Adv. SpaceRes. Vol. 7, No. 8, pp. (8)39—(8)47, 1987 0273—1177/87$0.00+ .50Printedin GreatBritain. All rights reserved. Copyright© 1987 COSPAR

OBSERVATIONS OF THE VARIATIONS OFTHERMOSPHERIC WINDS IN NORTHERNSCANDINAVIA BETWEEN 1980 AND 1986: ASTUDY OF GEOMAGNETIC ACTIVITYEFFECTS DURING THE LAST SOLAR CYCLE

D. Rees,*N. D. Lloyd,* T. J. Fu11er~Rowe11*andA. Steen**5Departmentof PhysicsandAstronomy, University CollegeLondon,Gower Street, London WC1EOBT, U.K.* *Kiruna GeophysicsInstitute, Kiruna, Sweden

ABSTRACT

Since late 1980, a ground—based Fabry—Perot interferometer has been in operation near Kiruna,Sweden (20.40 E, 67.80 N). This instrument has recorded upper thermospheric neutral windsunder a wide range of geophysical conditions which have occurred during six observing winters

These data cover the period from the time of maximum geomagnetic activity during the lastsolar cycle, 1980/82, to the 1985/86 period of generally quiet geomagnetic activity near sun-spot minimum. A statistical analysis of the data from about 400 nights of observationprovides a graphic description of the generally rapid time—dependence of upper thermospheric

winds in the vicinity of the mean auroral oval to individual geomagnetic disturbances.The data also provide an excellent data base describing the auroral oval. The data describeion convection patterns as they respond to variable geomagnetic activity, and also the meandistribution of 01 630 nm emission as a function of local time, latitude and geomagnetic

activity. These results can be used to examine the geomagnetic input parameters to a globalthermospheric model (for example the semi—empirical global models of magnetospheric convect-ion) as required to bring the simulations of thermospheric circulation into overall improvedagreement with the observations.

INTRODUCTION

Responses of polar thermospheric winds to geomagnetic activity have been reported over anumber of years through observations with high altitude rocket chemical trails /1,2/, ground—based Fabry—Perot interferometers /3,4,5/, incoherent Scatter Radar /6/ and by satellite—

borne wind measurements /7,8,9/. There have been systematic studies of the polar winds forlimited time periods, such as the combined satellite and ground—based observations for Dec.1981 /10/. Recently, Sica et al /11/ reported the statistical analysis of observations overa period of time from a ground—based Fabry—Perot interferometer located near Fairbanks,

Alaska.

The physical interpretation of these winds rests on the effects of solar heating atlow latitudes, creating a global circulation system with a dominant diurnal variation, anowith strong seasonal modulation /12,13/. Major wind perturbations occur within the auroraloval and polar cap as a result of momentum transfer (ion drag) from convecting plasma drivenby the basically two—cell magnetospheric convection system /14,15/. Ion drag is strongly en—ha~ced as the result of ionospheric plasma increases due to energetic particle precipitationassociated with the aurora. Intense heating, due in part to precipitation, and in part toenhanced Joule/frictional heating, resulting from the combination of strong electric fields

and enhanced conductivity, can also induce major effects /16,17/, changing the local pressuregradients, and thus modifying the resulting wind patterns.

As the geomagnetic inputs vary, so do the thermospheric responses /5,8,11,14/. Major global—scale changes of wind ciruclation occur within 1 hour of the onset of major geomagnetic dis-turbances, and the response to strong localised heating can be even more rapid, with large 10to 20 sin, period responses to major disturbances such as geomagnetic substorms and WestwardTravelling Surges /5,11, 16,17/.

This study describes the average geomagnetic response of the auroral oval wind in NorthernScandinavia over a period of some 6 winters, from 1980/81 to 1985/6, as observed by a ground—based Fabry—Perot interferometer. The wind data refers to an average altitude of about 240km. The observation period covers a range of solar and geomagnetic activity, from the

combined high solar and geomagnetic activity during 1980—1982, to the quiet periods (1984—1986), leading up to the solar cycle minimum. There is a large wind variance within each ofthe average samples, largely reflecting strong time—dependent wind changes induced by rapid

(8)39

(8)40 D. Rees et al.

fluctuations of the geomagnetic input. There are also, undoubtedly, seasonal and solar UV/EUV activity variations within the data base which have not been resolved by the presentanalysis.

