Radar auroral observations and ionospheric electric fields

AJv. Space Re~. Vol.2, No.7, pp.131-144, 1983 O273-I177/83/O70131-14507.OO/O Printed ~n Great Britain. Alt rights reserved. Copyright © COSPAR

RADAR AURORAL OBSERVATIONS A N D IONOSPHERIC ELECTRIC FIELDS

E. Nielsen* and J. D. Whitehead**

*Max-Planck-Institut far Aeronomie, 3411 Katlenburg-Lindau, F.R.G. **Department of Physics, University of Queensland, St. Lucia, Q4067, Australia

ABSTRACT

The ground-based systems STARE and SABRE utilize radar auroral phenomena to estimate ionospheric electric fields. Some of the assumptions underlying these systems have been tested and general agreement with expectations have been found. However, as the results have been analysed in detail, it has become clear that the error in the irregularity drift velocity can at times amount to I00 ms -I. Direct comparisons with other E-field measurements, as well as assessments of the results of applications of the Stare data clearly demonstrate that the electric field, calculated on the basis of the irregularity drift velocity, is a useful estimate of the actual horizontal electric field in the ionosphere and is sufficiently accurate for a great variety of geophysical studies.

INTRODUCTION

The radar auroral technique as applied in Stare (Scandinavian Twin Auroral Radar Experiment) and Sabre (Sweden and Britain Radar Experiment) is used to estimate ionospheric electric fields (e.g. Nielsen [I] 5. It is, we believe, well documented by a long list of publi- cations that observations made with the Stare system are fruitful and have led to a better understanding in several areas of ionospheric and magnetospheric physics. However, it is also clear that in order to make best use of the systems it is important to know in detail how accurate the E-field estimates are. The purpose of this report is to sun~narize evidence in this respect. The evidence consist of comparisons of electric fields estimated from Stare data with observations obtained simultaneously by other techniques, which yield information, direct or indirect, about the electric fields in the ionosphere. Furthermore, some of the assumptions made in the theories of radar aurora and in the design of the radars have been investigeated experimentally.

Briefly, the theoretical background of the Stare/Sabre systems is as follows. The radars are operated at about 140 MP~, so that the transmitted signal is backscattered from electron density fluctuations of a scale length of about one meter at auroral E-layer heights (Unwin and Gadsen [2] ). The linear dispersion relation of the density fluctuations associated with the two-stream and gradient-drift plasma instabilities (Buneman [3] ; Farley [4] ; Reid [5] ; Rogister and D'Angelo [6] ) has the form

top = k . ~ / (|+~5 (15

where k is the radar wave number, .~ = E x B is the bulk electron drift velocity, ~d = ~- /~i~e with ~i and ~e the ion ~nd ~lectron collision frequencies, and ~i and w e

°~ ~ . . . . the ion and electron gyro-frequencles. Thls equatxon xs often referred to as the cosine- relationship because it states that the phase velocity of the electrostatic plasma waves, in a direction given by k, equals the component of the electron drift velocity in that direction. Since the phase velocity of the instabilities is related to the observed Doppler displacement of the transmitted radar frequency, the electron drift velocity can be deduced when ~p is observed in at least two different directions. Actually, in the Stare/Sabre systems a double-pulse technique is used, which yields an average Doppler velocity of the instabilities. This velocity is equal to the desired mean Doppler velocity when the power spectrum of the backscattered radar signal is narrow and/or symmetric (Rummier [7] 5. Equation | was derived in the linear approximation, but it has been argued that it is also valid when secondary effects are considered (Sudan et al [8] ; Greenwald [9] ; Andr~ [I0]). Equation I is used to interpret data from the Stare/Sabre systems in terms of electric fields.

In the fluid theory approximation it is assumed that the growth-rate of the instabilities is much smaller than their phase velocity (Sudan et al. [8] 5. Schle~el and Nielsen [IJ1 questioned the validity of this approximation for the high latitude ionosphere, where large

131

132 E. Nielsen and J.D. Whitehead

electron drift velocities can occur. Furthermore, in the fluid theory the electron and ion gases are assumed to have the same temperature. Schlegel and St.-Maurice [12] found the electron temperature to be higher by a factor of 3 or 4 than the ion temperature at an altitude of 110 kmwhen high drift velocities are present. St. Maurice et al [13] accounted for the electron temperature enhancement with a quasi-llnear kinetic theory of the two-stream instability. Schlegel and Nielsen [ii] invoked the kinetic theory in the interpretation of the radial Doppler velocities observed with Stare. The result is illustrated in Figure 1. They found the predictions of the fluid and kinetic theory to be in agreement for small velocities. But for large velocities (> 800 ms -I) nearly directed towards one of the radars, the fluid theory treatment was found to underestimste the magnitude of the electron drift velocity by up to 25 %. The two theories predict the same direction of the irregularity drift also for large velocities. Should the kinetic theory in the future be proven as a better approximation to reality than the fluid theory we would use it to transform Doppler velocity measurements to electron drift velocities.

