below 1011 m3(105 cm3) with a relatively narrow, deep-blue trough


9.4 Validation of models 569

Figure 9.19. The USU “Assimilating Ionospheric Model” images taken once per hour for ageomagnetically quiet day (12 November 1993). The approximate great-circle paths fromWales to Seattle and to Fairbanks are shown as two sloping lines in the bottom left-handpanel (after Schunk and Sojka, 1999).

feature. A caveat of importance at this point is that AIM is currently running in aclimatology mode driven by only the Kp and 10.7-cm indices, since no ionosphericdata are available for assimilation. Furthermore, the model is being run in adefault mode with zero topside flux. Therefore, night-time maintenance and hencedensities are probably too low.

AIM is being modified to correct this shortcoming. In analyzing the PENEXdata for comparison with AIM, it should be mentioned that AIM models the O

density and molecular-ion density from 100 to 1000 km and includes the Hardyet al. (1987) empirical electron-precipitation model. D-region absorption effects(which can be profound on disturbed days) are, however, not included. Therefore,we shall examine the behavior of the Wales–Seattle circuit (a mid-latitude path) inthe framework of the AIM model. The peak amplitudes of the 5.6-MHz signaloccurred at 1300 UT (0300 AST) and 1530 UT (0530 AST), when the pathappears to be roughly aligned with and a few degrees equatorward of the troughfeature. The propagation from 1600 to 2400 UT on 11.0 and 16.8 MHz is, ingeneral, related to increasing electron densities at F2max height. Thus theWales–Seattle (“mid-latitude”) propagation path on this quiet day seems to be inat least qualitative agreement with AIM.

In contrast with Figure 9.19, Figure 9.20 represents a disturbed day (day 308)and the most noteworthy features are

(1) the very clear night-sector auroral region,

(2) the very marked “deep” night-sector trough,

(3) the extremely sharp equatorward trough boundary in the afternoon sector(0200–0600 UT),

(4) higher noon, sunlit densities, and

(5) Storm-enhanced densities at 0200 and 0300 UT. These are the high den-sities prior to the precipitous drop into the trough.

On the disturbed day, propagation occurred on all three frequencies from 0000to 1000 UT, with no propagation from 1100 to 2400 UT. The AIM plots from0000 to 0600 UT are consistent with the observed propagation, but from 0700to 1000 UT the path lies in a deep Ne trough. It is reasonable to assume that AEionization could augment the propagation from 1000 to 1100 UT (0000–0100AST), since this path lies well within the auroral oval at this time. After 1100UT, auroral absorption could contribute to the loss of the PENEX signal.

So, to a qualitative first approximation, the observed behavior of the PENEXHF propagation on these quiet and disturbed days is in agreement with theAIM outputs, as shown in Figures 9.19 and 9.20 and as outlined in the review ofhigh-latitude radio propagation by Hunsucker (1992). It is to be hoped that futureefforts will include ray-tracing through the AIM outputs and then validation byusing HF-propagation-mode structure.


9.4 Validation of models 571

Figure 9.20. The same as in Figure 9.19, but for a geomagnetically disturbed day (afterSchunk and Sojka, 1999).

9.5 Mitigation of disturbance of HF propagation

9.5.1 Early attempts

It has been known since early-1950s studies that the reliability and predictabilityof HF high-latitude propagation were lamentable, and Gerson (1962a, 1962b;1964) presented an interesting qualitative evaluation of various communicationsmodes as shown in Table 9.6.

It should be emphasized that the evaluations and cost estimates in Table 9.6were Gerson’s in the early 1960s and are subject to other reasonable estimates, andthat the communications-satellite mode was not available for comparison at thattime. Nevertheless, it is interesting to note that the VHF scatter mode has sincebeen abandoned because of high costs and that VLF/LF was not really a commu-nications mode. The row of totals indicated that the submarine cable and UHFtropospheric propagation modes rated the best in this evaluation, but, because ofthe high cost and difficulty in establishing a long-range UHF relay system, thelatter was not considered. Some serious consideration was given to laying subma-rine cables, as evidenced by the routes indicated in Figure 9.21. Communicationsand navigational satellites have greatly reduced the use of VLF/LF and HFsystems at high latitudes even with the high cost of such systems and their vulner-ability to some high-latitude ionospheric effects.

The use of forward-sounding circuits and link switching to ameliorate prob-lems with high-latitude HF propagation was discussed in the 1960s (e.g. Fenwickand Villard, 1963; and Hunsucker and Bates, 1969) and later Fenwick and


Table 9.6. Gerson’s (1964) comparison of various communications modes on high-latitude paths on a scale of 1–9 (1excellent, cheapest cost, most reliable, leastproblems, etc.)

SubmarineVHF

UHFParameter cable VLF/LF HF Scatter Meteor Tropospheric

Reliability 2 1 7 2 5 1Bandwidth 2 6 4 4 4 1Potential for 1 2 4 3 2 1Interference“Jammable”? 1 6 8 3 2 1Problems at solar 3 2 6 2 2 1maximum?Initial cost 6 4 3 5 5 6Operating cost 2 1 3 3 3 6Total 17 22 35 24 23 17

Woodhouse (1979) described the extensive US Navy HF frequency-managementsystem using a world-wide network of chirp sounders.

9.5.2 Mitigation using solar–terrestrial data

All of the HF-propagation-prediction programs listed in Chapter 3 and in Table 9.3provide for use of the sunspot number (or solar flux) and a geomagnetic-activityindex (usually Kp) in addition to time of day, month, and year as inputs. The

9.5 Mitigation of disturbance 573

Figure 9.21. A polar map indicating locations of proposed submarine cable routs fromScotland and Norway to Alert and thence to Moosonee and Barrow (from Gerson, 1964).

programs which predict field strength also require the antenna gain, transmitterpower, sensitivity of the receiver, noise levels of the receiver area, etc. as input infor-mation and, as shown in Table 9.4, these programs do not yield sufficiently accuratepredictions – especially at high latitudes. See Sailors and Rose (1993) for a discus-sion of how seven of these programs calculate the field strength. At present, onlythe PRISM/VOACAP and the PROPMAN® (Hu et al., 1998) prediction programsutilize additional solar–terrestrial inputs, and possible improvements due to theseadditional parameters and the use of improved algorithms have not yet been eval-uated (to the best of the authors’ knowledge). Use of the PRISM model with eitherthe ICECAP or AMBCOM prediction programs and realtime solar–terrestrialinputs would probably be more effective at high latitudes than PRISM/VOACAPand such a combination should, of course, be validated. There is some question,however, regarding whether even these models can adequately describe in sufficientdetail the near-realtime high-latitude ionosphere to permit ray-tracing or virtual-geometry calculations sufficient for calculations of HF propagation – especiallyfield-strength predictions.

Another approach to mitigation in predicting reliability of communication forHF through UHF propagation is utilized in the US Navy Radio FrequencyMission Planner (RFMP) described by Brant et al. (1994). RFMP is a suite ofradio-propagation and terrain-modeling programs in an object-oriented interfaceon a work station that allows the user to translate communication-mission objec-tives into user-understandable results. Features include the integration of visual-ization tools, digital mapping, rule-based selection of propagation models,presentation of a model’s results as stochastic values, and estimation of the successof a mission presented in the geographic context. Real-time inputs to RFMPinclude measurements of tropospheric moisture density, GPS-derived TEC, elec-tron-density profiles from ionospheric tomography and vertical-incidence sound-ers, plus satellite-measured solar–terrestrial parameters. RFMP is currentlydeployed on several platforms and is in the process of being validated.

9.5.3 Adaptive HF techniques

Some adaptive HF techniques are briefly described in Chapter 3 of this book andextended descriptions are given in Goodman (1992, Ch. 7) and in the book byJohnson et al. (1997). An essential part of any adaptive HF system is the auto-matic link-evaluation (ALE) scheme which is seen in the hierarchical diagram inFigure 9.22.