OBSERVATIONSOF THERMOSPHERICWINDS IN THE NORTHERNAURORALOVAL

The Fabry—Perot interferometer operated at Kiruna Geophysical Institute and its associatedinstrumentation and modes of operation have been described in previous publications /5,9,14/.The data which will be presented here have resulted from the statistical analysis of approx-imately 400 nights of observations, where the individual wind data have been accumulated intosets distinguished by the daily sum (K—sum) of the geomagnetic K index derived for Kiruna.Data for clear—sky periods have been selected with reference to the all—sky record from KGI,and to operational logs.

The index (K) and levels of local geomagnetic activity have been selected to obtain signifi-cant samples of wind observations, within the selected intervals of K—sum. Due to the highgeographic latitude, there is unavoidable UT bias in the data: Data at early (14 to 18 hoursUT) and late (04 to 07 hours UT) are under—represented in the total data sample. Data duringthese UT periods are only from mid—winter observations. Midnight observations (18 hours UTto Oil houra UT) are more uniformly sampled from the entire observing period, extending fromlate August to mid April. Since local magnetic midnight is close to 21 UT, while solar mid-night is close to 01 UT, the “summer” observations are of post—magnetic midnight convection,precipitation and wind systems. This causes a further lack of uniformity in the overall datasampling: no observations of evening auroral oval winds are made before mid September orafter late March.

PRESENTATIONOF DATA

Wind data are presented as Wind Speed and Average Direction. 15 minute averages are usedthroughout, and the variance (standard deviation) in each sample is indicated by the lengthof the vertical line. Individual wind errors are such smaller than this variance, typically10 ms—i. Six levels or bands of local geomagnetic activity have been selected:

BAND K—sum Av.Local 3—hour K Figure No.

I 0 <K—sum < 15 0 — 2~

II 15 <K—sum < 25 2_ — 3 2

III 25 <K—sum < 30 3 —

IV 30 <K—sum < 35 4_ — 4+

V 35 <K—sum < 40 4f — 5 3

VI 40 <K—sum < 72 5 — 9

The mean location and variance (meridian angle) of peak 630 nm (01) emission as a functionof UT for each of the activity levels are also available, but will not be shown or discussedhere.

The range of 3 hour K index for each band is also indicated. For a variety of operationaland statistical reasons, the first and last bands are less well distributed than would bedesirable: The wind errors tend to be rather larger at very low local geomagnetic activitylevels (low 01 630 emission intensities), while rather few very disturbed events occur wherethe K index averages above 5—6, for a complete 21! hour period. The selection by clearweather intervals reduces the number of such events for which satisfactory observations havebeen made. No attempt, within this data base, has been made to correlate directly with theIMF. Certainly, such effects are present within the total data sample, and are highly sign-ificant /14/. The extent of the data base, and the poor or non—uniform availability ofdirect IMP data since 1982/3 have precluded a straightforward analysis, however.

The most dramatic change of thermospheric wind pattern occurs between the first and secondactivity levels. At level I, the evening winds and early morning winds are anti—sunward,characteristic of mid—latitude systems driven by solar UV and EUV heating at low latitudes.Between 17 and 22 UT, there is a rapid change of wind direction to southward/southwestward.This is the only imprint of geomagnetic effects at this low level of activity, when geo-magnetic inputs should be confined to higher latitudes than the region sampled from Kiruna.Wind amplitudes are always low at this activity level, very rarely above 200 m s—i.

At level II, corresponding to K between 2 and about 3, the evening winds are between west-ward and northwestward, and then rotate anti—clockwise. After 20 UT, the winds are similarto those at activity level I, and follow similar magnitude and speed changes later in thenight, with slightly higher wind speeds and variability. The westward wind is induced byion drag within the evening auroral oval, and occurs although the peak DI 630 nm emissionlevels remains well poleward of Kiruna.

Observations of the Variations of Thermospheric Winds (8)41

Progressing through activity levels III, IV, and V, there is a modest and consistent increasein the equatorward winds in the period around and after magnetic midnight with increasingactivity. The evening winds remain with the same direction and direction changes as atactivity level II, and there is a continuous and considerable increase in the wind speed,reaching a mean value of 350—400 m/sec between 1~4 to 15 UT at activity level V (K~), withseveral observations in the range of 600 to 900 rn/sec. At the highest activity level, theevening wind speeds decrease dramatically, though the direction is the same as at levels IIto V. At the higher activity levels, the wind direction in the evening hours is remarkably

consistent. The wind speed, however, shows considerable variability. At higher levels ofgeomagnetic activity, the geomagnetic input is far from constant, and within any of theperiods of higher activity identified within a specific activity level, many violent changesof activity will occur. Earlier studies /1,5,16/ have shown that the wind speed in theevening auroral oval has a relatively rapid response to local geomagnetic inputs, particular-ly those due to ion drag under variable auroral convection.