~,~ •

D3

O~o <Q-

ol

h=110 km

o

0 .40

e i ! ~ ..... i i i ! i i i l , ~ ! 0.60 0.80 1,00 1.20 1.40 1.GO 1- 0 2.00

VDRIFT (KM/S)

Fig. I For an altitude of 110 km the curves display the relationship between the observ- able phase velocity (VPHASE) of the plasma instabilities and the electron drift velocity (VDRIFT) according to the fluid theory (f) and the kinetic theory (k).

The Stare system was described by Greenwald et al [14] . The Sabre system is essentially a copy of the Stare system (Jones and Nielsen [15]). One system consists of two radar stations located about 10OO km apart. The radiation diagrams of the receiving antennas are characterized by multiple narrow beams. The narrowness of the beams ensure good directional information for ~p and the many beams (8), together with a range resolution of 15 km along a beam, ensure good spatial resolution (20 x 20 km 2 ) over a large area (~ 200 OOO km2). Figure 2 shows the geographic locations of the four radar stations, together with the associated receiver antenna patterns. An example of Stare/Sabre observations is shown in Fig.3.

Radar Observations and Electric Fields 133

6 5 °

60"

3~0 ° 350 ° 0 ° 10 ° 20 ° 30"

I = HANKASALM . I Me = MALVlK -- ~ ~)~.r~ ?0 °

}" i Up = UPPSALA/--y,,#--k'~ ' ' , , , J ~ A L " - I

2\\\\\ 16s° iF / / / / / ~ \ ' \ \ \ \ \

60"

0 10 20

Fig. 2 The geographic locations and fields of view of the Stare radars (in Malvik~ Norway and Hankasalmi, Finland) and of the Sabre radars (in Wick, Scotland and Uppsala, Sweden).

70

L R G9 T I T 6 e u D (

6 7

&6

1 0 0 0 n s S - ,

Y ( R R : 1 5 8 2 DRY~ 9 2 HOUR: 13 f l l H : 4 2 S ( C : • I H T : 2 0

S T R R E - S R | R E IRREGULRRITY D R I F T UELOCITY --,.

~-".~-~-"S.-- " ' -- ' ~ ~ ....

-- L- ~ - / . . . . ~ ~

I .I . . . . .

64

i,, i |

I 4 8 12 14; 2 0 2 4 t.ONG I TUOK, GG

Fig. 3 An example of Stare/Sabre data: The irregularity drift velocities are displayed in regions where the signal-to-noise ratio is larger than I dB.


The irregularity drift velocities were calculated from the observed radial Doppler velocities using equation I, An integration time of 2Os is typical. In order to arrive at a quantitative estimate of the accuracy of our observations, a series of experiments have been carried out. These experiments test some basic assumptions underlying the Stare/Sabre systems. The results of these investigations are outlined in the following two sections. Finally, we compare electric fields deduced from irregularity drift velocities to E-fields measured with other field detectors.

,/ N

CONFIGURATION FOR 3-RADAR VELOCITY MEASUREMENT

Fig. 4 Experimental configuration of 3-radar velocity measurement. N, S and F point toward Ma, Up and Ha, respectively 9 in Figure I.

THE COSINE-RELATIONSHIP

In the Stare and Sabre systems radial Doppler velocities are measured in two directions from a given area in the E-region. This provides the necessary information to derive the irregularity drift velocity from equation Ip the cosine-relatlonship. To test this relationship we have obtained a set of measurements with the Doppler velocity observed not from two but from three directions. We have examined how well the irregularity drift veloicity derived from the 3-velocity measurements agree with the vector determined by the normal procedure (Nielsen et al []6]).

The Sabre radar located in Sweden has an antenna pattern that places the furthest ranges in the eastern--most beam (not shown in Figure 2) within the Stare field of view. Figure 4 is schematic illustration of the experimental setup for the 3-radar velocity measurements. V_ and V. represent the normal Stare Doppler velocity measurements. The additional velocity

N . measurement Is labelled V S.