The basic ALE operation of establishing a link between two stations proceedsas follows: (1) the calling station addresses and sends a call frame to the calledstation; (2) if the station “hears” the call, it sends a response frame addressed tothe calling station; and (3) if the calling station receives the response, it nowknows that a bilateral link has been established with the called station. The“polled” station does not yet know this, however, so the calling station sends an



Figure 9.22. Hierarchical layers of a HF radio system (from Johnson et al., 1997).

acknowledgement frame addressed to the called station. At the conclusion of thisthree-way “handshake,” a link has been established, and the stations may com-mence transmission of voice or data traffic, or simply note that communicationis possible, and then drop the link. The following protocol is taken from Johnsonet al. (1997, pp. 9–10).

The ALE standard also describes net and group calls and sounds as follows.

(1) A net call is addressed to a single address that implicitly names allmembers of a prearranged collection of stations (a net). All stationsbelonging to the net that hear the net call send their response frames inprearranged time slots. The calling station then completes the handshakeby sending an acknowledgement frame as usual.

(2) A group call works similarly, except that an arbitrary collection of stationsis named in the call. Because no prearranged net address has been set up,each station must be individually named. Called stations respond in timeslots, determining their slot positions by reversing the order in which sta-tions were named in the call. The calling station sends an acknowledge-ment as usual.

(3) A sound is a unidirectional broadcast of ALE signaling by a station toassist other stations in measuring channel quality. The broadcast is notaddressed to any station or collection of stations, but merely carries theidentification of the station sending the sound.

An example of the utilization of ALE techniques on a trans-auroral HF pathwas presented by Bliss et al. (1987), and a fairly detailed description of this experi-ment follows (in order to document this technique). The Trans-Auroral-HFExperiment (TAHFE) was conducted in 1986–1987 on a 4765-km path fromBarrow, Alaska to Cedar Rapids, Iowa, as shown in Figure 9.23 in relation to a“disturbed” auroral oval (Q7).

The TAHFE utilized a remote terminal at Barrow including a CollinsHF-8070A transceiver, a 1-kW power amplifier, a selective calling and scanningunit (Collins HF-8096 SELSCAN® with test-signal-generation capability), FSKmodems, a control microcomputer (PC), interface equipment, and a telephonemodem. The similarly equipped receiving station was located at Cedar Rapids.The SELSCAN® unit was used as a control device to allow transmission of inter-nally generated advanced-link-quality-analyzer (ALQA) tones and 300- and75-bps binary FSK data (via auxiliary ports) and for automatic connectivity tests.The ALQA is a patented Rockwell developmental three-tone generation-and-analysis subsystem for measurement of HF channel parameters, utilizing narrow-band signals within assigned radio channels.The SELSCAN® unit was controlledby a PC for sequencing purposes (time, frequency, and duration), which is pro-grammed by remote control.

At the receiver end of the circuit at Cedar Rapids, the HF-channel characteris-tics quantified are the several parameters measured directly by the ALQA (listed


in Table 9.7). The objectives of the TAHFE are given in Table 9.8 and the experi-mental data base is listed in Table 9.9. The configuration of the TAHFE equip-ment is sketched in Figure 9.24 and the TAHFE test procedure is shown in Figure9.25 and Table 9.10.

We present selected data from one of the disturbed days during the 80-dayduration of the TAHFE program to illustrate the types of data which wereobtained. Figure 9.26 shows the frequencies from 6 to 21 MHz propagated duringthe day 12 November 1986 (SSN15.2; Kp3) with the corresponding three-frequency sounding cycles shown in Figure 9.27. Some of the important signalparameters obtained during 12 November 1986 (SNR, Doppler spread, FSK bit-error rate and multipath spread) are shown in Figures 9.28–9.30. It was planned


Figure 9.23. The Great-circle path (D4765 km) from Barrow, Alaska to Cedar Rapids,Iowa on 14 October 1986 at 0500 UT in relation to an auroral oval for Q7:1, BRW(71.30, 156.80); MP, mid-point (60.26, 109.78); and 2, CDR (42.02, 91.38) (after Blisset al., 1987).


Table 9.7. Selected HF-channel parameters recorded during the TAHFE (afterBliss et al., 1987)

Parameter Units Description

DOY Days Day of yearTOD Hours Time of day (universal time) in decimal hoursFRQ Megahertz Operating frequencySNoR dB Hz Signal-to-noise power-density ratioa

DS Hertz Doppler frequency spreada

MP Milliseconds Multipath time-delay spreada

BER75 Ratio Bit error rate for 75-bps data (10000 bits)BER300 Ratio Bit error rate for 300-bps data (10000 bits)

Notes:ALQA (advanced link-quality analyzer) measurement

Table 9.8. Objectives of the Trans-Auroral HF Experiment (TAHFE) (after Blisset al., 1987)

To collect data for a trans-auroral HF-channel data baseTo investigate (i) the correlation between TAHFE data and solar/geophysical data

(ii) the correlation of oval and propagation predictions to TAHFEdata

To characterize (i) HF-channel characteristics by stepsounding with advanced link-quality analyzer (ALQA)

(ii) FSK data bit-error rates with ALQA(iii) deduced ionospheric states with solar geophysical data

Table 9.9. The TAHFE data base (after Bliss et al.,1987)

Identifier Collection interval

TAHFE 1A 16 September 1986 to 29 October 1986TAHFE 1B 5 November 1986 to 12 December 1986TAHFE 2 11 March 1987 to 11 April 1987

Note:a Preliminary analysis on TAHFE 1B.


Figure 9.24. The TAHFE equipment configuration.

Figure 9.25. The TAHFE test procedure.

to convolve a large solar–terrestrial data base into the results of the TAHFE-mea-sured data, but no funding was available for this effort.

9.5.4 Realtime channel evaluation

The most promising technique for ameliorating deleterious effects on HF high-latitude communication circuits is realtime channel evaluation (RTCE), which is


Table 9.10. TAHFE test procedures (after Bliss et al.,1987)

Typical data file sent to remote (approximately 100 bytes)

XXXX START TIME015 ALQA MEASUREMENT TIME (SEC)050 300 BAUD FSK (SEC)150 75 BAUD FSK (SEC)N CALL AT END OF CYCLE?CDR CALL STATION ADDRESS OR SCAN (NCL)0000 SCAN OR CALL TIME (SEC)LO POWER SETTING03260 F1 FREQUENCY (KHz)

::

29700 Fn

Figure 9.26. The diurnal frequency variation for 12 November 1986 (from Bliss et al.,1986).

22.00

20.00

18.00

16.00

14.00

12.00

10.00

8.00

6.00

FR

EQ

UE

NC

Y (

MH

z)

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00

Time of Day (Hours UT)

described in detail in Chapter 7 (122 pages) of Goodman (1992) and in CCIRReport 889-1 (1966). The technique basically consists of three stages for HF fre-quency management: long-term forecasting, short-term forecasting, and nowcast-ing. Specific classes of RTCE are oblique-incidence sounding (OIS), channelevaluation and calling (CHEC), vertical-incidence sounding (VIS), backscatter


Figure 9.27. Three-frequency step-sounding cycles to be characterized (after Bliss et al.,1986).

Figure 9.28. Typical SNR, frequency, and multipath spread versus time of day for 12November 1986 (from Bliss et al., 1986).


Figure 9.29. Doppler spread (RMS) characterization for 12 November 1986 (from Bliss etal. 1987).

Figure 9.30. A frequency-shift-keying (FSK) error-rate comparison for 75 and 300 BPS for12 November 1986 (from Bliss et al., 1986).

sounding (BSS), frequency monitoring (FMON), pilot-tone sounding (PTS), andan error-counting system (ECS) – the acronyms are those used by Goodman(1992). The CCIR definition of RTCE is

realtime channel evaluation is the term used to describe the processes ofmeasuring appropriate parameters of a set of communications channels inreal time and employing the data thus obtained to describe quantitatively thestates of those channels and hence the capabilities for passing a given class, orclasses, of communication traffic.

