how the thermospheric circulation a}ects the ionospheric...

\PERGAMON Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

S0253Ð5715:87:, ! see front matter Þ 0887 Published by Elsevier Science Ltd[ All rights reservedPII] S 0 2 5 3 Ð 5 7 1 5 " 8 7 # 9 9 9 5 1 Ð 4

How the thermospheric circulation a}ects the ionosphericF1!layer

H[ Rishbeth�Department of Physics and Astronomy\ University of Southampton\ Southampton SO06 0BJ\ U[K[

Accepted 19 April 0887

Abstract

After a historical introduction in Section 0\ the paper summarizes in Section 1 the physical principles that govern thebehaviour of the ionospheric F1!layer[ Section 2 reviews the physics of thermospheric dynamics at F!layer heights\ andhow the thermospheric winds a}ect the neutral chemical composition[ Section 3 discusses the seasonal\ annual andsemiannual variations of the quiet F1 peak at midlatitudes\ while Section 4 deals with storm conditions[ The paperconcludes by summing up the state of understanding of F1!layer variations and reviewing some important principlesthat apply to ionospheric studies generally[ Þ 0887 Elsevier Science Ltd[ All rights reserved[

0[ Introduction

The _rst suggestion of the ionosphere appears to havebeen made by Gauss "0728# who said]

{{It may indeed be doubted whether the seat ofthe proximate causes of the regular and irregularchanges which are hourly taking place in this ðter!restrial magneticŁ force\ may not be regarded asexternal in reference to the Earth [ [ [ But the atmo!sphere is no conductor of such ðgalvanicŁ currents\neither is vacant space[ But our ignorance gives usno right absolutely to deny the possibility of suchcurrents^ we are forbidden to do so by the enig!matic phenomena of the Aurora Borealis\ in whichthere is every appearance that electricity in motionperforms a principal part||[

In a remarkably forward looking paper\ Stewart "0772#speculated on causes of the daily geomagnetic variations\and concluded the most likely cause to be electric currentsgenerated by dynamo action in the upper atmosphere[Considering that the electron was not discovered till 0786\Stewart could have no knowledge of the physics of thesecurrents\ and it seems reasonable to regard the physical

� Corresponding author[ Tel[] 9933 0 692 481962^ Fax] 99330 692 482809^ E!mail] hrÝphys[soton[ac[uk

theory of the ionosphere as starting with the commentaryby Lodge "0891#]

{{Re Mr Marconi|s Results in day and night Wire!less Telegraphy] The observed e}ect\ which if con!_rmed is very interesting\ seems to me to be due tothe conductivity [ [ [ of air\ under the in~uence ofultra!violet solar radiation [ [ [ No doubt electronsmust be given o} from matter [ [ [ in the solarbeams^ and the presence of these will convert theatmosphere into a feeble conductor[||

The systematic study of the ionosphere really beganwith the measurement of the heights of the re~ectinglayers by Breit and Tuve "0814# and Appleton and Bar!nett "0814#\ followed by the discovery of the existenceof at least two separate ionized layers "Appleton\ 0816#"subsequently named the E and F layers# and the intro!duction of the term {ionosphere| by Watson!Watt "0818#[0

0 At this point\ it may be noted that the term {region| "D\ E\F# denotes parts of the atmosphere\ the D:E boundary being at89 km height and the E:F boundary at 049 km[ The term {layer|refers to ionization within a region\ e[g[ the F0 and F1 layers liewithin the F region\ and E1 and Es layers\ as well as the normalE layer\ lie within the E region[ Although there are manyoccasions on which the terms {region| and {layer| are inter!changeable\ this is not always the case\ and the o.cial distinctionis worth preserving[ The term {ionosphere| was proposed\ appar!ently independently\ by Watson!Watt and by Appleton in 0815[

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð03910275

The subject developed in three interrelated ways] themagneto!ionic theory of radio wave propagation\ the useof the ionosphere for communications\ and ionosphericphysics[ Scientists in several Western countries con!tributed to all these aspects[ As recounted by Wilkes"0886#\ the basis of magneto!ionic theory was well estab!lished by Eccles\ Larmor and Lorentz at the time iono!spheric research really began in the middle twenties\ thetheory being then applied to the ionosphere by physicistsand mathematicians[ The theory of the formation of ion!izing layers in the atmosphere was developed in Germanyby Lassen "0815#\ in America by Hulburt "0817#\ inDenmark by Pedersen "0818# and in Britain by Chapman"0820#\ who produced the de_nitive work on the subject[It may seem unfair that Appleton and Chapman sub!sequently received more credit than the others mentionedabove\ but there is a valid reason[ Only in their case didthe early work lead to continuing research that spannedmany decades[

Key landmarks in the establishment of ionosphericphysics in particular\ and the wider _eld of solar!ter!restrial physics in general\ occurred in the early thirties]

0820 Regular ionospheric sounding began at Slough0820 Chapman|s theory of ionized layers0820 Chapman|s theory of airglow0820 Chapman!Ferraro] Solar particles and the Earth|s

magnetic _eld*First idea of magnetosphere<0821 Start of Kp geomagnetic index0822 Swept!frequency ionograms at Slough[

After World War II ionospheric physics took newdirections\ which combined theoretical atomic physicswith the old radio tradition[ The new theories gave abasic understanding of important topics] the chemistryof the production and loss processes "Bates and Massey\0835\ 0836^ Nicolet\ 0838#\ electrical conductivity "Cow!ling\ 0834^ Maeda\ 0841^ Baker and Martyn\ 0841^ Chap!man\ 0845#\ electrodynamics and the E!layer {dynamo|and F!layer {motor| "Martyn\ 0842^ Maeda\ 0844#\ andthe di}usion of ions and electrons through the neutralair "Ferraro\ 0834^ Yonezawa\ 0844\ 0845^ Martyn\ 0845#[Major advances in knowledge came from the Inter!national Geophysical Year "IGY 0846Ð0847# and the fol!low!up research programmes that continued through theInternational Quiet Sun Year "IQSY 0853Ð0854#[ Thesalient features were]

, Worldwide experiments, Rockets and satellites, Solar radiation measured, World Data Centres, Start of computer age, Incoherent scatter radar, Topside sounding, Laboratory aeronomy, The data explosion, Theoretical modelling

Through these advances the worldwide ionosphere wasbetter explored\ and the structure and composition of theneutral air began to be known[ Better knowledge of theneutral atmosphere paved the way to better under!standing of the ionosphere\ and to the converse idea thatthe ionized plasma*the ions and electrons*can act asa {tracer| for the ambient neutral atmosphere\ in thationospheric measurements can be used to derive infor!mation about the neutral air[ In this way ionosphericscience became integrated into the larger science of aeron!omy and indeed the wider _eld of solar!terrestrial physics[The negative side of this evolutionary process was thatthe {physics| and {communications| sides of ionosphericscience tended to diverge[

By the seventies a new understanding of the ionospherewas well established "Rishbeth\ 0863#[ All the main iono!spheric layers are created by solar ionizing radiationand*particularly the F1!layer*are in~uenced by theglobal thermospheric circulation[ The driving forces ofthis circulation are _rst\ and most important\ heating dueto solar photon radiation^ second\ the solar wind\ energyfrom which appears in the high latitude ionosphere in theform of electric _elds or energetic particles^ and third\tides and waves transmitted upwards from the middleatmosphere[ As a succinct if oversimpli_ed description\suggested by Giraud and Petit "0867#\ the ionosphere|svertical structure depends on the solar spectrum\ its lati!tude structure on the geomagnetic _eld[

1[ General principles of F!layer physics

1[0[ Basic equations

The behaviour of the ionospheric plasma and the neu!tral thermosphere is subject to the equation of state "per!fect gas law# and the general conservation equations formass\ momentum and energy]