After removing the linear trend from a time series of velocity data, the average Stare irregularity drift v~locity and its r.m.s, error (rms(2)) were determined. A typical value of rms(2) was 60 ms- . For the same time interval a vector was least square fitted to the 3-veloclty data and the r.moS, error (rms(3)) calculated. A typical value of rms(3) was 75 ms -I . Two examples of the result of this procedure are shown in Figure 5. The two estimates of the irregularity drift velocity are in good agreement. ~at the error rms(3)


is somewhat larger than than rms(2) indicates a small systematic disagrement between the estimates. The value of rms(2) must be seen in relation to the r.m.s error inherent in Doppler velocity measurements using the double-pulse|technique. For a spatially homogeneous backscatter intensity this error is typically 50 ms for an integration time of 20 seconds as used during these observations. Fluctuations in time and space as well as noise in the receiver system will tend to increase this value. Thus, the 60 ms -| is probably a realistic lower estimate of the r.m.s, error. The reason for the systematic small difference in tile two error estimates is at present not known. Nevertheless, the result of this comparisons is satisfying and encouraging, because the irregularity drift velocities determined using Stare data alone are in a satisfactory agreement with the results of the more accurate 3-radar velocity measurements.

This test has demonstrated that the cosine-relationship provides a consistent framework in which to interpret observed radial Doppler velocities. The resulting irregularity drift velocity is a reasonably well defined parameter.

3-RADAR VELOCITY COMPARISON.

698 °N 300 /

1200 ms "1 900 600 300 \ . . . . f . . . . --4- . . . . . . . , ~

-300

' N 600

Fig. 5 Two results of the 3-radar velocity measurements. The vector marked STARE is determined in the normal way. The vector marked LSQ is determined from the 3-radar velocity measurements. The diameters of the circles indicate the r.m.s. errors. The configuration of the three stations is given. It is optimum for velocities directed poleward or equatorward, as these directions would cause large radial velocities, and therefore more accurate measurements, to be made at all three radar stations. However, velocities with such directions were not observed during the campaign.

POWER SPECTRA

The test of the cosine-relationship provides no direct information on the relationship between irregularity drift velocity and electron drift velocity or electric field. This aspect has been investigated by studying the power spectra of the backscattered radar signal (Whitehead et al. [17] ). From the simple theory, the power spectrum is expected to be symmetric around the mean frequency which is directly related to the component of the electron drift velocity along the radar k - vector. The double-pulse technique used in Stare/Sabre provides a measure of the mean Trequency for a symmetric spectrum (Ru~mler [7] ). The power spectral observations were carried out with the Stare radars operating in a mode that allowed measurements to be made of the complex autocorrelation function of the radar auroral signal. A series of eleven sets of double pulses with increasing separation were transmitted to yield eleven points of the real and imaginary part of the autocorrelation function. This pulse scheme was repeated about one hundred times in the 30 second integration time and the averaged data Fourier transformed to yield the power spectrum.

An example of symmetric power spectra is shown in Figure 6. Also shown are the associated line spectra, determined by fitting an infinity of Gaussian curves to the spectra. The width of the Gaussians, governed by the growth and decay rate of the plasma instabilities as well as by turbulence in the medium, was chosen such as to produce a single line spectrum or an asymmetric distribution of line spectra. A symmetric distribution of line spectra was transformed into a single line by increasing the line width. A line width of 3 ,Ls was used for Norwegian data and 2 ms for Finnish data. These lines widths were adequate for all the data available. One notices a rather close agreement between the observed (single-double-pulse) Doppler velocity (D-P.-EXP), the theoretical expected velocity based on the observed spectrum and the double-pulse technique (D.F.-THEO) as well as with the mean velocity. Differences are no more than about 30 ms -I .


STARE POWER SPECTRA GEOGRAFIC LONG (E) AND LAT iN): 21.0.69.6 1981 187 T IME 1"7 100UT INT. TiME: 600s AND OVER 9 GRID POINTS

N0eURV

D.P . - {XP . : 2f, S S o . P . - T H [ O . : 2&3 - BEAN UEL. : 234 -

r t HL n r I o SH~ " : " t a Og O.P . -EXP . : -SO n /$ O .P . - 1HEO. : -71 - M[AH VEL. : -9B -

- I Z - 6

a)

",2 J ~"."c~ 2 6 6 -~j,~ - frequency I 100,Hz b) n c y i 1 0 0 , H z

Fig. 6 Symmetric power spectra measured from the two Stare radars. The line spectrum is also shown.

STARE POWER SPECTRA GEOGRAFIC LONG(E) AND LAT(N): 200,69.6 19B1 ~87 T IME 17 2'9 0 UT INT. TIME:180sAND OVER 25 GRID POINTS

NORU~Y . . . . . . X~ O . P . - C x P . : , 19e S D . ~ , " I ~EO. : 5 ? 4 - rICAN UEL. : 48B

J frequencyl1OO,Hz

o)

/ !