The CCIR classes of RTCE are listed in Table 9.11, along with some examples.A relatively long-term (December 1994–summer 1996) investigation of HF

communication channels (some at high latitudes) that utilized a FMCW sound-ing network was reported by Goodman et al. (1997). Propagation parametersincluding ionospheric-mode information, MOFs, SNR, and availabilities of chan-nels for digital data communications were derived and archived. Figure 9.31 is amap showing the HF propagation paths used during this experiment. One of theultimate aims of the RTCE effort, according to Goodman et al. (1997), is toexplore the potential for development of a practical HF data link (HFDL), evenfor high latitudes.

The frequencies used were in the aeronautical mobile band (3.0, 3.5, 4.6, 6.6,9.0, 10.1, 11.4, 13.3, 18.0, and 22.0 MHz) during a period when the number of sun-spots was generally below 50. Data were compared with the minimum values ofSNR required to pass traffic at 300–1800 bits s1. Figure 9.32 illustrates the per-centage availability of signals received at Iceland and transmitted from four sta-tions (Iqaluit and Jan Mayens being the most “auroral” of the paths). Figure 9.33shows the percentage availability of HFDL service for each path and for frequencygroups of 11, eight, six and four frequencies, respectively, illustrating the advan-tage of combining paths.


Table 9.11. The CCIR classes of RTCE

Class one: Remote transmitted signal preprocessinga. Oblique-incidence sounder (OIS)

1. Pulse type2. Chirp type

b. Channel evaluation and calling (CHEC)

Class two: Base transmitter signal preprocessinga. Vertical-incidence sounding (VIS)b. Backscatter sounding (BSS)c. Frequency monitoring (FMON)

Class three: Remote received signal processinga. Pilot-tone sounding (PTS)b. Error-counting system (ECS)


Figure 9.31. The geometry of HF propagation paths in the Northern Experiment (fromGoodman et al., 1997).

Figure 9.32. The percentage availability of signals in the Aeronautical-Mobile bandsreceived at Iceland and transmitted from four stations shwon in Figure 9.29 from 13December 1994 to February 1995, SNR3dB (from Goodman et al., 1997).

Fig

ure

9.3

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In conclusion, this study illustrated the advantages of the availability of a widespectrum of HF frequencies, oblique frequency-sounding, spatial and frequencydiversity, and dynamic frequency and link switching, even at high latitudes.Caveats include that the data were obtained during low sunspot activity and mod-erate geomagnetic activity, and that an essentially “mid-latitude” ionospheric cli-matological model (IONCAP/VOACAP) was used for prediction.

9.5.5 Recent advances in assessment of HF high-latitudepropagation channel

Angling et al. (1998) presented results of measurements of Doppler and multi-path spread on oblique high-latitude HF paths and their use in characterizingdata- modem performance on the basis of four high-latitude HF communica-tions paths. The data were analyzed in a manner pertinent to the design of robustHF data modems. The channel sounder utilized was the Doppler and MultipathSounding Network (DAMSON) (Davies and Cannon, 1993). The DAMSONsystem operates from remote sites on preselected frequencies from 2 to 30 MHz.It is based on commercially available equipment (HF transceivers, PCs, etc.) andmakes extensive use of DSP techniques and uses GPS for system timing – pro-viding reception and transmission synchronized to within better than 10 s andto allow time-of-flight (TOF) measurements to be made. DAMSON uses severalsounding waveforms, such as delay-Doppler, a Barker-13 sequence modulated at2400 bps onto a biphase carrier, and passive noise measurements, as well as othermodes.

Figure 9.34 shows the geometry of the DAMSON paths studied in this investi-gation in relation to the auroral oval for low and high magnetic activity and thepath-lengths are listed in Table 9.12.

Rhombic and sloping-V antennas and power levels of about 250 W were uti-lized for this DAMSON investigation and the data were displayed in the formatshown in Figure 9.35. Sample results from this DAMSON experiment are givenin Figures 9.36–9.38 and Tables 9.13 and 9.14

The authors state that, in addition to measuring the multipath and Dopplerspread and SNR conditions on HF paths, DAMSON data may also be used to


Table 9.12. DAMSON HFpropagation paths

Path Length (km)

Svalbard–Tuentangen 2019Svalbard–Kiruna 1158Harstad–Tuentangen 1019Harstad–Kiruna 194

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ure

9.3

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Figure 9.35. A schematic illustration of DAMSON analysis program display (from Anglinget al., 1998).

Figure 9.36. The bit-error-rate (BER)response of a MIL-STD-188-110A 75-bps modemto Doppler spread (80%power region) and multi-path spread measured at0 dB SNR (fromAngling et al., 1998).

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evaluate the reliability of a circuit by using different modems on the same paths.The modems tested in this DAMSON experiment appeared to be rather robust,with availabilities of up to 95% on subauroral paths, dropping to 64% on theauroral paths. It was claimed that, using the lowest-frequency sub-band andproper frequency-selection, an auroral-path “availability” of 92.5% would bepossible.

9.6 Other high-latitude propagation phenomena andevaluations

9.6.1 Large bearing errors on HF high-latitude paths

Warrington et al. (1997a) and Rogers et al. (1997) have presented results of an HFdirection-finder (HFDF) experiment conducted at high latitudes, in which they

9.6 Other phenomena 591

Table 9.13. A summary of Doppler/SNR plots for multipath spreads of 0–5 ms(from Angling et al., 1998)

Path

Time Frequency S–T S–K H–T H–K

All frequencies 14.0 17.0 14.8 15.02.8–4.7 MHz 15.5 17.5 11.5 9.5

00–24 UT6.7–11.2 MHz 5.0 8.0 8.0 15.0

CNR14.4–21.9 MHz 17.0 20.0 17.5 23.0

(3 kHz)All frequencies 11.5 13.0 13.0 15.5(dB)2.8–4.7 MHz 8.0 12.0 4.0 4.5

19–01 UT6.7–11.2 MHz 6.5 9.5 10.0 11.014.4–21.9 MHz 15.0 16.0 16.0 20.0


Doppler

00–24 UT6.7–11.2 MHz 9.0 8.9 1.9 27.7

spread14.4–21.9 MHz 5.7 15.2 3.9 50.9

(Hz)All frequencies 9.7 12.3 3.8 30.92.8–4.7 MHz 13.0 8.9 2.6 8.1

19–01 UT6.7–11.2 MHz 9.2 11.2 4.0 36.314.4–21.9 MHz 7.6 18.9 4.7 50.0

Notes:S–TSvalbard–TuentangenS–KSvalbard–KirunaH–THarstad–TuentangenH–KHarstad–Kiruna


Table 9.14. A summary of Doppler/multipath plots for SNR of 5 to 5 dB (fromAngling et al., 1998)

Path

Time Frequency S–T S–K H–T H–K


00–24 UT6.7–11.2 MHz 2.6 2.5 2.5 10.714.4–21.9 MHz 0.6 1.1 0.6 5.1

No guardAll frequencies 3.2 4.1 3.2 7.52.8–4.7 MHz 4.2 4.2 3.1 1.9

Composite19–01 UT

6.7–11.2 MHz 1.7 1.9 2.6 9.2

multipath 14.4–21.9 MHz 0.6 1.1 0.6 6.3

spread All frequencies 4.9 6.1 5.4 10.7(ms) 2.8–4.7 MHz 5.3 6.1 4.6 5.1

00–24 UT6.7–11.2 MHz 3.1 7.4 9.1 11.214.4–21.9 MHz 0.6 3.1 0.7 5.2Guard

All frequencies 4.2 4.6 5.1 8.20–1.67 ms

2.8–4.7 MHz 4.3 4.619–01 UT

6.7–11.2 MHz 2.9 4.1 9.314.4–21.9 MHz 0.6 4.1 6.4


00–24 UT6.7–11.2 MHz 12.0 12.5 4.3 30.314.4–21.9 MHz 7.2 22.2 3.0 54.6

No guardAll frequencies 11.3 15.5 4.8 44.72.8–4.7 MHz 12.8 7.0 2.5 4.9

Composite19–01 UT

6.7–11.2 MHz 8.8 10.5 6.0 32.9

Doppler 14.4–21.9 MHz 9.3 25.8 3.9 53.0

spread All frequencies 16.4 24.2 9.7 36.0(Hz) 2.8–4.7 MHz 15.8 14.9 2.8 8.3

00–24 UT6.7–11.2 MHz 17.9 23.3 22.0 31.014.4–21.9 MHz 12.4 30.0 11.4 55.0Guard

All frequencies 16.0 25.3 13.5 46.50–1.25 Hz

2.8–4.7 MHz 15.5 11.219–01 UT

6.7–11.2 MHz 11.2 14.8 34.114.4–21.9 MHz 17.7 31.9 53.4

Notes:S–TSvalbard–TuentangenS–KSvalbard–KirunaH–THarstad–TuentangenH–KHarstad–Kiruna

