Ade.SpaceRes.Vol. 11,No. 10,pp. (iO)109—(10)112,1991 0273-1177/91$0.00+ .50Pnntedin GreatBritain. All rightsreserved. Copyright© 1991 COSPAR

THE HIGH LATITUDE LOWERIONOSPHEREOBSERVEDBY EISCAT

S.Kirkwood* and P.N. Collis**

*EJSCA7’,Ramfjorcfrnoen,9027Ramfjordborn,Norway**EJSCAT,Box705,S-98127Kiruna,Sweden

ABSTRACT

Routine observations of lower ionosphere electron densities are now availablefrom the EISCAT radar facility. High resolution (3km in altitude) powerprofile measurements are obtained from the altitude interval 55-170km forapproximately 400 hours per year, during continuous operations of 24 hours ormore. Variations of D and E region electron densities as a function of solarzenith angle, during undisturbed conditions, and as a function of simulated30MHz riometer absorption, during disturbed conditions, are presented.

INTRODUCTION

The purpose of this paper is to examine the possibilities for modelling thehighly variable electron densities in the auroral zone D and E regions as afunction of the easily available parameters solar zenith angle, 3—hourmagnetic K index and 30MHz riometer absorption. This has previously beenattempted using rocket measurements /1/. The data base used here is,however, over a hundred times larger, so a more thorough study is possible,including a study of possible variations in the shape of the electron densityprofiles with increasing disturbance or with local time.

DATA BASE

The electron density profiles used are from the EISCAT UHF incoherent scatterradar (69.6°N,19.2°E), experimental program CP-1-F. The radar is convenientlydescribed by Folkestad et al. /2/ and the Common Program operating modes byBaron and Persson /3/. One year of measurements have been used, from November1984 to November 1985, as detailed in Table 1, giving a total of 4656electron density profiles, each corresponding to a 5-minute average andcovering all times of day, seasons and magnetic conditions. The measurementsare very evenly distributed in local time and the distribution with respectto other parameters is shown in Figure 1.

Measurements are made by the power-profile technique, using an uncoded 2Opstransmitted pulse and 50kHz Gaussian receiver filters. The method ofreduction to electron density and the particular problems and limitations inusing this technique in the lower ionosphere are described in detail byMatthews /4/. In deriving electron density we have here assumed equal ion andelectron temperatures. Simultaneous measurements of these temperatures by the

TABLE 1. Dates and times (UT) of measurements

26/27 November 1984 1300—1300 06/07 August 1985 0800—080012/13 December 1984 1200—1200 13/14 August 1985 1000—150028/29 January 1985 1200—1200 03/04 September 1985 0800—220014/15 February 1985 1200—1200 10/11 September 1985 0800—220016/17 April 1985 1100—1100 29/30 October 1985 0900—090014/15 May 1985 0800—0800 05/06 November 1985 0900—090021/22 May 1985 0800—1200 12/13 November 1985 0900—230025/26 June 1985 1200—1200

(10)109

(10)110 S. KirkwoodandP.N.Collis

1500

1000 1000 200, 200

soo1’flfl..,~,soo ~

01234567 .0 .2 .4 .6 .81.0 1. 2. 3. 4. 5dB 1000 am 50°pm 100°K index absorption solar zenith angLe

Fig. 1. Histograms showing the numbers of electron density profilesavailable and their distribution as a function of 3-hour K-index,simulated 30 MHz absorption and solar zenith angle (for daytime)

multipulse technique /5/ indicate that this approximation will not lead toany significant error in the altitude interval used (70-120km) except duringsome short periods of very distubed conditions. Another possible source ofsystematic error is the radar system constant. However, this has now beendetermined to good accuracy (5%) /6/. From SNR considerations and the basicintegration time used, we expect statistical uncertainties for indiy~dua~profiles of about 1% fpr ~ 1011 m~