The time of the anti—clockwise directional swing from westward/northwestward to south/south-eastward is consistent at all levels of activity above level II. During the period aroundand after magnetic midnight, the wind speed increases steadily from 100 rn/sec to 200 m/sec

at level VI. The variability in speed also increases, with midnight equatorward wind speedsof 400 s/sec occurring some 20% of the time at level VI. At all activity levels, there is

a consistent clockwise swing of the wind from south — southeastward to westward about 04 UT.In this “dawn” phase, the wind speed is almost invariably less than 100 m/sec, even at levelVI. At activity level VI, the duration of nighttime equatorward winds is rather longer, andthe swing from evening westward to midnight equatorward winds sharper and perhaps earlier,than at more moderate activity levels.

GLOBAL THERMOSPHERICSIMULATIONS

Four model simulations will be used in comparison with these observations. A low geo-magnetic activity model (GM Model I ) includes no precipitation within the auroral oval andpolar cap, and a modest3o KV cross—polar cap potential over a poleward—contracted auroraloval. There is very little high latitude Joule heating, and no particle heating, in thissimulation. The momentum transfer to the neutral gas in the winter polar region is small,but significant, resulting from ion drag due to magnetospheric convection in the auroraloval (30 Ky) and the low plasma densities within the polar region in the Chiu model. Figure5 shows the wind variations at 320 km altitude, as a function of latitude and Universal Time,

at a longitude of 18° E, close to that of Kiruna (22° F).

A second model (GM Model II, Figure 6) includes a more average polar electric field (A2/B2)/14/, but retains the Chiu global ionosphere. This model has a larger momentum transfer tothe neutral gas than GM ModelI , resulting from the expanded auroral oval, and larger ores—

polar cap potential. Joule/frictional heating are also increased.

A third model (GM Model III, Figure 7) has the same polar electric field (A2/B2, /i4/).This model further increases momentum transfer to the neutral gas by including, in a non

self—consistent way, the effects of auroral electron precipitation. This enhances iono-spheric plasma densities, and thus ion drag and Joule heating, as well as adding the directenergy deposition of the auroral electrons. The description of the electron precipitationis taken from /18/, as described in more detail by /14/.

To simulate high geomagnetic activity, a model of polar convection described by /5/, with anexpanded auroral oval, and a cross—polar cap potential of some 160 KV has been used in GMModel IV (Figure 8). The auroral oval precipitation has been increased significantly inthis simulation, with considerable enhancements of ionospheric plasma densities (notcalculated self—ccnsistently), ion drag and Joule heating, in addition to the direct particleheating.

COMPARISONOF DATA AND MODELS: MODIFICATIONS OF GEOMAGNETICINPUTS

For the lowest level of geomagnetic activity represented in the data, the evening winds, up

to 18 UT, are in good agreement with the low activity model (Figure 8). Around the magneticmidnight period, however, the observed winds swing eastward and southward/southwestward,

continuing a clockwise rotation sense, more than 900 further than the model. Figure 6 showsthis is the characteristic signature of the midnight auroral oval. We conclude that GMModel I, despite its inclusion of a 30 KV cross—polar cap potential, considerably under-estimates the total geomagnetic input under conditions appropriate to K—sum<15. Inclusionof a low level of auroral precipitation consistent with /18,13/ could provide the additionalpolar heat input, and enhancement of ion drag to explain the observed wind distribution.

(8)42 D. Reeset al.

There is no compelling evidence that the cross—polar cap potential, or size of the auroraloval, need be increased. GM Model II provides approximately the correct geomagnetic inputby using too large an auroral oval and cross—cap electric field, and underestimating (i.e.

neglecting) the particle precipitation and consequent enhancementsof energy input andmomentum coupling.

Post magnetic midnight, observed winds are rather more southward than the quiet ‘symmetric’model. This is also characteristic of increased input of geomagnetic energy into the polar

region. In the morning hours, 03 to 07 UT, observed winds swing westward, where the GMModel I winds undergo a ‘shear’ reversal. Such rotational reversals do occur in models with

an asymmetric polar electric field pattern, corresponding to IMP BY positive.

At the next activity level, (average 2 < Ep < 3), the observed winds have changed to a flowpattern which is preserved for all higher geomagnetic activity levels. The auroral oval hasexpanded significantly equatorward and winds of the early evening are now directed westwardto northwestward. The wind vector rotates anticlockwise during the early part of theevening and night, ending up between southward and southeastward from 21 to 03 UT. Thisaverage behaviour agrees well with model simulations: those corresponding to IMF BY positive/i4/ produce more southward winds around midnight, while when the IMP BY component isnegative (Figure 6,7), the wind is directed more southeastward in this period.