report finding azimuthal deviations on paths tangential to the auroral oval up to100° from the great-circle path (GCP), as predicted by Bates et al. (1966). Themeasurements were made using a wide-aperture goniometric direction-findingsystem with a dual-band antenna array, a single receiver, and a computer-baseddata-collection and -processing system on frequencies from 3 to 30 MHz. TheVOACAP program (basically a mid-latitude data base) was used to predict modestructure on the high-latitude propagation paths. Rogers et al. (1997) concludethat 50° bearing deviations and deviations as large as 100° from the GCP areprimarily due to lateral reflection from the walls of the mid-latitude ionospherictrough – this was also confirmed by Warrington et al. (1997a).

In another paper, Warrington et al. (1997b) also report bearing deviations ofup to 100° from the GCP on paths contained within the polar cap. Reception oftransmissions on frequencies near 8 MHz from Iqaluit, Canada (D2100 km)and Thule, Greenland (D670 km) were analyzed for the period from Januarythrough April 1994 (near the minimum of solar cycle 22). The authors attributethese large bearing deviations to lateral reflections from large, drifting electron-density structures such as dense plasma and sun-aligned arcs. It is also interestingto speculate that these large bearing deviations might be caused by reflection fromlarge-scale TIDs, which have been observed propagating in the polar-cap F region(see Rice et al., 1988; and Williams, 1989).

Smaller bearing deviations from the GCP were reported by Warrington (1997)using the DAMSON HF experimental system on circuits between Svalbard andCricklade, UK (D3073 km) and a 1383-km polar-cap path between Isfjord andAlert, Canada. Measurements on the trans-auroral Isfjord–Cricklade path weremade on a frequency of 14.4 MHz for 7 days in late 1995 and early 1996 duringthe interval 1100–1600 UT and measurements on the Isfjord–Alert circuit weremade from 0145 to 1342 UT on 22 January 1996. Indicated bearing deviationsup to 2.5° were found, while, for signals on the polar-cap path from Isfjord toAlert, standard deviations of bearing deviation of up to 35°were observed. A vari-ation in bearing with Doppler shift was frequently evident and interpreted as evi-dence that the signal was scattered from ionospheric irregularities drifting acrossthe reflection points. The 1997 HFDF measurements of large bearing deviationsfrom the GCP are a verification of results based on time-delay measurementsreported by Bates et al. (1966).

9.6.2 Effects of substorms on auroral and subauroral HFpaths

Effects of an auroral substorm and ionospheric modification on HF signals prop-agated in February 1996 were reported by Blagoveschchenskaya et al. (1998). HFtransmissions from London on 9.410 and 12.095 MHz were received directly atSt Petersburg along with a signal reflected from the heated ionosphere over


Tromsø. Dynamic Doppler spectra on these received signals showed the presenceof well-defined field-aligned scattered signal components that peaked during themaximum substorm phase. The proposed scattering mode is illustrated in theionospheric ray-tracings in Figures 9.39 and 9.40.

Substorm effects on HF propagation on four paths (transmissions from Quito,Havana, Ottawa and London received at St Petersburg) were also reported byBlagoveschchensky and Borisova (1998). The principal substorm effects are a sub-stantial growth in strength of the signal several hours before the expansion phaseof the substorm and a more significant influence of the ionospheric irregularitiesinside the poleward edge of the main ionospheric trough on the structure of thesignal.

9.6.3 Use of GPS/TEC data to investigate HF auroralpropagation

Hunsucker et al. (1995) presented results of an investigation utilizing GPS TEC“signatures” to forecast AE ionization on a 950-km east–west 25.5-MHz prop-agation path inside the auroral oval as shown in Figure 9.41. The strength andduration of the signal from the AE experiment (Hunsucker et al., 1996) werecorrelated to “signatures” obtained when the propagation path from GPSthrough the satellite was recorded at Fairbanks. Figure 9.41 shows the TEC sig-nature types, data showing the structure over the mid-point of the path, an illus-tration of one particular signature, and the result of “filtering” of TECsignatures.

During the period from December 1993 through January 1995, 58 passes of theGPS prn 28 satellite whose LOS path to Fairbanks was near the mid-point ofE-region propagation path for the 25.5-MHz Wales–Fairbanks propagation pathwere analyzed. The GPS/TEC indications of AE, along with the strength andduration of the AE signal are shown in Figure 9.42.

From analysis of these GPS passes in winter 1993–1994, it appears that it maybe feasible to predict propagation of high-HF-to-low-VHF signals on a near-real-time basis if the mid-points of the E-region propagation paths are within theauroral oval.

An extension of the technique described above to detect auroral activity withinthe oval was reported by Coker et al. (1995). An example of the latitudinal distri-bution of 1-min GPS satellite LOS tracks through the E region for 1 December1993, detections of AE, and the College, Alaska magnetometer H trace are shownin Figure 9.43.

Verifications of the relation between GPS/TEC detection of AE and TIROSprecipitating particle auroral energy flux for Kp values of 1, 2, 3 and 4 are shownin Figure 9.44. Figure 9.45 illustrates how the GPS/TEC data can define the equat-orward boundary of the auroral oval.



Figure 9.39. Simulated ray-tracing of the field-aligned scattered HF signals from the Esregion on the London–St Petersburg propagation path for 12.095 MHz for the geophysicalconditions of 17 February 1996 at 2030 UT (Kp3); (a) height of Es 110 km; and (b) height130 km (after Blagoveschenskaya et al., 1998).

To quote the authors,

Tremendous potential exists for monitoring the effects of auroral substorms(space weather) in real-time. A single GPS station or network of stationscould track the motion of the equatorward edge of the oval, which is animportant boundary for understanding magnetospheric processes.


Figure 9.40. The same as Figure 9.38, but for f9.410 MHz (after Blagoveschenskaya etal., 1998).

9.6.4 The performance of HF modems at high latitude usingmultiple frequencies

In a recent paper, Jodalen et al. (2001) evaluated the performance of two of the“robust-Waveform” modems at 75 bps (STANAG 4415/4285) and 2400 bps(STANAG 4285) along with Morse and voice transmissions on short and mid-range high-latitude paths. Data were acquired from April to December 1995 forsmoothed numbers of sunspots 35–70, with average K indices of 0–3 at Kiruna,but including one disturbed period (K5). The paths are shown in Figure 9.46and the method of comparisons between DAMSON measurements and simulatedperformance of modems is shown in Figure 9.46. Figures 9.47 and 9.48 show theoverall availability of modems when the frequency set consists of 1, 2, . . . , 10 fre-quencies for the Isfjord–Tuentangen path (2019 km) and the Harstad–Kirunapath (194 km), respectively.

The authors concluded from this investigation that the data rate must be sacri-ficed if high availability is required. Specifically, when there is mode support onthe 2019-km path, the availability of two robust modems was 60%–70% higherthan that achievable from the 2400-bps modem. On the 194-km path the availabil-ity was typically 75% higher. Also, the 75-bps modem benefits from being betterable to cope with scattered and off-great-circle modes, therefore providing fre-quency availabilities above the MUF. The maximum overall availability achievedwith a certain number of frequencies of the robust modems was high for bothpaths for all seasons, but a degradation of 5%–10% was observed on the short pathduring a geomagnetic disturbance.

Using the robust modem (75 bps), the overall availability needed only one fre-quency during summer and winter and four frequencies during the disturbedperiod on both paths. The 2400 bps modem needed three or four frequencies onthe short path and four to six frequencies on the long path for all periods.