Continuity equation]

"Density change# � "Production#

−"Loss#−"Transport#

Equation of motion]

"Acceleration# � "Force#

−"Drag#−"Transport#

Heat equation]

"Temp change# � "Heating#

−"Cooling#−"Conduction#

In the F1!layer the continuity equation for the electrondensity "or electron concentration# N takes the well!known form

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391 0276

1N:1t � q−bN−div"NV# "0#

where q is the production rate\ b is the linear losscoe.cient\ and V is the plasma drift velocity[ Moredetailed discussion can be found in ionospheric textbooks"e[g[ Rishbeth and Garriott\ 0858^ Rees\ 0878^ Harg!reaves\ 0881#[ Kelley "0878# deals with the plasma physicsthat governs the small!scale structure of the ionosphere\which is not described in this paper[

1[1[ The quasi!equilibrium F1 layer

The F1 layer is usually in {quasi!equilibrium| in thesense that 1N:1t is smaller than other terms in the con!tinuity eqn "0#[ To recapitulate brie~y the rules that applyto the behaviour of the F1 peak]

"a# By day\ at heights below and up to the F1 peak\ theproduction and loss terms are roughly in balance] qdepends mainly on the atomic oxygen concentrationðOŁ\ b depends mainly on the molecular nitrogen con!centration ðN1Ł with some contribution from molec!ular oxygen ðO1Ł[ The steady!state electron density isgiven by

N ½ q:b ½ I�ðOŁ:"k?ðO1Ł¦kýðN1Ł# "1#

where k?\ ký are rate coe.cients and I� is proportionalto the ~ux of solar ionizing radiation\ which varieswith the solar cycle[ Both q and b decrease with theupward decrease of gas concentration\ but the ratioq:b\ which depends on the atomic:molecular ratio ofthe neutral air\ increases upwards so N increases too[

"b# The upward increase of N stops because\ at greatheights\ gravity controls the ion distribution[ The F1peak lies at the height where the transport terms arecomparable to the production and loss terms\ i[e[where chemical control gives way to di}usive "gravi!tational# control[ If D is the coe.cient of di}usionof the ions through the neutral air "inversely pro!portional to the ion!neutral collision frequency# andH is the atmospheric scale height\ the peak lies at theheight at which

b ¹ D:H1 "2#

"c# The height of the peak "known as hmF1# tends to lieat a _xed pressure!level in the atmosphere\ i[e[ a _xedvalue of the reduced height z de_ned in Section 1[2["Garriott and Rishbeth\ 0852#[

"d# The height of the peak can be shifted by a neutral airwind or an electric _eld[ A horizontal wind U blowingtowards the magnetic equator drives the ions andelectrons up geomagnetic _eld lines "dip angle I# atspeed U cos I\ of which the vertical component is

Fig[ 0[ The steady!state F1 layer[ Left] Idealized log N"h# pro_lesof electron density versus height[ The height of the peak isdetermined by a balance between di}usion and loss\ eqn "2#[ Inthe presence of an upward vertical drift W\ the pro_le is displacedfrom the full curve to the dashed curve and the peak rises byDh\ eqn "3#[ Right] Sketches showing how a horizontal wind Ublowing equatorward "above# or poleward "below#\ produces a_eld!aligned ion drift V � U cos I\ the vertical component ofwhich is given by W � V sin I � U cos I sin I[ The thin slopinglines show the direction of the geomagnetic _eld "dip angle I#[

W � U sin I cos I "see right!hand sketches in Fig[ 0#[This upward drift raises the peak and increases thepeak electron density NmF1\ which changes approxi!mately in accordance with the value of the ratio q:bat the displaced level of the peak[ Very roughly\ thechange in peak height is given by

Dh ½ WH:D ½ "H:D#U sin I cos I "3#

Opposite e}ects are produced by a poleward wind["e# Vertical drift can also be produced by a zonal elec!

trostatic _eld E[ The drift velocity is E×B:B1\ and itsvertical component is upward for eastward E\ down!ward for westward E[ This {electromagnetic drift| ismost e}ective at equatorial latitudes\ because at mid!latitudes the vertical drift speed is greatly reduced bythe reaction of the neutral air\ the so!called {ion!drage}ect| "Dougherty\ 0850#[

"f# Above the peak\ the plasma distribution is gravi!tationally controlled\ with N decreasing exponentiallyupwards with the plasma scale height[

The sketches of Fig[ 0 illustrate the N"h# distribution thatconforms to rules "aÐf#[

1[2[ Real hei`ht and reduced "pressure!level# hei`ht

The rates of ionospheric processes "production\ lossand di}usion# are largely controlled by the density ofneutral constituents[ Their variation with height mayconveniently be expressed in terms of {pressure!levels|\ aconcept that is commonplace in meteorology[ The ver!tical distance in which the air pressure decreases by afactor of e is the atmospheric scale height


H � kT:m` � RT:M` "4#

where T is gas temperature\ ` the acceleration due togravity\ m and M are the mean molar mass of the airexpressed in kg and in atomic mass units\ and k and Rare Boltzmann|s constant and the universal gas constant"so M:m � R:k#[ It is convenient to de_ne a dimen!sionless parameter\ {reduced height|\ which represents thenumber of scale heights above a selected base level h9[{Reduced height| z is related to {real height| h "km# by therelations

z � "h−h9#:H � gh

h9

"dh:H#[ "5#

Conversely\ the real height of a given pressure!level zis found by inverting eqn "5# to give

h"z# � h9¦Hz � h9¦gh

h9

Hdz[ "6#

The linear relations in eqns "5# and "6# hold if H isindependent of height\ but the integral forms "in whichthe integration runs from height h9 "z � 9# up to theheight in question# are required if H varies with height[The base level h9 may be chosen in di}erent ways[ InChapman theory "Chapman\ 0820# h9 is measured fromthe height of peak production for overhead sun\ but incomputations that involve thermospheric structure itmay be useful to take h9 at the mesopause\ i[e[ the baseof the thermosphere[

1[3[ Composition in a static thermosphere

Well above the turbopause\ each major constituent issaid to be in {di}usive equilibrium|\ meaning that it isdistributed with its own scale height\ so that eqns "5# and"6# hold for each constituent separately[ Exceptions tothis situation should be noted] _rst\ up to about 019 kmthe distributions of O and O1 are partly controlled byphotochemistry\ and second\ departures from {di}usiveequilibrium| occur if there are strong vertical motions"Section 2[3#\ as especially happens during magneticstorms[ If the composition\ which may be speci_ed eitherby the mean molar mass M or by the molecular:atomicconcentration ratio\ remains _xed at the lower boundaryh9\ then it remains _xed at any _xed pressure!level zeven if the temperature changes\ i[e[ the relation M"z# isunchanged^ see Appendix[ But since a temperaturechange a}ects the real height of a given pressure!levelabove the lower boundary\ according to eqn "6#\ it chan!ges the composition at a _xed height h[

At the lower boundary h9\ the mean molar mass "orthe molecular:atomic ratio# can be raised by increasedturbulence[ This may happen\ for example\ if gravity

wave activity is strong enough to cause nonlinear {break!ing| with consequent mixing[ An increase of M at thelower boundary leads in turn to an increase of the molarmass versus reduced height\ M"z#\ at all greater heights[Shimazaki "0861# pointed out that such mixing is lesse}ective than vertical mass motion in in~uencing ther!mospheric composition\ but this point is not further pur!sued here[