- t 2 -G

b)

F INLAND S"R "Z'- ~.| DB O.P . -E IXP . : -7G n."$ D .P . - T l f f n . : - gg - MERM VEL . : - g4

I 6 IZ

f r e q u e n c y / IO0 ,Hz

Fig. 7 As)nmnetric power spectra measured from the two Stare radars. The line spectra are also shown (velocity splitting).

At times the spectra are asymmetric. An example is shown in Figure 7. In such cases two sig- nificant line spectra appear. For example, for the Norwegian data the two maxima are located at 165 ms -I and 623 ms -1. Owing to the varying aspect sensitivity across the field of view, the antenna side-loops may contribute a not insignificant part to the total signal. This might in principle give rise to an asylmnetry of the power spectrum. However, an attempt at explaining the asymmetry as a result of this effect was unsuccessful.

Unwin and Johnston [18] have shown that asymmetric spectra are caused by backscatter from two layers at distinct heights in the E-region separated by about 5 kilometers. The layers are characterized by differing line-of-sight velocities. No theory to explain this effect has been offered. It is reasonable to expect the asymmetric spectra observed with Stare to be associated with the same phenomena. This, however, raises a problem in the interpretation of the velocity data. Since a different radial velocity is associated with each spectral

Radar ObservatiOnS and Electric Fields 137

peak, it is not clear which is the relevant one to use in estimating the horizontal ionospheric electric field. Let us assume that the line with maximum intensity (in Figure 6) is the one relevant to the E-field. The difference between the velocity of this spectral line and that determined from the double-pulse technique is in this case of the order of 1OO ms -I • The irregularity drift velocity deduced from the observed Doppler velocities would be underestimated by 5% and the direction would be 15 degrees away from the drift velocity obtained using the Doppler velocities of the line-spectra with maximum power. This interpretation of the observed power spectra indicates that the error estimates of the electron drift velocity associated with an asymmetric spectrum are larger (1OO ms -I ) than for a symmetric spectrum (30 ms-l).

The phenomenon of velocity splitting=my be simply an asymmetry of the power spectrum, but it is more likely to indicate different velocities at different heights. The phenomenon lasts between 3 and 30 minutes, and occur for about 30% of the time. The splitting occurs over an area with a typical scale of 300 km. This area has an equatorward directed velocity of about 500 ms -I .

DIVERGENCE OF IRREGULARITY DRIFT VELOCITIES

Because Stare measures drift velocities over a large area with good spatial resolution, it is possible to calculate their divergence. This provides another test of the accuracy with which the irregularity drift velocities approximate the electron drifts (Whitehead et al~ [17]). For spatial and temporal constancy of the geomagnetic field, the divergence equals the curl of the electric field, which under these conditions should be almost zero. During the power spectral observations the magnetic field varied about 200 nT in 7000 s, setting a limit for the divergence of the velocity at IO-5s -I .

DIVERGENCE OF STARE IRREGULARITY VELOCITIES DAY 189. 1981

100 ms -1 RANDOM VELOCITY,---

I l I I I

-6 -4 -2

TIME. UT

• %'."

173"0 .o ~ ee

• •e

• e • • .% % •

• ~.". -.....

P • - • o •

• -)'/.."" 1630 .... • s"

;! • e °

, f ' : I I I 0 2 4

DIV V x 10 ~ s -1 f

L•

>- k--

u

o

Fig. 8 The temporal variation of the velocity divergence through a closed curve in the Stare field of view.


Leaving out the details of the calculations we show in Figure 8 an example of the temporal variation of the divergence of t h e Stare drift velocities through a given closed curve. The divergence was only calculated when velocity measurements were available at the whole circumference of the curve. The scatter in the data points during short time intervals is to a large extent caused by the r.m.s, error on the observations, which is typically 1OO ms -I for these measurements. The full spectrum measurement increases the inherent noise in the measurement. The length of the horizontal bar at the top of the figure indicates the expected scatter in the divergence for a random velocity of this magnitude. There are, however, also long term variations. The divergence is consistently positive between 1 6 2 0 - 1645 UT and between 1720 - 1730 UT. Outside these time intervals the mean divergence is not inconsistent with the expected value. The vertical bars at the right of the figure mark the times when the power spectrum was asyu~etric and a splitting of the veloclties occurred. Clearly, asyu~etric spectra appear to be associated with positive values of the divergence. In order to reduce the divergence to a reasonably small value a velocity correction of about 25 ms -I must be applied along the curve of integration. This provides a typical magnitude of the correction that has to be applied to convert the measured irregularity drift velocity to a drift velocity consistent with the magnetic field variations. It should be emphazised that the divergence is sensitive only to the spatial variations of the irregularity drift velocity and not to its absolute value. However, the compatibility in magnitude of the velocity corrections determined in the divergence analysis and in the spectral analysis, suggest that the assomption made earlier that the electric field is associated with the line spectrum with maximum power is valid.