Roesler and Carmichael (2000) have reported that error-free transmissions ofdata approaching 9600 bps in a 2-kHz bandwidth and 19200 bps in a 6-kHz inter-symbol-interference-bandwidth mode have been achieved using a quadrature-amplitude-modulation waveform and the STANAG 5066 modem in anautomatic-request-for-repeat system (see Section 9.5.4). These data rates weremeasured on HF paths from Cedar Rapids, Iowa to Ottawa, Canada (1336 km)and Cedar Rapids to San Diego, California (2467 km).

9.7 Summary and discussion

It is obvious that a large amount of research has been carried out in the last fourdecades on radio propagation at high latitudes (mostly HF ionospheric propaga-tion), but much of it is to be found in relatively obscure reports and conferenceproceedings. For that reason, we feel justified in including a considerable number


25 20 15 10 5 0 –50

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of examples of data obtained by these research programs, especially since circuitbehavior displays such a profound variation with solar-terrestrial conditions, pathorientation, frequency, and even type of modulation. Specific examples have beenincluded to illustrate these variations.

Ionospheric modeling and propagation-prediction techniques have beenimproved significantly since the early 1970s for mid-latitudes, but most of theextant models are still inadequate for realistic portrayal of the auroral and polarionosphere. It is important to note that practically all of the models and predic-tion programs are “climatological” in nature and do not really predict the“weather” aspects of propagation. In spite of this caveat, attempts to compare pre-dictions with realtime data continue to be made. Fortunately, recent advances inavailability of “space-weather” data, improvements in ionospheric data bases andnew modeling theory have somewhat improved the situation.

The advent of “realtime channel evaluation (RTCE), automatic link-quality(ALQ) evaluation, robust modems, and computer-controlled frequency manage-ment provide order-of-magnitude improvements in reliability of HF high-latitudepropagation. If sufficient resources are available, probably the best approach is touse the above techniques to achieve high reliability on HF high-latitude circuits,instead of expending resources on improving ionospheric data bases, modeling,and prediction techniques.


Prediction AEI strength AEI duration

Pas

ses

no maybe yes 0 21 0 34dB min

Figure 9.42. A compari-son of predicted andmeasured AEI for 60GPS satellite passes(from Hunsucker et al.,1996).

Recent accurate measurements of HF bearing deviation have revealed that, asdemonstrated by time-delay analysis in 1966, deviations as great as 100° from thegreat-circle-path (GCP) often occur on HF high latitude paths. Since most prac-tical HF communication systems utilize antennas with 50°–70° azimuthalbeamwidths, it may be useful at times to have the capability to rotate the antennabeam to take advantage of this non-great-circle (NGC) mode. This could furtherimprove the reliability of HF high-latitude circuits, since research in the 1960sindicated that the MOF on the circuit was sometimes carried by the NGC modeand the NGC mode is not an uncommon occurrence.

Promising areas of research include validation of ionospheric models and pre-diction programs using quantitative HF circuit data appropriate to the outputs ofthe models (relatively little has been accomplished so far); near realtime availabil-ity of space-weather data needed for specific radio-propagation characterization,


Figure 9.43. The latitudinal distribution of 1-min GPS satellite LOS tracks through theauroral-E region for 1 December 1993 (from Coker et al., 1995).

Fig

ure

9.4

4.

The

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S pa

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).


Figure 9.45. The latitudinal distribution of 1-min GPS satellite LOS tracks through the Eregion for 1 December 1993 (top). Oval detection compared with a model of the equator-ward boundary and individual TIROS passes for 1 December 1993 (bottom) (from Cokeret al., 1995).

improvement of the spatial and temporal resolution of ionospheric models, andutilization of existing three-dimensional ionospheric ray-tracing techniques toverify high-latitude propagation modes by understanding magnetospheric pro-cesses. Similarly, GPS stations could monitor areas of intense AE ionization dueto particle precipitation, which support high-HF and low-VHF propagation.


604

Figure 9.46. The methodof comparison betweenDAMSON measure-ments and the simulatedperformance of modems(from Jodalen et al.,2001).


Figure 9.47. The overall availability of modems when the frequency set consists of 1, 2, . . .,10 frequencies for the Isfjord–Tuentangen path (2019 km) (from Jodalen et al., 2001).


Figure 9.48. The same as Figure 9.45, but for the Harstad–Kiruna path (194 km) (fromJodalen et al., 2001).


Section 9.1Hunsucker, R. D., Rose, R. D., Adler, R. W., and Lott, G. K. (1996) Auroral-E modeoblique HF propagation and its dependence on auroral oval position. IEEE Trans.Antennas Propagation 44, 383–388.

Nishino, M., Gorokhov, N., Tanaka, Y., Yamagishi, H., and Hansen, T. (1999) Probeexperiment characterizing 30 MHz radio wave scatter in the high-latitude ionosphere.Radio Sci. 34, 833–898.

Section 9.2Aarons, J., Kersley, L., and Rodger, A. S. (1995) The sunspot cycle and “auroral”F-layer irregularities. Radio Sci., 30, 631–638.

Anderson, D. N., Buonsanto, M. J., Codrescu, M., Decker, D., Fesen, G. G.,Fuller-Rowell, T. J., Reinisch, B. W., Richards, P. G., Schunk, R. W., and Sojka, J. J.(1998) Intercomparison of physical models and observations of the ionosphere.J. Geophys. Res. 103, 2179–2192.

Bent, R. B., Llewellen, S. K., Nesterczuk, G., and Schmid, P. E. (1975) The develop-ment of a highly successful worldwide empirical ionosphere model and its use incertain aspects of space communication and in worldwide total electron content inves-tigations. In Proc. IES75 (ed. J. Goodman). US Government Printing Office,Washington DC.

Bibl, K. (1998) Evolution of the ionosonde. Annal: de Geofisica 41.

Bilitza, D. (1999) IRI 2000. In Proc. IES99, pp. 348–351.

Bishop, G. J. et al. (1999) The effect of the protonosphere on the estimation of GPStotal electron content: validation using model simulations. Radio Sci. 34, 1261.

Burtch (1991) A comparison of high-latitude ionospheric propagation predictionsfrom ICEPAC with measured data. M. S. Thesis. Naval Postgraduate School,Monterey, California.

Bust, G. S. and Coco, D. (1999) CIT analysis of the combined ionospheric campaign(CIC).Proc. IES99, pp. 508–518.

Chiu, Y. T. (1975) An improved phenomenological model of ionospheric density.J. Atmos. Terr. Phys. 37, 1563–1570.

Davé, N. (1990) The use of mode structure diagrams in the prediction of high-latitudeHF propagation. Radio Sci. 30, 309–323.

Davies, K. (1965) Ionospheric Radio Propagation. National Bureau of Standards,Washington DC.

Decker, D. T. et al. (1999) Longitude structure of ionospheric total electron content atlow latitudes measured by the TOPEX/Poseidon satellite. Radio Sci. 34, 1239.

Feldstein, Y. I. and Galperin, Yu. I., (1985) The auroral luminosity structure in thehigh-latitude upper atmosphere: its dynamics and relationship to the large-scale struc-ture of the Earth’s magnetosphere. Rev. Geophys. 23, 217.

Ferguson, J. and Snyder, F. P. (1986) The segmented waveguide program for long wave-length propagation calculations. NAVOCEANSYSTEM Report TD-1071.


Ferguson, J. A. (1995) Ionospheric model validation at VLF and LF. Radio Sci. 30,775–782.

Ferguson, J. and Snyder, F. P. (1989) Long wave propagation assessment. InOperational Decision Aids for Exploiting or Mitigating Electromagnetic PropagationEffects (eds. Albrecht and Richter). AGARD-CP-453.

Ganguly, S. and Brown, A. (1999) Real time characterization of the ionosphere usingdiversity data and models. Proc. IES99, pp. 365–376.

Gikas, S. S. (1990) A comparison of high-latitude ionospheric propagation predictionsfrom advanced PROPHET 4.0 with measured data. M. S. Thesis. Naval PostgraduateSchool, Monterey, Calfornia.


Hunsucker, R. D. and Owren, L. (1962) Auroral sporadic-E ionization. J. Res. NBS D66, 581–592.