2[ Neutral!air winds and plasma drifts

2[0[ Upper atmosphere winds

The thermosphere is a vast heat engine driven byenergy from solar\ auroral and interplanetary sources\with tidal and wave input from the underlying middleatmosphere[ The heat inputs from these sources producehorizontal gradients of temperature and pressure\ asshown schematically in Fig[ 1[ The pressure gradientsdrive horizontal winds "King and Kohl\ 0854^ Kohl andKing\ 0856# with typical speeds of 49 m s−0 at F!layerheights[ These winds\ together with the associated verticalupcurrents and downcurrents\ form a global circulationthat carries energy away from the heat sources and lib!erates it elsewhere[ Vertical cross!sections of this cir!culation are sketched in Fig[ 2"A\ B#\ for {quiet| and{storm| conditions[ The sketches represent averages overlocal time\ so they show {prevailing winds|[ A daytime{snapshot| of the circulation would look quite di}erent\with the horizontal winds diverging from the vicinity ofthe Sun|s latitude\ and a nighttime {snapshot| would showthe winds converging in the winter hemisphere[ The sket!ches C\ D are discussed later in Section 3[4[

The wind velocity U resulting from the horizontal pres!sure gradients depends on the Coriolis force due to theEarth|s rotation "angular velocity V#\ on the molecularviscosity of the air\ and on the {ion!drag| due to collisionsbetween air molecules and the ions[ Ion!drag existsbecause the ions\ being constrained by the geomagnetic_eld\ cannot move freely with the wind[ Omitting theviscosity term\ the equation of motion for the horizontalwind is

dU:dt �F−1V×U−KN"U−V# "7#

where F is the driving force due to the horizontal gradientof air pressure p\ and is given by

F � −"0:r#9horizp "8#

where r is air density\ K is a collision parameter and KNis the neutral!ion collision frequency[ Molecular viscosityis important in the F region because it smooths out thevertical variation of wind velocity\ so that dU:dh : 9 atgreat heights[ Lower down in the thermosphere\ however\


Fig[ 1[ Sketch map of quiet day wind patterns at F1!layer heights at northern summer "June#\ in local time and geographic latitude[Heavy dashed curves represent the auroral ovals[ The dash!dot curve represents the terminator "sunriseÐsunset line#[ Thin dashedcurves represent both isobars and isotherms[ Points D9 show the locations of maximum and minimum temperature[ Arrows representapproximate wind directions[

Fig[ 2[ Sketches of the prevailing "local!time!averaged# meridional circulation in the midday thermosphere at solstice\ with the Sun �displaced from the geographic equator Eq[ Dark squares AO represent the dayside auroral ovals[ Arrows represent upwelling anddownwelling\ thin lines indicate the general direction of the meridional air ~ow[ The main feature is the summer!to!winter ~ow\ drivenby solar heating and reinforced by the e}ect of the summer auroral oval[ Heating in the winter auroral oval drives a subsidiarycirculation[ The dashed arrows showing winds blowing into the polar caps are speculative[ Upwelling "decreased O:N1 ratio# occurs inlow latitudes and the summer hemisphere^ downwelling "increased O:N1 ratio# occurs at high midlatitudes where the two circulationsmeet[ Sketch "A# represents quiet conditions\ sketch "B# represents storm conditions with more active auroral ovals and strongercirculations[ Sketches "C\ D# show the quiet!day situation for two longitudes\ one near to the winter magnetic pole where thedownwelling is at a moderate geographic latitude^ the other far from the winter magnetic pole "but containing the summer magneticpole#\ where the winter downwelling takes place at a high geographic latitude remote from the Sun[


small!scale velocity gradients are not smoothed out byviscosity[

The wind direction depends on the ratio of Coriolisforce to ion!drag[ This may be seen by considering specialsteady!state cases of eqn "7# with dU:dt � 9[ If Coriolisforce is dominant and ion!drag is small\ as in the lowerionosphere\ the wind blows at right angles to the pressuregradient[ Then at geographic latitude f

U � F:"1VV sin f# "U_F# "09#

This is the situation familiar in weather maps for thelower atmosphere\ with the {geostrophic| wind blowingalong the isobars of constant pressure[ In accordancewith Buy Ballot|s Law\ the wind blows clockwise aroundpressure {highs| "anticyclones# and anticlockwise around{lows| in the northern hemisphere\ and in the oppositesense in the southern hemisphere[ A very di}erent situ!ation exists in the daytime F layer\ where ion!drag is largeand the wind is almost parallel to the pressure!gradientforce\ so that

U � F:"KN sin I# "U>F#[ "00#

In general\ both ion!drag and Coriolis force are sig!ni_cant and the wind is inclined to F at the anglec � arc tan "1VV sin f:KN#[ To illustrate this\ the sche!matic wind vectors shown in Fig[ 1 are nearly parallel tothe gradients of temperature "and pressure# by day\ butare de~ected by Coriolis force at night[ As the directionof the driving force F changes continually with local time\the wind direction changes too\ but always lags behindits steady!state direction because of inertia "Rishbeth\0861#[

The ion velocity V in eqn "7# is caused mainly byelectrostatic _elds\ as in "e# of Section 1[1[ The driftingions accelerate the air horizontally by ion!neutral colli!sions\ and if V is large\ the ion!drag term in eqn "7# actsas a driving force for neutral air winds[ This happensespecially in the polar ionosphere\ where strong electric_elds originate from the magnetosphere "and ultimatelyfrom the solar wind#[ For di}erent reasons\ electric _eldsalso play an important role in the equatorial F!region^see Kelley "0878#[

In polar latitudes\ the winds have a dominant day!to!night pattern\ driven partly by the large!scale mag!netospheric electric _eld "Maeda\ 0866# and partly bysolar heating which produces the day!to!night pressurevariations "Kohl and King\ 0856#[ As in the midlatitudewind system\ the direction of the cross!polar wind isdetermined by the ratio of Coriolis force to ion!drag[Localized heating in the auroral oval produces local pres!sure gradients\ which drive smaller scale convective windsaway from the auroral oval\ as indicated rather specu!latively in the sketches of Figure 2[ But\ since the sketchesrepresent a local!time average\ they do not show the

cross!polar wind just mentioned\ because its 13!houraverage is small or even zero[

As discussed by Kohl et al[ "0857# and others\ the windsystem shown in Fig[ 1 a}ects the height and electrondensity of the midlatitude F1!layer\ in the mannerdescribed in "d# of Section 1[1 and shown in Fig[ 0[ The_eld!aligned drift V\ shown in the _gure\ varies with localtime^ it is essentially downward by day when the wind ismostly equatorward\ and upward at night when the windis mostly poleward[ Note that U in eqn "3# represents thewind component parallel to the magnetic meridian\ whichis inclined to the geographic meridian at the declinationangle D[ It follows that the phase of the local time vari!ation of the wind!induced drift depends on D[ As shownby Kohl et al[ "0858#\ this explains the {declination e}ect|in NmF1 found by Eyfrig "0852#[

2[1[ Neutral air continuity

The neutral air is subject to equations of continuityand energy\ as well as the equation of motion eqn "7#[Production and loss processes are unimportant for themajor constituents of the neutral air\ so the continuityequation for the concentration n reduces to

1n:1t � −div"nU#[ "01#

The pressure distribution and the wind velocity con!tinually adjust themselves to satisfy eqn "01#\ because anyconvergence of horizontal wind produces a {pile!up| oraccumulation of air and the resulting increase of pressuremodi_es the winds[ In the F!region\ this situation maybe caused by a localized enhancement "or depletion# ofelectron density\ which increases "decreases# the ion!dragand hence slows "quickens# the wind\ as in the idealizedcase studied by Dickinson et al[ "0860#[ This rapidreadjustment of pressure gradients and wind speed hassome resemblance to the way in which the voltages acrossthe components of an electric circuit depend on theirimpedances[