Later in the night, a characteristic of observations under all but the most disturbed

conditions, the winds swing clockwise from southward/southeastward to westward. This onlyappears to be characteristic of the IMP BY positive simulations. It may indicate that, asthe auroral oval swings poleward of Kiruria irs the early morning hours, the shape and locationof the dawn oval around 06 to 09 MLT is, on average, more consistent with convection patternscorresponding to IMF BY positive. In very few cases does the observed wind vector swinganticlockwise, through north, in this time period.

Observed wind magnitudes generally increase with geomagnetic activity levels, as indicatedin the simulations. At activity levels III — IV (Kp”il_), they match the moderately dis-turbed model (Figure 7).

At activity level V (Kp’~5), while the wind directions are generally similar to those of

levels II — IV, the early evening wind speed increases sharply. This is best simulated bythe model shown in Figure 7, where the winds are enhanced as a result of increasing the iondrag throughout the auroral oval by including the effects of auroral electron precipitation,and thus plasma densities (and Joule heating etc).

The major discrepancy is perhaps that average equatorward winds in the midnight hours areobserved to be around 150 m/sec, rather than the predicted 250 rn/sec. Since the midnight

winds of the moderately disturbed model (Figure 6) are rather lower than those observed, themain conclusion is that the more disturbed model has overestimated the total geomagnetic in-put at this activity level. However, the meridional wind decreases rapidly with decreasinglatitude,so that the simulations are quite sensitive to the location of the auroral oval.In general, it appears that the modelled location of the auroral oval in the midnight region,

for moderately disturbed conditions, is approximately correct.

At the most disturbed level (VI, ~p~7) stronger winds are observed around the magnetic mid-night period (average 200—400 m/mec, sometimes 500 to 700 a/see), but the westward winds ofthe early evening are actually lower than at activity level V. We have simulated thesestorm—time winds by increasing the diameter of the auroral oval and the cross polar cappotential (Figure 8), in addition to increasing precipitation etc. The model produced verylarge westward winds (600—800 a/see), and the latitude variation hints at the explanation of

observed winds under the most disturbed conditions. The auroral oval, under activity levelVI conditions, has expanded even further than in GM Model IV. Kiruna is thus on the pole—

ward edge of the auroral oval, or even within the polar cap, in the early evening. Theinduced winds are smaller than at the centre of the dusk auroral oval. However, much dis-turbed periods are renowned for rapid fluctuations of activity. The “K—sum” method chosenhere for the statistical analysis of the data does not reflect these rapid fluctuations ofactivity within the individual 24 hour periods.

In individual case studies /5,i4/ it has been possible to use empirical data (polar electricfields etc) to model the auroral oval response during periods of intense activity. Thesedetailed studies indicate that good agreement between thermospherie simulations and actualobservations can be found when the magnetosphere inputs are moderately well known. Theconsiderable equatorward expansion of the auroral oval is indicated by the continuation ofsouthward winds throughout the night at activity level VI. We have not yet attempted amodel simulation using a cross—polar cap potential (>200 Ky) and polar cap diameter (>50°),with corresponding precipitation input, as is indicated by analysis of the wind data atactivity level VI.

Observations of the Variations of Thermospheric Winds (8)43

ACKNOWLEDGEMENTS

The cperation of the UCL Fabry—Perot interferometer at Kiruna Geophysical Institute issupported jointly by the U.K. Science and Engineering Research Council and by KirunaGeophysical Institute.

REFERENCES

1. D. Rees, Ionospheric Winds in the Auroral Zone, J. Brit. Interplan. Soc. 24,233 (1971)

2. J.P. Heppner and M.L. Miller, Thermospheric winds at high latitude from chemical release

observations, J. Geophys. Res. 87,1633 (1982)

3. P.B. Hays, T.L. Killeen, and R.G. Roble, Nighttime thermospheric winds at high latitudes,J. Geophys. Rem. 84,1905 (1979)

1!. G. Herriandez and R.G.Roble, Thermospheric response to geomagnetic storms, J.GeophyS.Res. 87,9181 (1982)

5. D. Bees, T.J. Fuller—Rowell, H.P. Smith, B. Gordon, T.L. Killeen, P.B. Hays, N.W. Spencer

L. Wharton, and N.C. Maynard, The westward jetstream of the evening auroral oval, Planet.Space Sci. 33,415 (1985)

6. J.W. Meriwether, P. Shih, T.L. Killeen, V.W. Wickwar, and R.G. Roble, Nighttime thermo—

spheric winds over Sondre—StromFjord, Greenland, G.R.L. 1,931 (1984)