Hunsucker, R. D. (1965) On the determination of the electron density within discreteauroral forms in the E-region. J. Geophys. Res. 70, 3791–3792.

Hunsucker, R. D. (1971) High-frequency propagation predictions and analysis for cir-cuits from the USCG San Francisco radio station to ships and aircraft operating inthe North Pacific area. OT/TRER 15. Boulder, Colorado.

Hunsucker, R. D. (1982) Atmospheric gravity waves generated in the high latitude ion-osphere: a review. Rev. Geophys. Space Phys. 20, 293–315.

Hunsucker, R. D. and Delana, B. S. (1988) High Latitude Field-strength Measurementsof Standard Broadcast Band Skywave Transmissions Monitored at Fairbanks, Alaska.Geophysical Institute, University of Alaska, Fairbanks, Alaska.


Hunsucker, R. D. (1999) Final Report on PENEX Data Analysis Project for the NavalPostgraduate School. Naval Postgraduate School, Monterey, California.

Jones, R. M. and Stephenson, J. J. (1975) A Versatile Three-Dimensional Ray TracingComputer Program for Radio Waves in the Ionosphere. USGPO, Washington DC.

Lane, G. (1993) Voice of America coverage analysis program (VOACAP). USInformation Agency, Bureau of Broadcasting Engineering Report 01-93, p. 203.

McNeal, G. D. (1995) The high frequency environment at the ROTHR Amchitkaradar site. Radio Sci. 30, 739–746.

Mather, R. A., Holtzclaw, B. L., and Swanson, R. W. (1972) High-latitude HF signaltransmission characteristics. In Radio Propagation in the Arctic Conference Proc. CP-97.

McDowell, A. I., Breakall, J. K., and Lunnen, R. (1993) Project PENEX InterimReport – 1993, High-frequency Receiving Site Research Center at Rock Springs.Applied Research Laboratory, Pennsylvania State University State College,Philadelphia.

Milan, S. E., Lester, M., Jones, T. B. and Warrington, E. M. (1998) Observations ofthe reduction in the available HF band on four high latitude paths during periods ofgeomagnetic disturbance. J. Atmos. Terr. Phys. 60, 617–629.


Omura, J. K., Schultz, R. A., and Levitt, B. K. (1985) Spread SpectrumCommunications, volumes I–III. Computer Science Press, Rockville, Maryland.

Rose, R. B. (1982) An emerging propagation prediction technology. In Effects of theIonosphere on Radiowave Systems (IES81) (ed. J. Goodman). US GovernmentPrinting Office, Washington DC.

Rose, R. B. (1993) Project PENEX: Polar, Equatorial, Near Vertical IncidenceExperiment – Methodology Document, Rev. 1. Naval Command, Control and OceanSurveillance Center, RDT&E Division, San Diego, California.

Rush, C. M. et al. (1984) Maps of fôF2 derived from observations and theoreticaldata. Radio Sci. 19, 1083.

Sailors, D. B. and Rose, R. B. (1993) HF Skywave Field Strength Predictions.NraD/NOSC, RDT&E, San Diego, California.

Sailors, D. B. (1995) A discrepancy in the international radio consultative committeereport 322-–3 radio noise model: the probable cause. Radio Sci. :30, 713–728.

Schunk, R. W. (1996) Solar–Terrestrial Energy Program: Handbook of IonosphericModels. STEP Report Center for Atmospheric and Space Science, Utah StateUniversity, Logan, Utah.

Smith, R. W. (1988) Low latitude ionospheric effects on radiowave propagation.Dissertation. Naval Postgraduate School, Monterey, California.

Szuszczewicz, E. P., Blanchard, P., Wilkinson, P., Crowley, G., Fuller-Rowell, T.,Richards, P., Abdu, M., Bullet, T., Hanbaba, R., Lebreton, J. P., Lester, M.,Lockwood, M., Millward, G., Wild, M., Pulinets, S., Reddy, B. M., Stanislawska, I.,Vannorini, G., and Zoleski, B. (1998) The first real-time worldwide ionospheric predic-tions network: an advance in support of space borne experimentation, on-line modelvalidation and space weather. Geophys. Res. Lett. 25, 449–452.

Thrane, E. V., Jodalen, V., Stewart, E., Saleem, D., and Katan, J. (1994) Study ofmeasured and predicted reliability of the ionospheric HF communication channel athigh latitudes. Radio Sci. 29, 1293–1309.

Tsolekas, M. D. (1990) A comparison of high latitude ionospheric propagation predic-tions from IONCAP-PC 2.5. M. S. Thesis. Naval Postgraduate School, Monterey,California.

Warber, C. R. and Field, E. C. Jr (1995) A long wave transverse electric–transversemagnetic noise prediction model. Radio Sci. 30, 783–797.

Wilson, D. J. (1991) A comparison of high-latitude ionosphere propagation predic-tions from AMBCOM with measured data. M. S. Thesis. Naval Postgraduate School,Monterey, California.

Section 9.4Hardy, D. A., Gussenhoven, M. S., and Brautigan, D. (1987) A statistical model ofauroral ion precipitation 2. Functional representation model of the average patterns.J. Geophys. Res. 96, 5539–5547.



Section 9.5Angling, M. J., Cannon, P. S., Davies, N. C., Willink, T. J., Jodalen, V., and Jundborg,B. (1998) Measurements of Doppler and multipath spread on oblique high-latitudeHF paths and their use in characterizing data modem performance. Radio Sci. 33,97–107.

Bliss, D. H., Roessler, D. P., and Hunsucker, R. D. (1987) Preliminary results from atrans-auroral HF experiment. Proc. MILCOM87.

Brant, D., Lott, G. K., Paluszek, S. E., and Skimmons, B. E. (1994) Modern HFmission planning combining propagation modeling and real-time environmental moni-toring. Proc. IEE94.

Davies, N. C. and Cannon, P. S. (1993) DAMSON – a system to measure multipathdispersion, Doppler spread and Doppler shift on multi-mechanism communicationschannels. Presented at AGARD Electromagnetic Wave Propagation Paths: TheirCharacteristics and Influences on System Design, Rotterdam.

Fenwick, R. B. and Villard, O. G. (1963) A test of the importance of ionosphere–ionosphere reflections in long distance and around-the-world HF propagation.J. Geophys Res. 68, 5659–5666.

Fenwick, R. B. and Woodhouse, T. J. (1979) Real-time adaptive HF frequency man-agement. In Special Topics in HF Propagation, AGARD Conference Proc. No. 263 (ed.V. J. Coyne).

Gerson, N. C. (1962a) Radio Wave Absorption in the Ionosphere, p. 113. PergamonPress, London.

Gerson, N. C. (1962b) Polar radio noise. In Arctic Communications (ed. B.Landmark). Pergamon Press, New York.

Gerson, N. C. (1964) Polar communications. In Arctic Communications (ed. B.Landmark). Pergamon Press, New York.


Goodman, J. M., Ballard, J. and Sharp, E. (1997) A long-term investigation of the HFcommunication channel over middle and high latitude paths. Radio Sci. 32,1705–1715.

Hu, S., Bhattacharjee, A., Hou, J., Sun, B., Roesler, D., Frierdich, S., Gibbs, N., andWhited, J. (1998) Ionospheric storm forecast for high-frequency communications.Radio Sci. 33, 1413–1428.

Hunsucker, R. D. and Bates, H. F. (1969) Survey of polar and auroral region effects onHF propagation. Radio Sci. 4, 347–375.

Jodalen, V., Bergsvik, T., Cannon, P. S., and Arthur, P. C. (2001) The performance ofHF modems on high latitude paths using multiple frequencies. Radio Sci. 36, 1687.

Johnson, E. E., Desourdis, R. I., Jr, Earle, G. D., Cook, S. C., and Ostergaard, J. C.(1997) Advanced High-frequency Radio Communications. Artech House, Boston.

Section 9.6Bates, H. F., Albee, P. R., and Hunsucker, R. D. (1966) On the relationship of theaurora to non-great-circle HF propagation. J. Geophys. Res. 71, 1413–1420.