The {geostrophic| winds described by eqn "00# arenearly divergence!free\ and are ine}ective in removingthe horizontal pressure di}erences that drive them[ Thisis the situation in the lower atmosphere\ where pressure{highs| and {lows| can persist for days[ If the winds weredirected across the isobars instead of along them\ the{highs| and {lows| would disappear in a matter of minutes[

2[2[ Vertical winds

In the real thermosphere\ air motion is three!dimen!sional\ and any divergence "or convergence# of the hori!zontal winds is at least partly balanced by upward "ordownward# winds\ so the magnitude of div "nU# is smallerthan it would otherwise be[ Vertical winds are importantfor the energy balance\ because air is heavy and raising


it involves doing work against gravity\ while downwardair motion releases energy[ To give a numerical illus!tration] to raise the whole neutral F!region atmospherefrom 049 km upwards at the typical vertical wind speedof 0 m s−0 requires a power input of order 0 mW m−1\which is a signi_cant fraction of the daytime solar input[For further discussion\ see Smith "0887#[

The vertical wind velocity may be regarded as the sumof two components "Dickinson and Geisler\ 0857\ Rish!beth et al[\ 0858#

Uz � WB¦WD[ "02#

The {barometric| component represents the rise and fallof constant pressure!levels\ due to thermal expansion orcontraction[ It may be expressed as

WB � "1h:1t#p[ "03#

Consider _rst a simple one!dimensional situation\ inwhich the expansion or contraction involves only verticalup or down motion\ with no horizontal ~ow[ Bearing inmind that the pressure at any point in the atmosphere isjust the weight of the column of air above it\ the amountof air above a given pressure!level must be constant\ andthe air does not move with respect to the pressure!levels"assuming that the vertical acceleration ð `#[ As men!tioned in Section 1[3 and shown in the Appendix\ such{barometric| motion does not change the chemical com!position at any given pressure!level[

The more realistic three!dimensional situation includesboth vertical and horizontal winds[ Since the neutral airin the thermosphere is neither produced nor destroyed\the upward motion must be accompanied by horizontaldivergence of air at great heights\ and horizontal con!vergence of air at the bottom of the thermosphere[ Tobalance this divergence and convergence\ the air mustmove through the pressure!levels with a vertical velocity\called the {divergence velocity| and given by

WD � −"0:r`# "dp:dt#[ "04#

2[3[ Effect of vertical and horizontal winds on neutral aircomposition

Unlike the {barometric| motion described above\ the{divergence| component does change the chemical com!position at a _xed pressure!level z[ Evaluation of its e}ectis complicated by the fact that the composition varieswith height even in a static thermosphere\ because eachmajor constituent has its own scale height[ To meet thisdi.culty\ Rishbeth et al[ "0876# de_ned a parameter Pwhich is height!independent in a static atmosphere withtwo components\ atomic oxygen and molecular nitrogen]

P � 17 ln ðOŁ−05 ln ðN1Ł¦01 lnT "05#

where the square brackets indicate gas concentrations[

This parameter may be useful in studying the relationbetween vertical motion and composition changes[

To summarize the general rules that relate ionosphericbehaviour to vertical air motions\ bearing in mind thatthe electron density N depends on the ðO:N1Ł ratio]

, Barometric motion "positive or negative WB# changesthe ðO:N1Ł ratio and the mean molar mass at a _xedheight\ but not at a _xed pressure!level[

, Upwelling "positive WD# decreases the ðO:N1Ł ratio andincreases the mean molar mass\ so tends to decrease N^

, Downwelling "negative WD# increases the ðO:N1Ł ratioand decreases the mean molar mass\ so tends to increaseN[

These e}ects are in addition to any changes of com!position at the lower boundary h9 due to increased tur!bulence or gravity wave activity "Section 1[3#[

The composition changes\ produced by verticalmotions\ are propagated horizontally by the global windsystem sketched in Figs 1 and 2A[ The global transport ofair is determined by the {prevailing| "local!time averaged#wind\ which at midlatitudes is typically 29 m s−0 directedfrom summer to winter[ As 29 m s−0 corresponds toabout 1499 km per day\ the prevailing wind carries airfrom summer to winter midlatitudes in a few days[ Theprevailing zonal wind\ also of order 29 m s−0 at midla!titudes\ carries the air from west to east in the winterhemisphere\ east to west in the summer hemisphere\ byabout 1499 km per day "which\ depending on latitude\ isroughly 29> of longitude or two hours of local time#[Superimposed on the prevailing winds is the day!to!nightoscillation\ with a typical meridional and zonal windamplitude of 49 m s−0 which causes a daily excursion of249 m s−0×"0 day:1p# ¼ 799 km[ Although not negli!gible\ this oscillation is smaller in scale than the summer!to!winter prevailing motion\ so its e}ect on the large!scale transport of air should be relatively minor[1

3[ Quiet!day variations of the midlatitude F1 peak

3[0[ The observed {anomalies|

Figure 3 shows four solar cycles of solar!terrestrialdata[ The two upper panels show the international rela!tive sunspot number R "formerly called the Zu�rich sun!spot number#\ which dates back to 0638\ and the solarradio ~ux density at 09[6 cm wavelength\ which datesback to 0837[ The centre panel\ showing the intensityof the interplanetary magnetic _eld\ is not particularly

1 It is interesting that this daily excursion resembles the dis!tance scale of 499Ð0999 km over which F!layer behaviour isthought to be correlated[


Fig[ 3[ 34 years of monthly mean data "Willis et al[\ 0883#[ Top to bottom] Relative Zu�rich "or international# sunspot number RZ orRI^ solar decimetric ~ux density at 09[6 cm wavelength^ strength of the interplanetary magnetic _eld measured by the IMP!7 probe inthe vicinity of the Earth^ noon ionospheric critical frequencies foF1\ foF0 and foE at Slough "41N# and Port Stanley "41S# "MHz#[Courtesy] Rutherford Appleton Laboratory[

relevant to the present discussion[ The two lower panelsshow the monthly median noon critical frequencies foF1recorded at two stations at equal and opposite latitudes\Slough and Port Stanley[ It is immediately obvious that\at both stations\ foF1 has a pronounced solar!cycle vari!ation^ that Slough foF1 has a dominant annual variation^and that Port Stanley foF1 has a dominant semiannualvariation[ More detailed inspection shows that greatestfoF1 occurs in winter at Slough but at the equinoxesat Port Stanley^ and that at solar minimum there is aperceptible semiannual variation at Slough[2

These features\ which are observed to di}erent degreesat other places\ have come to be known as {F1 layeranomalies|[ The term {anomaly| originally meant anydeparture from {solar!controlled| behaviour\ in which thecritical frequency foF1 "proportional to zNmF1# variesregularly with the solar zenith angle x\ as it does in thewell!known Chapman layer[ The term is also applied

2 Historically\ this may have been the earliest anomaly to bereported[ The _rst complete year of Slough data to be analysedwas in 0822Ð0823\ near solar minimum "Appleton and Naismith\0824#[ The seasonal anomaly only became prominent with theincrease of solar activity in 0824Ð0825[

to the well!known {equatorial F1!layer anomaly| "notdiscussed in this paper#[ The midlatitude F1!layer ano!malies may be characterized as follows]