3, increasing to 8% for Ne ~‘ 10’” mand 40% for Ne ~ 3.1O~ m~. Statistical uncertainties for averages oveseveral profiles will be rather less, however average densities below 2.10cannot be correctly measured as the statistical variation would imply asignificant number of negative densities in the population, which are notrecorded on the data tapes. Further, the measurements make use of the totalscattered power within the receiver bandwidth and thus contain both the ion-line and part of the electron-line /2/. Reduction to electron density hasbeen made assuming that the electron-line contribution is negligible,cor

9spoz~ging to a likely overestimate of electron density of about 10% at Ne~ 10 m , increasing rapidly for lower electron den~ities. We thereforediscard all mean electron densities below Ne = 2.10~ m’.

VARIATION WITH SOLAR MIGLE ZENITH IN UNDISTURBEDCONDITIONS

In order to examine the variation of electron density with solar zenith anglewe must first eliminate all profiles during disturbed conditions. Wehavefirst used the 3-hour KHflZ indices from the Sodankyla Geophysical Observatory(N. Finland) and discar~ed all times when this has a value of 2 or more.However, it is known that particle precipitation can also occur duringmagnetically quiet conditions so profiles have also been rejected if thesimulated absorption (expected absorption at 3014Hz calculated from themeasured electron density profile from 75km altitude upwards ) is morethan 0.05 dB above the quiet level. The quiet level has been determined byexamining the distribution of simulated absorption for all the profiles as afunction of solar zenith angle. A quiet level of 0.25dB for a solar zenithangle of 470 is found, decreasing essentially linearly to 0 dB for 98° solarzenith angle.

The undisturbed profiles have been averaged in ~0 solar zenith angle bins togive the results shown in Figure 2a. The averages shown are for post-noonprofiles but no significant difference has been found for pre-noon profiles.The figure shows the strong variation with solar zenith angle above 100kmaltitude, with electron densities varying by more than a factor of 10 betweensunset and the lowest zenith angle attained (98’ and 470 ,respectively),which is slightly more than previously suggested /1/. Below 100km electrondensities fall rapidly, even for the lowest solar zenith angles, reaching thethreshold of our measurement technique by 85km altitude.

VARIATION WITH ABSORPTIONLEVEL IN DISTURBED CONDITIONS

In order to examine the electron densities occurring during disturbedconditions, we have selected 3 groups of profiles which might be expected to

LowerIonosphereObserved by EISCAT (10)111

I I III liii I II I I I 11111 I I ill hug’ I I Ill IIIç~ I Jill!)

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70 I I 1111111 I I 1111111 I I 111111 I I III I I I 1111111 I I 11111io~ 1010 1011 i010 iOu

electron density (m3)

Pig. 2. Average electron density profiles for daytime. (a) shows profilesfor different solar zenith angles during undisturbed conditions and (b)shows profiles for different levels of disturbance as characterised bythe simulated 30 MHz absorption.

show characteristic differences : night (unilluminated), pre.-midnightnight, post-midnight ; day (sunlit). The first distinction is made because ofthe difference in precipitated electron energy spectrum which might beexpected between the two time sectors / /. UT boundaries 1700-2230 for thepre-midnight and 0000-1200 for the post midnight sector have been used (localmagnetic midnight is around 2130 UT). There are almost no disturbed profilesfound between 1200 and 1700UT. The sunlit conditions are treated separatelybecause of expected differences in recombination coefficients. Almost all ofthe disturbed, daytime profiles occur in the post-midnight sector so it isnot possible to make the same local time distinction as for the night-time profiles.

Within each of the groups, profiles are averaged according to simulatedabsorption (in excess of the quiet level) for the intervals 0.00-0.05 dB,0.45—0.55 dB, 0.95—1.05 dB, 1.75—2.25 dB, 2.75—3.25 dB, giving the resultsshown in Figures 2b, 3a and 3b. If we consider the daytime profiles first(Figure 2b) we see that even a rather small level of disturbance (0.5 dB)corresponds to a very large increase in electron density at the loweraltitudes. One feature which is clea~

0y d~splayed here is the occurrence offairly high electron densities ~ 101 m~) even below 75km altitude. Theseare not seen in the night-time data (Figure 3) and are a signature of themuch lower daytime recombination rate.