Blagoveshchenskaya, N. F., Korienko, V. A., Brekke, A., Rietveld, M. T., Kosch, M.,Borisova, T. D., and Krylosov, M. V. (2000) Phenomena observed by HF long-distance diagnostic tools in the HF modified auroral ionosphere during a magnetos-pheric substorm. Radio Sci. 34, 715–724.

Blagoveshchensky, D. V. and Borisova, T. D. (2000) Substorm effects of ionosphereand HF propagation. Radio Sci. 35, 1165.

Coker, C., Hunsucker, R. and Lott, G. (1995) Detection of auroral activity using GPSsatellites. Geophys. Res. Lett. 22, 3259–3262.

Hunsucker, R. D., Coker, C., Cook, J., and Lott, G. (1995) An investigation of the fea-sibility of utilizing GPS/TEC “Signatures” for near-real-time forecasting of auroral-Epropagation at high-HF and low-VHF frequencies. IEEE Trans. Antennas Propagation43, 1313–1318.

Hunsucker, R. D., Rose, R. D., Adler, R. W., and Lott, G. K. (1996) Auroral-E modeoblique HF propagation and its dependence on auroral oval position. IEEE Trans.Antennas Propagation 44, 383–388.

Rice, D. D., Hunsucker, R. D., Lanzerotti, L. J., Crowley, G., Williams, P. J. S., Craven,J. D., and Frank, L. (1988) An observation of atmospheric gravity wave cause andeffect during the October 1995 WAGS campaign. Radio Sci. 23, 919–930.

Roesler, D. P. and Carmichael, W. R. (2000) The implications and applicability of theQAM high data rate modem. IEE (in press).

Rogers, A. S., Warrington, N. C., Jones, E. M., and Jones, T. B. (1997) Large HFbearing errors for propagation paths tangential to the auroral oval. IEE Proc.Microwaves, Antennas and Propagation 144, 91–96.

Warrington, E. M. (1997) Observations of the directional characteristics of ionospher-ically propagated HF radio channel sounding signals over two high latitude paths.Proc. 2nd Symp. on Radiolocation and Direction Finding. SwRI, San Antonio, Texas.

Warrington, E. M., Jones, T. B., and Dhanda, B. S. (1997a) Observations of Dopplerspreading on HF signals propagating over high latitude paths. IEE Proc. MicrowavesAntennas Propagation 144, 215–220.

Warrington, E. M., Rogers, N. C., and Jones, T. B. (1997b) Large HF bearing errorsfor propagation paths contained within the polar cap. IEE Proc. Microwaves AntennasPropagation 144, 241–249.

Williams, P. J. S. (1989) Observations of atmospheric gravity waves with incoherentscatter radar. Adv. Space Res. 9, 65–72.


Appendix: some books for general reading

Each of the following titles addresses a good range of the geophysical topics thathave concerned us, including, in particular, chapters or articles on the upperatmosphere, the ionosphere and magnetosphere, and the aurora and substorms.They are therefore especially useful as works of general reference. Obviously, eachwill reflect the state of knowledge at the time it was written. While the more recentshould be the most up to date, the older ones should not be neglected for they arecloser to the development of the basic ideas and knowledge upon which the fieldstands today. Mitra’s famous book of 1952 is well worth re-reading. The auroralclassics by Harang (1951) and Stormer (1955) are cited in Chapter 6.

Brekke, A. Physics of the Upper Polar Atmosphere. Wiley, Chichester, New York,Brisbane, Toronto and Singapore (1997).

Deehr, C. S. and Holtet, J. A. (eds.) Exploration of the Polar Upper Atmosphere.Reidel, Dordrecht (1981).

Hargreaves, J. K. The Solar–Terrestrial Environment. Cambridge University Press,Cambridge (1992).

Hines, C. O., Paghis, I., Hartz T. R., and Fejer, J. A. (eds.) Physics of the Earth’s UpperAtmosphere. Prentice-Hall, Englewood Cliffs, NJ (1965).

Jacobs, J. A. (ed.) Geomagnetism; volume 3 and 4. Academic Press, London (1989,1991).

Mitra, S. K. The Upper Atmosphere. The Asiatic Society, Calcutta (1952).

Scovli, G. (ed.) The Polar Ionosphere and Magnetospheric Processes. Gordon andBreach, New York (1970).

612

absorption cross-section 15, 24

acoustic gravity waves and the aurora 331–332

acoustic gravity waves, theory 52–57

adiabatic invariants 79

aeronomy, physical 13–23

airglow and atmospheric cooling 9

Alfvén Mach number 72

Alfvén wave 103

all-sky camera 291

alpha-Chapman layer 18

antennas 115–116basic principles 115design 116

atmospheric composition 10–13

atmospheric heating 8–9

attachment coefficient 18

attenuation 115atmospheric absorption 122ionospheric absorption 151measurement techniques 203–210deviative techniques 151non-deviative techniques 144

aurora australis 291

aurora borealis 291

auroraaltitude of 296diffuse 300–301discrete 300–301intensity of 299–300luminous 285, 291–302mantle 300radar 285, 326–329theta 288, 302

auroralactivity predictions 367–371electrojet 312–314forms 296green line 302infrasonic waves 300–331red line 302

auroral oval 286–288and radio scintillation 308boundaries of 289–290models of 288–291

auroral radar 326–329

auroral radar echoes,occurrence of 328–329polarization of 328

auroral radio absorption 285, 304, 339–382and geomagnetic activity 365and HF propagation 365–367conjugacy of 379–382co-rotation in 363–365duration of 351–354dynamics of 354–365global movement of 358–359preceeding bay in 345–347, 359–361profiles of 373pulsations in 347, 382sharp onset in 341–342substorm onset in 377–379substorm dynamics in 377–379slowly varying 347, 363spatial extent of 351spike event in 341–345statistics of 350–354zone 350–351

auroral spectroscopy 302auroral substorm 285, 308–311

and ionospheric effects 311–312break-up of 309expansion phase of 309growth phase of 309–310pseudo-breakup of 310recovery phase of 309

auroral X-rays 285–304auroral zone 286

magnetic bays in 305magnetic disturbances in 285

barometric equation 5

Bartels musical diagram 97–98

beta-Chapman layer 19

613

Index

Bragg curve 107

Bremsstrahlung X-rays 106–107

Brunt–Väisala frequency 54

bursty bulk flow 318

C layer 36

Canadian-border effect 261

Chapman production function 15

character figure 45

charge-exchange reaction 26

chemical transport (of heat) 9

conductivityHall 49height variation of 50of the ionosphere 48–50of the ground 48Pedersen 49with magnetic field 48–49with no magnetic field 48

continuity equation 14

coronal hole 67

coronal mass ejection (CME) 67–69

current,Birkeland 85–86, 96, 312–314ring 84–85

currents in substorm 312–315

current system,equivalent 86, 312Sp

q 87SD 95–96

current wedge, wedgelet 313–314

D regionat high latitude 337–339electron flux 373–377production of 31profiles in the auroral zone 373recombination coefficient 400summer mesospheric echo in 406–409

Dalton’s law 7

diffusion coefficient 20ambipolar 22

diffusion in the ionosphere 20–23

dipole field 61–63

distribution height 21

E layer at night 27

E region at high latitude 322–332aeronomy of 26–31disturbed auroral 323–326polar 323quiet auroral 323sporadic-E phenomenon in 27–31

eddy diffusion in the atmosphere 9

electric current,Birkland 51Cowling 51

electric currents in the ionosphere 50

electric field and co-rotation 92

electrojet,auroral 75–76equatorial 51

EM noise and interferenceatmospheric noise 127–139galactic noise 133solar noise 134–139

emissions, electromagnetic 285

escape temperature 8

EUV (extreme ultra-violet) 14

exobase, definition of 5

exosphere 7–8definition of 5

F regionblobs 245–249in the auroral oval 240–242in polar cap: U.T. effect 235–237in polar cap: the tongue 234–236patches 244–245scintillation production in 249–260storm-time variation of 46–47