Winter or seasonal ano! Greatest NmF1 "or foF1#maly in winterAnnual or non!seasonal Greatest NmF1 "or foF1#anomaly in DecemberSemiannual anomaly Greatest NmF1 "or foF1#

at equinox

Torr and Torr "0862# constructed global maps thatshow regions where noon foF1 is greatest in summer\ orat equinox\ or in winter[ A simpli_ed version of their mapis shown in Fig[ 4[ The three maps represent a veryhigh solar maximum "0847#\ a moderate solar maximum"0858#\ and solar minimum "0853#[ Leaving aside thepolar regions\ to which the present discussion does notapply\ the most obvious feature is the belt of strongseasonal anomaly "winter maximum# at high northernmidlatitudes[ This feature is most pronounced in the Eur!opean:North American sector but extends\ more weakly\over most of the temperate zone in the northern hemi!sphere[ A much smaller region of seasonal anomaly existsin the Australasian sector[ Low latitudes and southernmidlatitudes show a semiannual variation "maximum at


equinox#[ At lower solar activity\ there are regions ofsummer maximum "i[e[ no anomaly# in equatorial andsouthern latitudes[ It is noticeable that the di}erentregions tend to be delineated by magnetic rather thangeographic latitude[ The method of plotting\ however\does not clearly distinguish areas of no data\ and itsreliability is clearly a}ected by the very uneven dis!tribution of stations over the globe[

The annual anomaly "Berkner et al[\ 0825^ Yonezawa\0860# is related to the observation that\ taking the worldas a whole\ the overall level of NmF1 appears to begreater in December than in June[ It can alternatively bedescribed by saying that the {{seasonal anomaly is greaterin the northern hemisphere than the southern||[ Thise}ect may be discerned in Fig[ 4\ in that the regions ofsummer maximum "i[e[ no seasonal anomaly# are mainlysouth of the equator[ It should be mentioned that there isalso a {winter anomaly| in the D!region "e[g[ Las³tovic³ka\0861#\ taking the form of large electron densities on indi!vidual days or groups of days superimposed on anenhanced winter background[ This pattern is di}erentfrom the more consistent winter:summer di}erences inthe F1!layer[ The D!region winter anomaly is associatedwith chemical changes "enhancements of neutral NO#\but these do not seem to be linked to the thermosphericcomposition changes that cause the F!layer anomaly"Section 3[3#[

3[1[ Geometrical explanations

In principle\ the simplest explanations of the anomaliesare {geometrical|\ i[e[ related to the shape of the Earth|sorbit\ or to some other factor that a}ects the ~ux ofionizing radiation on the Earth|s upper atmosphere[ Inthese terms the annual anomaly might appear the easiestto explain\ because of the 2) variation in SunÐEarthdistance[ Simple as it is\ this explanation is not necessarilycomplete\ because the annual variation of NmF1 seemsto be rather more than would be caused by the 5) di}er!ence in the ~ux density of solar ionizing radiation receivedby the Earth "Yonezawa and Arima\ 0848^ Yonezawa\0860#[ However\ the phase is correct\ so in this sense theannual variation may not be an {anomaly| at all[ Thesame might be said of the semiannual variation at thegeographic equator\ where the noon solar zenith angledoes vary semiannually[

For the semiannual variation outside the tropics\ otherexplanations must be sought[ One idea "Burkard\ 0840#is that the Sun|s ionizing radiation is emitted aniso!tropically\ so that the ~ux received by the Earth dependson its heliographic latitude\ which attains its greatestvalues of 26> in early March and September\ but thisidea has not been accepted[

3[2[ Thermal explanations

The earliest theory of the seasonal anomaly was simple[Appleton "0824# suggested that the upper atmosphere

would be hotter\ and therefore more expanded\ in sum!mer than in winter[ It had to be supposed that\ goingfrom winter to summer\ the thermal expansion of the F!layer "which tends to decrease NmF1# more than com!pensates for the decrease of solar zenith angle x "whichwould increase NmF1#[ It is now known that the ther!mosphere at F!layer heights is indeed hotter in summerthan in winter\ but only by about 19) at midlatitudes\quite inadequate to explain the observed seasonal anom!aly[ Furthermore\ since thermal expansion just redis!tributes the ionization\ it cannot account for the observedfact that the height!integrated total electron content ÐNdh is greater in winter than in summer[ Another sugges!tion was that ionization ~ows along geomagnetic _eldlines from the hotter summer ionosphere to the coolerwinter one "e[g[ Rothwell\ 0852#\ but this process is muchtoo slow to be e}ective[

3[3[ Chemical explanations

As an alternative idea\ it was suggested by Rishbethand Setty "0850# that the seasonal anomaly is caused bychanges in the chemical composition "i[e[ the ato!mic:molecular ratio# of the neutral air[ This idea arosefrom a detailed study of the rate of increase dN:dt of F1!layer electron density just after F!layer sunrise[ At thistime\ dN:dt depends largely on the production rate q\which depends on the ðOŁ concentration\ "a# of Section1[1[ Rishbeth and Setty "0850# and Wright "0852# realizedthat a change in the atomic:molecular ratio wouldaccount not only for the anomaly in sunrise dN:dt butalso for the anomaly in noon NmF1\ though the relativeimportance of the production and loss terms would bedi}erent at sunrise and noon[ Later work "Rishbeth etal[\ 0884# showed that the loss term is quite importanteven near sunrise\ so the change in the coe.cient b\ whichdepends on the ðN1Ł and ðO1Ł concentration "eqn 1#\ alsocontributes to the seasonal sunrise anomaly[ Transportprocesses are of lesser importance[ Through the associ!ated changes of scale height\ the composition changesalso account for the fact that the increase of N startsearlier\ with respect to ground sunrise\ in winter than insummer[

At that time\ the atomic:molecular ratio at F!layerheights was not well known\ nor was there any evidenceas to whether it changes with season[ Johnson "0853# andKing "0853# suggested\ by qualitative arguments\ thatthe atomic:molecular ratio is in~uenced by summer!to!winter transport[ Subsequently Duncan "0858# took thisidea further\ by suggesting that both storm e}ects andseasonal e}ects in the F1!layer are due to compositionchanges\ which in turn are caused by the vertical andhorizontal winds associated with the global ther!mospheric circulation[

The experimental evidence came years later from inco!herent scatter radar data "Waldteufel\ 0869^ Cox and


Evans\ 0869^ Alcayde� et al[\ 0863# and from rocket andsatellite experiments "O}ermann\ 0863^ Pro� lss and vonZahn\ 0863^ Mauersberger et al[\ 0865#[ Duncan|s ideathat the composition changes are caused by a globalcirculation was further developed by Mayr et al[ "0867#\and was eventually con_rmed by global modelling\ whichevaluated the seasonal composition changes induced bythe prevailing summer!to!winter meridional wind"Fuller!Rowell and Rees\ 0872#[ This work basicallyexplained the existence of the seasonal anomaly\ but notthe fact that its strength varies with latitude and longitudein a way that appears to be geomagnetically controlled[Nor does it have any obvious connection with the semi!annual e}ect\ so this question must now be explored[

3[4[ Explainin` the semiannual effect

None of the possible causes considered in Sections 3[0Ð3[3 o}er an obvious explanation of the F1!layer semi!annual e}ect\ which remained a puzzle[ Possible clues aresuggested by other semiannual phenomena in the upperatmosphere\ as reviewed by Ivanov!Kholodnii "0862#]

"0# the equinoctial maxima of density of the neutral ther!mosphere\ _rst detected by observations of the atmo!spheric e}ects on satellite orbits "Paetzold andZscho�rner\ 0850#^

"1# the equinoctial peaks in geomagnetic activity^"2# the semiannual oscillations in the lower or middle

atmosphere^"3# the semiannual e}ect in the height hmF1\ found by

Becker "0856#[

Of these "3# seems likely to be related to "0# becausehmF1 tends to follow _xed pressure!levels\ "c# of Section1[1^ "1# is unpromising for the simple reason that mag!netic activity normally depresses NmF1\ so would notproduce equinoctial maxima of NmF1 "though it maywell be a complicating factor in any other explanation#^and the complexity of mesosphere:thermosphere dynami!cal coupling makes "2# a formidable problem to inves!tigate[ However\ Millward et al[ "0885# advanced a theorythat depends on none of these\ but invokes only dynami!cal processes within the thermosphere itself\ and seemscapable of explaining at least the main features of theseasonal and semiannual variations of NmF1[ Recently\Fuller!Rowell "0887# has developed a theory of the ther!mospheric semiannual variation\ "0 above#\ based on theidea that\ as a global average\ the thermosphere is moremixed at solstice than at equinox[