7. P.B. Hays,T.L. Killeen, yLW. Spencer, L.E. Wharton, R.G. Roble, B.A. Emery, T.J. Fuller—Bowell, D. Bees, L.A. Frank, J.D. Craven, Observations of the dynamics of the polar

thermosphere, J. Geophys. Res. 89,5547 (1984)

8. T.L. Killeen, P.B. Hays, M.W. Spencer, and L.E. Wharton, Neutral winds in the polarthermosphere as measured from Dynamics Explorer. Geophys. Res. Lett. 9,977 (1982)

9. D. Rees, T.J. Fuller—Rowell, B. Gordon, T.L. Killeen, P.B. Hays, L.E. Wharton, andN.W. Spencer, A comparison of wind observations of the upper thermosphere from theDynamic Explorer satellite with the predictions of a global time—dependence model,Planet. Space Sci. 31,1299 (1983)

10. T.L. Killeen, R.G. Roble, R.W. Smith, N.W. Spencer, J.W. Meriwether, D. Bees,G. Hernandez, P.B. Hays, L.L. Cogger, D.P. Sipler, M.A. Biondi, and C.A. Tepley, Mean

neutral circulation in the winter polar F—region, J.G.R. 91 ,A2, 1633 (1986)

11. R.J. Sica, M.H. Reem, G.J. Romick, G. Hernandez, and R.G. Roble, Auroral zone thermo—spheric dynamics, I, Averages J.G.R. 91,A3,3231 (1986)

12. ~ Fuller—Rowell and 0. Rees, A three dimensional time—dependent, global model of thethermosphere, J. Atmos. Sci. 37,2545 (1980)

13. D. Rees, The response of thehigh latitude thermcsphere to the geomagnetic activity,Advances in Space Research, Vol. 5, No.4,267 (1985)

14. D. Rees, T.J. Fuller—Rowell, B. Gordon, H.P. Smith, N.C. Maynard, J.P. Heppner,N.W. Spencer, L.W. Wharton, P.B. Hays, and T.L. Killeen, A theoretical and empiricalstudy of the response of the high latitude thermosphere to the Sense of ‘1’ Componentof the Interplanetary Magnetic Field, Planet. Space Sd. Vol. 3~4,1—4O (1986)

15 T.L. Killeen, and R.G. Roble, An analysis of the high latitude thermospheric windpattern calculated by a thermospheric general circulation model, 1, Momentum Forcing,

J.G.R. 89,7509 (1984)

16. D. Rees, R.W. Smith, P.J. Charleton, F.G. McCormac, N. Lloyd, and Ake Steen, Thegeneration of vertical thermospheric winds and gravity waves at auroral latitudes — I,observations of vertical winds, Planet. Space Sci. 32,667 (1984)

17. 0. Ream, M.F. Smith, and R. Gordon,The generation of vertical thermospheric winds andgravity waves at auroral latitudes — II, Theory and numerical modelling of verticalwinds, Planet. Space Sci. 32,685 (1984)

18. R.W. Spiro, P.F. Reiff, and L.J. Maher, Precipitating electron energy flux and auroral

zone conductances — an empirical model, J. Geophys.Res. 87,8215 (1982)

(8)44 D. Roes Ct al.

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Observations of the Variations of Thermospheric Winds (8)45

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Fig. 6. Same as Fig. 5, for moderate geomagnetic activity conditions. This simulationincludes a cross—polar cap electric potential of 70 KV (ccrresponding to By positive). Theglobal Chiu ionospheric model is used, which has no enhancements corresponding to auroralprecipitation. Polar ionospheric conductivity and ion drag are thus very low.

Observations of the Variations of Thermospheric Winds (8)47

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Fig. 7. Same as Fig. 5, for moderate geomagnetic activity conditions. This simulation usesa cross—polar cap electric potential of 70 KV (corresponding to By pcsitive), and a corres-ponding level of auroral precipitation /5,18/, which enhances polar ionospheric conductivityOhmic heating, ion drag in addition to the energy of the particles themselves.

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Fig. 8. Same as Fig. 5, fcr high geomagnetic activity (storm) conditions. This simulationuses a symmetric cross—polar cap electric potential of 120 KV, with a corresponding highlevel of auroral precipitation /5,18/. Polar ionospheric conductivity, Ohmic heating and

ion drag are all strongly enhanced.

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