F1 layer, aeronomy of 26

F1 ledge 31

F2 region 37–38alpha–beta transition level in 37and composition changes 43and conjugate ionosphere 44and effect of neutral wind 43anomalies of 39–44Bradbury layer in 37seasonal anomaly of 40semi-annual anomaly of 40

field-line circulation 316

field-lines 62

flux-transfer event 91

frozen-in field 65

geomagnetic cavity 63

geomagnetic field 61–63

geopotential height 7

Harang discontinuity 95, 242

heliosphere, definition of 5

heterosphere, definition of 5

HF propagation at high latitudes 440–530adaptive HF techniques 574–580assessment of HF channels 586–590fixed-frequency tests 440–474absorption effects 467–474

614 Index

Alaska/Scandinavia 440–446other high-latitude paths 450–471, 503–511Alaska (Wales–Fairbanks) 523–530Alaska–continental USA 447–450Alaska–Greenland 471–474large bearing errors 591–593

mitigation of disturbances 572–574use of GPS/TEC 594–603modem use 597–606

PENEX 553–568realtime HF channel evaluation 580–586substorm effects 593–594swept-frequency tests 474–493auroral-E effects 480–482Andøya–College 478–480, 503–512Barrow–Boulder tests 506–516Canadian tests 517–522McMurdo–Thule path 514, 517Sodankylä–Lindau tests 494–502Sondrestrom–Keflavik tests 528–530Thule–College tests 474–478

models of high-latitude ionosphere 538–546,568–572

non-great-circle modes 477–480, 482–483“ducted” modes 490–493Doppler and fading characteristics 493–494ionospheric ray-tracing 538–541

homosphere, definition of 5

hydrated ions 32–35

hydromagnetic wave 103

hydrostatic equation 5

hydrostatic equilibrium 5–7

instabilityballooning 319gradient-drift 328Kelvin–Helmholtz 105, 318–319two-stream 104–105, 328

interplanetary magnetic field (IMF) 65ionization

by alpha-particles 107–108by energetic electrons 105–106by energetic protons 107–108

ionization efficiency 15, 24ionization potential 24

ionospheredefinition of 4naming of 1sluggishness of 27

ionospheric effectsof the aurora 302–305of the sunspot cycle 44–45of the thermospheric wind 23

ionospheric layerscritical frequency of 27definitions of 13–14heights of 25

ionospheric storm 46–47with sudden commencement 46

irregularity strength parameter 258

keogram 300

log-normal distribution 369–371

luminous aurora and the E region 325

Lyman-alpha line of solar spectrum 31

M region 285–286

magnetic bay 95–96

magnetic cloud 69

magnetic-field merging 90–91

magnetic field, reconnection of 90–91, 316

magnetic indexAp 96–97Dst 94Kp 96–97

magnetic indices 96–100AU, AL, AE 97–99

magnetic longitude 62

magnetic micropulsation 103–104

magnetic storm 93–102classical 94phases of 94practical effects of 100–102

magnetopause 69–71and image-dipole method 71definition of 5

magnetosheath 71–72

magnetosonic wave 103

magnetosphere,circulation of 86–90, 228–234, 316electric fields in 91–92definition of 5shock front of 71–72

magnetosphere boundary layer 73–74

magnetospheric substorm 315–319

magnetotail 70, 72–73behavior in substorm 316–318lobes 78plasma sheet 70, 73, 78

magnetotail,electric potential across 228ionospheric sources to 240neutral line 318

mean free path 7

mesopause, definition of 4

mesosphere, definition of 4

metallic ions in the atmosphere 12–13

negative-ion/electron ratio 20, 405

neutral line 90, 318

nitric oxide 31in the atmosphere 12

Index 615

optical depth 17, 25

oxygen, dissociation of in the atmosphere 10

ozone and the ozonosphere 12

plasmapause 75

plasmasphere 73–78depletion of 93dynamics of 92–93

plasmoid 317

polar arc 301–302

polar capcirculation patterns in 228–234electric field in 228potential across 91–92

polar-cap absorption 382–406and solar radio emissions 389–390and solar flares 389–390and proton flux 387day–night effects in 400–405duration of 384magnitude of 389midday recovery in 395–397occurrence of 384–387seasonal variation of 387–389uniformity of 395

polar-cap edge (in PCA) 393–395

polar cusps (clefts) 237–239and charged particles 237and luminosity 237and ionospheric heating 239

polar hole 260, 276–280

polar wind 74, 239–240

propagationconductivity 121Faraday rotation 149–151forward scatter 171HF at high latitudes 440–530LF and MF at high latitudes 430–439ionospheric 140–169ionospheric scatter 171–174line-of-sight 113–116lossy medium 120–121magnetoionic theory 140–145phase effects 148–149prediction programs 174predictions and validations 546–565scintillations 152–163terrain effects 125–127transionospheric 147–159VHF–microwave 530–531VLF/UHF at high latitudes 530–531VLF/ELF principles 163–167VLF/ELF at high latitudes 419–430whistlers 167–169HF-propagation-prediction programs

174–176noise and interference 127–139

proton effects in the neutral atmosphere 398–406

protonosphere 38–39base of 39definition of 5

protonsIMF effects on 390–392magnetospheric effects on 392–395

quiet-day curve 339

radar aurora 303–304

radar, basics of 116–119

radiation in the atmosphere 9

radio absorption in the D region 36

radio absorption, winter anomaly of 36

radio wavesinteraction with matter 122terrain effects 125–127

reaction rates, temperature dependence of 43,274–275

recombinationdissociative 19, 26, 33radiative 26

recombination coefficient 18effective 20

recombination processes, types of 18

reflection from the ionosphere 144–145relation between oblique and vertical

145–147reflection at a boundary 159–163

refractive effectstropospheric 119–120neutral atmosphere 122–125

relativistic electron precipitation 348

rigidity 393

ring current 73, 84–85

riometer 339–340

scale height, definition of 6

scale height of plasma 22

scatter from the ionosphere 169–174coherent scatter 169–171forward scatter 171incoherent scatter 171–174

scintillationand Fresnel zones 256modeling 258–260

scintillation,properties of 249–256S4 index of 255spectrum of 256

small irregularites, in situ measurement of257–258

solar wind 63–69and Kp 97ballerina model of 67

616 Index

composition of 63fast stream in 67garden-hose effect in 65sectors 65–66

space-weather-data use 565–574, 600

sporadic-Eand metallic ions 29and scintillation 31and wind shear 28–29at high latitude 29–30

Störmer theory 392–395

storm–substorm relations 321–322

stratopause, definition of 4

substorm 308–322current wedge 313rate 321theories of 318–319triggering 319–321

techniquesD-region absorption 203–210ground-based 181–214HF Doppler and spaced receiver 217–219incoherent scatter radars 203–205in situ measurements 216–217ionosondes 181–187ionospheric imaging 219–220modification by HF transmitters 210–214oblique-incidence HF/VHF sounders

187–202riometers (URSI A2 method) 206–208satellite beacons 215–216space-based measurements 214–217topside ionospheric sounders 216–217URSI A1a and A1b (HF) absorption methods

204–206URSI A3a and A3b (LF) absorption methods

208–210

temperature of neutral atmosphere 8–10

thermosphere, definition of 4

three-body reaction 35

trapped (Van Allen) particles 78–84longitude drift 83loss cone of 83mirror point of 83pitch angle of 83pseudo-trapping of 83

traveling ionospheric disturbance 57

tropopause, definition of 4

troposphere, definition of 4

trough and electron precipitation 270–271

troughin electron content 261, 266in the southern hemisphere 269main 260–275motion of 271–273orientation of 269–270poleward edge of 269–271principal properties of 263–265time and activity variations of 265–269causes of: heating 274–275causes of: plasma decay 273–274

troughs at high latitude 276–280

turbopause, definition of 5

turbosphere, definition of 5

Van Allen belts 78–84

vertical transport in the atmosphere 20–23

viscous interaction 86–88

VLF-wave reflection in the D layer 35–36

VLF whistlers and the plasmasphere 75

VLF whistlers, nose 75

wave–particle interaction 104

X-rays 14, 106–107

Index 617

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