The theory of Millward et al[ "0885# considers theinterface of the solar!driven\ low:midlatitude ther!mospheric circulation with the magnetospherically!driven high latitude circulation[ The latitude of this inter!face depends on longitude[ The high latitude circulation\being geomagnetically controlled\ extends to relatively

low geographic latitudes in the vicinity of the magneticpoles\ i[e[ in the North Atlantic "North American:WestEuropean# sector in the North and the AustralasianÐIndian Ocean sector in the South[ These will be called{near!pole| sectors "{pole| here meaning magnetic pole#[

The key to the explanation is the geographic latitudeof the {downwelling region| in the winter hemisphere "asshown in Fig[ 2#\ where the ðO:N1Ł ratio is enhancedas explained in Section 2[3[ Because of the interactionbetween the two circulations\ the downwelling is greatesta few degrees equatorward of the auroral oval[ In the{near!pole| sectors mentioned above\ on the dayside "Fig[2 C#\ this occurs at moderately high geographic latitudes"roughly 49Ð54>#[ At these latitudes\ the solar zenith angleat midwinter noon is 62Ð77> which\ though large\ doesgive enough photoionization to produce a high NmF1in the oxygen!rich winter thermosphere[ But in regionsremote from the magnetic poles\ here called {far!from!pole| sectors\ i[e[ the Asian and South Atlantic sectors\the situation is di}erent "Fig[ 2 D#[ The downwellingregion is at such high geographic latitude as to be intwilight or even in darkness at noon\ so there isinsu.cient ionizing radiation to produce a large electrondensity despite the large ðO:N1Ł ratio\ and thereforeNmF1 is smaller than in the {near!pole| sectors[

A contributory factor is the meridional wind\ which isequatorward during the day\ and therefore tends toreduce NmF1 through the e}ect of vertical drift U sin Icos I "Fig[ 0#[ At a given geographic latitude\ this e}ectis stronger in the {far!from!pole| sectors\ because therethe dip angle I is smaller\ and over large areas of themidlatitude ionosphere the factor sin I cos I is not muchbelow its greatest value of 9[4 at I � 34>[ In addition\ thehorizontal wind speed U tends to increase with distanceaway from the convergent downwelling region\ whichwould reinforce the dip angle e}ect[

How does this explain the semiannual e}ect< Considerthe changes going from midwinter towards equinox[Because of the seasonal change in the circulation pattern\the ðO:N1Ł ratio at midlatitudes also decreases\ whichtends to decrease NmF1[ But the noon solar zenith angledecreases too\ which tends to increase NmF1[ At the F1peak\ the production rate q depends strongly on x whenx is large "around 89>#\ but is rather insensitive to x whenx is smaller\ i[e[ less than about 79> "This follows fromthe properties of the Chapman function\ bearing in mindthat the F1 peak lies well above the peak of q\ except atgrazing incidence\ when x is near 89>#[

In the {far!from!pole| sectors\ where the downwellingregion is in darkness at winter noon and NmF1 is low\ asexplained above\ NmF1 increases from winter to equinoxbecause the e}ect of decreasing x exceeds the e}ect ofdecreasing ðO:N1Ł[ But in the {near!pole| sectors\ thedownwelling region is in daylight even in winter\ soNmF1 is high and the reverse applies] the decrease of xhas less e}ect than the composition change\ so NmF1decreases from winter to equinox[


Going from equinox noon to summer noon\ x is notparticularly large anywhere at midlatitudes\ and NmF1is insensitive to its changes[ In this case\ the changes inðO:N1Ł have a greater e}ect than changes in x in anylongitude sector\ so NmF1 decreases generally from equi!nox to summer[ Combined with the winter!to!equinoxchanges described above\ this leads to a predominantlyseasonal "winter:summer# variation in the {near!pole| sec!tors and a semiannual variation in the {far!from!pole|sectors[

At lower middle latitudes\ the semiannual e}ect pre!dominates more generally[ The solar!cycle changes in thedomains of the seasonal and semiannual e}ects\ as shownin Fig[ 4\ have yet to be explained\ but may be connectedwith changes in the strengths of the global circulation[Further study is needed as to the relative importance ofthe solar zenith angle and the ðO:N1Ł ratio in producingseasonal and semiannual variations[ It is thought thatF1!layer seasonal and solar cycle variations may also bein~uenced by changes in photochemical conditions*forexample by O¦ ions in metastable states\ and by thevibrational excitation of N1\ both of which modify thevalue of the loss coe.cient b in eqn "1# "Torr et al[\ 0879#[These questions have yet to be _nally settled[

3[5[ Summary] Solar and `eoma`netic control of the quiet!day thermospheric circulation

The explanations of the midlatitude seasonal and semi!annual e}ects\ given in Sections 3[3 and 3[4\ may besummed up as follows with reference to the schematiccirculations shown in Fig[ 2 "C\D#[ The notation "X# isused as a shorthand to denote {{the change in NmF1resulting from the change of X||[ For the quiet!day vari!ations\ the key features of the circulation are]

Summer midlatitudes] Upwelling: decreased ðO:N1Ł

Winter midlatitudes] Downwelling: increased ðO:N1Ł

The greatest downwelling occurs about 4> equatorwardof auroral oval[ The horizontal "meridional# wind U isweak near the downwelling\ but stronger at lower lati!tudes[ By day\ the meridional wind U is poleward andtherefore creates a downward drift of U sin I cos I whichtends to depress NmF1\ as explained in Section 1[1 "d#[This is secondary in importance to the compositione}ects\ but it does a}ect the shape of the local timevariation of NmF1[ The wind e}ect is longitude depen!dent because U depends on the proximity to the auroraloval\ while I depends on magnetic latitude[ The in~uenceof winds is not considered in detail here\ but has to beincluded in detailed modelling of the F1 layer[

"a# Consider the transition from summer to equinox[ Atmidlatitudes\ the thermosphere at F1!layer heights is

more molecular in summer\ and becomes less molec!ular towards equinox\ i[e the mean molecular massdecreases[ The noon solar zenith angle increasestowards equinox\ but its e}ect in the F1!layer is smallsince x has moderate values\ no more than about 59>[Hence the net e}ect in all longitude sectors is]

Summer : equinox] "compo# × "zenith angle#

"b# Now consider winter at about 49> geographic\ wherex × 69> at noon\ in a longitude sector near the mag!netic pole[ The noon sector is relatively close to theauroral oval\ so downwelling and the resulting e}ecton composition are strong[ The e}ect of the polewardwind is moderate] U is small\ I large\ cos I sin I ½ 9[14

Winter : equinox] "compo# × "zenith angle#

so NmF1] "winter# × "equinox# × "summer#

The predominant variation of NmF1 is seasonal["c# Now consider winter at about 49> geographic\ in

longitude sectors far from the magnetic poles[ Thenoon sector is a long way from the auroral oval\ sodownwelling and composition e}ects are small[ Thee}ect of the poleward wind is strong] U is large\ Ismall\ cos I sin I ½ 9[4