If we next consider the night-time profiles in Figure 3 we see broadlysimilar behaviour in the two time sectors, with the largest electron densityvariation occurring b~tween 75 and 105km altitudes, and with no measureableionisation (< 2.10’ m”) below 75km even for the most disturbed conditions. Acloser comparison of the results for the two time sectors shows rather largerdensities at 100km and rather lower densities between 80 and 90km for thepre-midnight sector, for the higher levels of disturbance (2.0dB or more), inagreement with the expectation of harder electron energy spectra in the post-midnight sector.

CONCLUSIONS

The very large data base provided by the EISCAT common program experimentscan be used for extensive study of electron density characteristics in the

(10)112 S. Kirkwood andP.N. ~o1lis

120 — I I I~lUll;’ I I 1111111 I I I III I I II iIiU( I I II 11111 I 1111(1

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~90~ ,‘ - . e’y. -

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80’ ~‘ - / -

(a) (b)

70 I 1111111 I I 1111111 I I 111111 I I 1111111 I I 1111111 I I ii uin

i010 1011 i09 i~10 iOuelectron density (m3)

Fig. 3. Average electron density profiles for night-time, for differentlevels of disturbance characterised by the simulated 30MHz absorption. (a)is for time sector 1700—2230 1ST, (b) for 0000-1 200 UT. The dashed linesshow the 0dB level for quiet magnetic conditions.

lower ionosphere in the auroral zone during a wide variety of magneticconditions and for all times of day and year. We have so far used this database to determine the undisturbed, solar-zenith dependence and to derivetypical profiles for various levels of disturbance. We have further been ableto quantify characteristic differences between profile shape for day (sunlit)and night (unilluminated), and for pre-midnight and post-midnightprecipit~ion

4 We find that solar illumination alone produces ionisationabove 10’ ni~ only above about 90km altitude. However, even rather smalldisturbance levels (0.5dB) correspond to very large increases in electrondensity from 80km altitude upwards. Further increases in the disturbancelevel (to 1-3 dB) correspond to electron density increases mainly in the 80-100km altitude interval, but also below 80km when the region is sunlit.

ACKNOWLEDGEMENTS

The EISCAT Scientific Association is supported by the Centre National de laRecherche Scientifigue of France, Suomen Akatemia of Finland, Max-PlanckGesellschaft of West Germany, Norges Almenvitenskaplige Forskningsrad ofNorway, Naturvetenskapliga Forskningsradet of Sweden and the Science andEngineering Research Council of the United Kingdom. We wish to acknowledgethe assistance of the director and staff of EISCAT.

REFERENCES

1. Friedrich,M., and K.M.Torkar,J.Atmos.Terr.Phys. 45, 127 (1983),2. Folkestad,K.,T.Hagfors and S.Westerlund, Radio Sci.18, 867 (1983).3. Baron,M. and K.Persson,EISCAT Technical note 85/43, EISCAT, Kiruna,

Sweden (1985),4. Mathews, J.D.,J.Atmos.Terr.Phys. 46, 975 (1984).5. Kirkwood,S.,, Seasonal and tidal variations of neutral temperatures and

densities in the high-latitude lower thermosphere measured by EISCAT,J.Atmos.Terr.Phys., in press (1986).

6. Kirkwood,S., P.N.Collis and W.Schmidt, Calibration of electron densitiesfor the EISCAT UHF radar, J.Atmos.Terr.Phys, in press (1986).

7. Collis,P..N. and A.Korth,J. Atmos. Terr. Phys.,47,327 (1986)-