Winter : equinox] "compo# ³ "zenith angle#

so NmF1] "winter# ³ "equinox# × "summer#

The predominant variation of NmF1 is semiannual[

4[ Storm variations of the midlatitude F1 peak

4[0[ {Positive| and {ne`ative| storm effects

In the early days of ionospheric research\ it was noticedthat geomagnetic disturbance is accompanied or quicklyfollowed by marked changes in the F1!layer[ Sometimes\especially in winter\ these changes take the form ofincreases of NmF1\ but more often there are severedecreases of NmF1 "Appleton and Ingram\ 0824^ Berkneret al[\ 0828#\ especially in summer and at equinox[ Thephenomena came to be known collectively as {F!layerstorms|\ the terms {positive| and {negative| being generallyused to denote whether NmF1 is increased or decreasedfrom its usual quiet!day value during the {main phase| ofthe ionospheric storm[ The {main phase|\ lasting typically13Ð25 h\ is preceded by an {initial phase| lasting a fewhours[ The {initial phase| is usually {positive|\ i[e[ NmF1is enhanced\ whether the subsequent {main phase| e}ectbe {positive| or {negative|[ Note that the terms {initialphase| and {main phase| are also used in connection withthe variations of the geomagnetic _eld during a storm\ asobserved by ground magnetometers\ though the geo!magnetic storm phases are generally shorter than the


corresponding F1!layer phases[ Many processes areinvolved in F1!layer storm phenomena\ including atmo!spheric waves and electric _elds transmitted from highlatitudes^ see for example the comprehensive review byPro� lss "0884#[

4[1[ Composition effects in ne`ative storms

Using the newly developed photochemical ideas\ as setout in Section 1[1\ Seaton "0845# advanced the idea thatnegative F!layer storm e}ects are due to an increased losscoe.cient b\ resulting from increased concentrations ofneutral molecular gases "though\ at that time\ it wasthought that the loss processes involved only O1\ ratherthan N1#[ Duncan "0858#\ linking this explanation ofnegative storms to the photochemical theory of the F1!layer season anomaly "Section 3[3#\ proposed that bothseasonal and storm e}ects are induced by a global ther!mospheric circulation\ primarily driven by the solar inputbut strongly modi_ed by high latitude {storm| inputs[Subsequent authors developed this idea in terms of athermospheric {storm circulation| superimposed on thesolar!driven {quiet!day| circulation "e[g[ Mayr andVolland\ 0861^ Matuura\ 0861#\ and Reddy "0863# foundevidence for a {return circulation| in the lower ther!mosphere during a storm[ Pro� lss "0879# reviewed theexperimental evidence for composition changes duringstorms\ as found by satellite!borne instruments[

As in the case of the quiet!day seasonal changes\ it wassome years before the idea that the {storm circulation|drives composition changes could be tested by full globalmodelling[ The detailed modelling was done by Burns etal[ "0878\ 0880\ 0884# and Fuller!Rowell et al[ "0883\0885#[ The e}ect of vertical motions was especially stud!ied by Burns et al[ "0884#\ but the full theory of therelationship between vertical motions and compositionchanges has yet to be established[ Fuller!Rowell et al["0883# further developed the {storm circulation| idea\envisaging that a {composition bulge| "a region ofenhanced molecular:atomic ratio# forms as a result of thestorm heat inputs[ Once formed\ the {bulge| migrates inlatitude and local time under the in~uence of the ther!mospheric wind system which\ within a day after the startof the storm\ usually returns more!or!less to the quiet!day pattern[ As pointed out in Section 2[3\ the diurnalamplitude of this migration is only of order 0999 km[

Detailed modelling by Fuller!Rowell et al[ "0885# andField et al[ "0887# suggests that both {negative| and {posi!

3000000000000000000000000000000000000000000000000000000000

Fig[ 4[ Annual and semiannual e}ects in noon critical frequency foF1\ simpli_ed from Torr and Torr "0862#[ The shading shows {Max[Summer| domains where "summer foF1 × equinox foF1#\ i[e[ {no seasonal anomaly|\ and {Max[ Winter| domains where "winterfoF1 × equinox foF1#\ i[e[ {seasonal anomaly| exists[ No shading means foF1 is greatest at equinox\ i[e[ {semiannual anomaly|[ The{Max[ Winter| domains are divided according to whether "winter foF1*equinox foF1# is less than 1 MHz "A# or greater than 1 MHz"B#[ Symbols mark the positions of the magnetic dip poles "crosses on black spots#[

tive| main phase e}ects in NmF1 are broadly explainedin terms of composition changes[ Very roughly\ if {s| and{q| denote {storm| and {quiet day| mean values]

NmF1s:NmF1q ¼ ðO:N1Łs:ðO:N1Łq[ "06#To investigate this relationship\ and following the

method introduced by Rodger et al[ "0878#\ Field andRishbeth "0886# derived values of storm:quiet ratio"NmF1s:NmF1q# for 42 stations and extracted the DCmean value NÞ averaged over local time\ the AC peak!to!mean amplitude N of the variation with local time t\ andthe local time t¼ of its maximum\ as in the equation

ln "NmF1s:NmF1q# � NÞ¦N = f "t−t¼#[ "07#The function f is arbitrary in shape\ but it has unit

amplitude and zero mean and attains its maximum atzero time[ These AC:DC results are derived from manydi}erent storms over the period 0846Ð0889\ and thusrepresent a substantial averaging of storm behaviour[Despite the averaging\ this is a useful way of showinghow the main phase e}ect varies with latitude and season[The curves in Fig[ 5 show the DC amplitude NÞ computedfor the 42 stations used by Field and Rishbeth "0886#\ forthree seasons[ NÞ is generally negative in summer and atequinox "i[e[ short dashes in the southern hemisphere\long dashes in the northern hemisphere\ and the full curvein both hemispheres# but is positive in winter "long dashesin the southern hemisphere\ short dashes in the northernhemisphere#[ The _gure shows the {storm seesaw e}ect|as the curves of NÞ swing from northern solstice throughequinox to southern solstice[ Although the individualdata points are rather scattered\ they mostly follow the{seesaw| trend at middle and low latitudes[ Field andRishbeth "0886# showed that this trend is consistent withthe composition changes given by the empirical MSIS!89model of the neutral thermosphere "Hedin\ 0880#\ whichis based on experimental data from many sources[ Fieldet al[ "0887# showed that the storm:quiet ratio of NmF1follows quite closely the storm:quiet ratio of ðO:N1Ł[ Thissupports the idea that the positive main phase e}ects\more common in winter\ are produced by compositionchanges\ and so are the negative main phase e}ects insummer and at equinox[

5[ Conclusion

5[0[ {State of the ionosphere|

This review has described how the quiet!day and stormvariations of the F1 layer are controlled by the global


Fig[ 5[ {Storm seesaw|[ The local!time "DC# average NÞ of the ratio "NmF1s:NmF1q# plotted against magnetic latitude\ based onionosonde data for 42 stations\ 0846Ð0889[ The curves are averages drawn through the data points of Fig[ 3 of Field and Rishbeth"0886#[ Long dashes] northern solstice "MayÐAug#^ full curve] equinox "Mar\ Apr\ Sept\ Oct#^ short dashes] southern solstice "NovÐFeb#[ The mean deviation of the data points is about 29[0[

quiet day and storm circulations in the thermosphere[The control mainly comes about through compositionchanges of the neutral air\ which are induced by thecirculation and largely determine the electron and iondensity[ It is well said that the F!layer is dominated locallyby photochemistry and globally by dynamics[

Ionospheric science is at the heart of the topical sub!jects of space weather\ predictions and forecasts[ Eventhe quiet ionosphere is heavily in~uenced by the Sun "i[e[\the observed solar cycle variations and 16!day vari!ations#^ the disturbed ionosphere is in~uenced also bythe solar wind and interplanetary magnetic _eld\ whichmodulate the energy sources of the storm circulation[Future work will also consider interactions "which mayturn out to be two!way# between the ionosphere and theweather\ as well as possible ionospheric e}ects of surfacetopography "oceans and mountains#\ events such asearthquakes and thunderstorms^ and the intriguing ques!tion of long!term global change in the upper atmosphere[

5[1[ Guidin` principles for ionospheric research

As a tailpiece\ here are some general ideas that may befound useful in thinking about the ionosphere*or\ forthat matter\ many other natural systems[

First\ always consider the scales of time and distancethat are involved[ The kinds of questions to ask are {{Howlong does ða processŁ take<||^ {{Over what distance does itoperate<||^ {{How fast does it move<|| To help decidewhether a particular explanation makes sense\ try com!paring a typical speed of motion with the ratio "distancescale 6 time scale# for the process under discussion[

Second\ remember that ionospheric parameters vary

much more rapidly vertically than they do horizontally[For large scale structure\ vertical scales "tens ofkm# ð horizontal scales "099Ð0999 km#[ However\ ver!tical motions are generally slower] e[g[ for wind systems\vertical speeds "½2 m s−0# ð horizontal speeds "½099 ms−0#[ But\ despite horizontal speeds being faster\ verticalmotions are more important for two reasons] "a# thetransport terms in the basic conservation equations "Sec!tion 1[0#\ which involve gradients and divergences\ areusually dominated by vertical motions^ "b# verticalmotions\ which involve gravity\ are more energetic thanhorizontal motions\ of which the energetics are associatedonly with inertia and frictional "collisional# processes[

Third\ it is di.cult to derive absolute values of iono!spheric parameters\ such as production and loss rates\from observations of electron density[ The reason is thatthe ionospheric layers\ even the F1!layer\ are almostalways close to equilibrium\ in the sense that the "d:dt#terms in the conservation equations are much smallerthan others[ For example\ measuring the F1!layer elec!tron density may give quite a good value of the ratio q:b\but neither parameter is separately well determined[ Thisis true even for apparently non!steady situations such assunrise\ sunset and eclipses\ and explains why so muchearly work gave unsatisfactory numerical results\ eventhough it did contribute to physical understanding[

Fourth\ a converse proposition] ionospheric modellingis often insensitive to adopted values of input parameters\which means that observations can often be _tted byquite a wide range of numerical parameters[ A good_t does not necessarily mean that the chosen values ofparameters are accurate[

Fifth\ it is commonplace that better space or time res!


olution in experiments leads to new science[ For example\much of the gross structure\ particularly in active systemslike the equatorial and auroral ionospheres\ depends onthe small!scale physics[ Future research will be increas!ingly directed towards this microstructure and the associ!ated plasma science[

Sixth\ many aspects of the science depend on the readyavailability of data[ Even {boring| routine data are mostimportant in experimental {campaign!based| science\ andare vital for systematic and long term research[ Hence theimportance of routine solar!terrestrial monitoring data\such as Kp\ F09[6\ and R\ and archives such as the WorldData Centres and national data and analysis centres[ SeeWillis et al[ "0883#[

Seventh\ use MKSA "SI# units; The old!fashioned cgsunits have served well for atomic processes\ but in theauthor|s opinion they are burdensome whenever elec!tricity or magnetism is involved\ and the tiresome factorsof c obscure the dimensional relationships between thephysical parameters[

Finally\ the ionosphere is a vital part of the solar!terrestrial system^ and although its basic mechanismsappear to be known\ surprises are possible in any livingand active area of science; Ionospheric science is aliveand well\ and still has importance for space and terrestrialcommunications\ as it approaches the start of its secondcentury in 1991[

Acknowledgements

This paper is based on a Tutorial Lecture given atthe National Science Foundation CEDAR Workshop atBoulder\ Colorado\ on 09 June 0886[ The author thanksthe National Science Foundation for _nancial support[Thanks are due to I[ Mu�ller!Wodarg for help with twoof the diagrams[

Appendix

To prove that composition is invariant at a `iven pressure!level

Consider an atmosphere in di}usive equilibrium at auniform temperature T\ consisting of atomic oxygen "gas0# and molecular nitrogen "gas 1#[ Let p0 and p1\ n0 and n1

denote the partial pressures and concentrations of thesegases[ Their scale heights are H0 � RT:05` andH1 � RT:17`\ where R � universal gas constant\ and` � acceleration due to gravity which\ for the sake ofsimplicity\ is assumed independent of height[ Consider abase level {O|\ situated at the ground or at another levelin the atmosphere\ at which the partial pressures areconstant\ namely p09 and p19\ so the total pressure isp9 � p09¦p19[ From the perfect gas law\ p09 � kT[ n09 and

p19 � kT[ n19 where k � Boltzmann|s constant and thegas concentrations are n09 and n19[ Let the ðO:N1Ł ratio atlevel {O| be denoted by r9 so that r9 � n09:n19 � p09:p19[

Now consider another constant pressure!level {A|\ fouroxygen scale heights or\ equivalently\ seven nitrogen scaleheights above level {O|[ Introduce the {reduced height| zas in eqn "5#\ which can be de_ned for each gas separately"z0 for O\ z1 for N1# as well as for the whole atmosphere[Then\ at the level {A|\ z0 � 3 and z1 � 6\ and the partialpressures are

p0A � p09 exp "−3#\ p1A � p19 exp "−6# "08#

and\ since the temperature T is the same as at level {O|\the concentrations are

n0A � n09 exp "−3#\ n1A � n19 exp "−6# "19#

The total pressure at {A| is pA � p0A¦p1A and theðO:N1Ł ratio is

rA � n0A:n1A � "n09:n19# exp "6−3# � r9 exp "2# "10#

Consider too a further constant pressure!level {B|\ at eightoxygen scale heights "z0 � 7# or equivalently fourteennitrogen scale heights "z1 � 03# above level {O|\ and alsoat temperature T[ The partial pressures and con!centrations at {B| are

p0B � p09 exp "−7#\ p1B � p19 exp "−03# "11#

n0B � n09 exp "−7#\ n1B � n19 exp "−03# "12#

The total pressure at {B| is pB � p0B¦p1B and the ðO:N1Łratio is

rB � n0B:n1B � "n09:n19# exp "03−7# � r9 exp "5# "13#

Now assume that the temperature is no longerconstant\ but varies in an arbitrary way with height\ thetemperature being T9 at level {O|\ TA at level {A|\ and TB

at level {B|[ The levels {A| and {B| are still de_ned by theirpressures pA and pB[ Whatever the temperature\ the scaleheights H0 and H1 are always in the ratio 6:3 at any levelin the atmosphere[ Applying eqn "5#\ in its general formin which the temperature varies with height\ it remainstrue that z0 � 3 and z1 � 6 at {A| and z0 � 7 and z1 � 03at {B|[ Hence eqns "08# and "11# still hold for the partialpressures at these levels\ but "using the perfect gas law#the gas concentrations at {A| are altered in the ratio"TA:T9# and at {B| in the ratio "TB:T9#\ and are now

n0A � n09"T9:TA# exp "−3#\ n1A � n19"T9:TA# exp "−6#

"14#

n0B � n09"T9:TB# exp "−7#\ n1B � n19"T9:TB# exp "−03#

"15#

As the temperature ratios disappear in deriving the ðO:N1Łratios\ the latter are the same as in the isothermal case\eqns "10# and "13#[


The same argument applies to any pressure level above{O|[ The ðO:N1Ł ratio depends only upon reduced heightabove {O| and of course on its value r9 at level {O|[ Thisconclusion is not a}ected if there are more than twogases "provided each has its own scale height and thermaldi}usion is negligible# or if allowance is made for thevariation of ` with height[

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