The studies of the cosine-relationship and of the power spectra indicate that the assumptions of the properties of the radial Doppler velocity measurements, underlying the Stare/Sabre systemsp are fairly well satisfied. Stare irregularity drift velocities deviate from those resulting from 3-radar velocity measurements and from spectral analysis by no more than about 100 ms -I. In order to arrive at a better understanding of the dlscrepancies, it would be interesting to carry out 3-radar measurements simultaneously with power spectral observations and also to attempt to measure the height dependence of the spectral components (Whitehead [19]). Such observations are being planned.

COMPARATIVE E-FIELD MEASUREMENTS

In this section, estimates of the ionospheric.electric fields deduced from Stare irregularity drift velocity measurements are compared with E-field observations obtained by different kinds of experiments.

In situ measurements with rockets. Sounding rockets launched from Andoya Rocket Range in No--~ay have trajectories that lie within or near the Stare field of view. Figure 9a shows a comparison of electric fields obtained with the Stare facility and from rocket borne double-probe measurements (Cahill et al, [20] ). The rocket data are circled in the figure. With the exception of the earliest data point at 19:19:40 UT, there exists good agreement between the two data sets. In Figure 9b another Stare-rocket data set is displayed, and again

S~*RE'.OC,~ ~ CC~A~I~a.

~o .tLL;UOLIS ~ - - -

, t t ; s

t, tt f[ t L~/_L,~.J [,!'~.',.~ l l t !_t£J :

{el

Fhght 18:1004 Slore E -F ,e l ds v$ ROC;~el E - r~e l ds ~, 50 m l l l i voJ l $ /m l - - I n f eg rohon T ime l sec } 20

71 1375ec 1575ec [77 sec

71 19?se t 217set 23?set

[ _ _ _ _ J 71 257sec 277se¢ 297 ~e:

~o t I < i i i . ,,_-:

Fig. 9 Comparisons of electric fields measured with the Stare auroral radar system and rocket-borne electrostatic probes (Figure 3a: courtesy of Cahill eta] [20] Figure 3b: courtesy ef Zanetti et al. [21]).


the two vector measurements compare well. For these data the fields on board the rocket were measured with the dual-probe technique (Zanetti et al. [21]). The westward boundary of the radar field of view is indicated in the figure.

A disturbance causing a southward transition of the fields appears to be drifting eastward, as it is first observed at the rocket (at 237 s) and later in the northerly part of the Stare field of view (at 257 s).

A closer examination of the data sets reveals some differences between field measurements. For example, in Figure 9a at |9:20 UT the Stare field is about 25% larger than the rocket field, whereas at |9:22:20 UT the rocket field is about 50% larger. To what extend these differences are real or an effect of the difference in spatial resolution of the two techniques is not clear.

The data set shown in Figure 9a yields information on the sensitivity of the radar technique. The double-probe fields were less than 20 mV/m between about 70.1 and 70.5 degree latitude. The rocket charged particle measurements suggest high electron densities in the E-region at these latitudes. The absence of backscatter indicates that plasma instabilities were not operative at the radar wavelength. It was therefore concluded that one can take 20 mV/m as an upper bound for the minimum electric field necessary to produce plasma instabilities one meter wavelength. Later during the flight, after 19:23 UT, large fields and low precip- itating flux intensity were measured on board the rocket. The absence of backscatter in this case was thought to be caused by an ambient electron density so low that the backscattered signals were too weak to be observed by the Stare radars. Cahill et al [20] therefore concluded that Doppler velocity measurements with the Stare system are possible when the electric field exceeds a threshold of about 20 mV/m and when the ambient electron density in the E-layer is large enough.

BALLOON-STARE E- FIELD COMPARISON

1 0 m V l m = ' , - - -

B A L L O O N

\\\1 STARE

1979j DAY 158

lJllll I I l

2300 2320 23/*0 0000 TIME, UT

BALLOON

II 1 \ \ \ \ \ \ \ \ STARE

I I I

0100 0120 0160 0200 TIME, UT

BALLOON

\ \ \ \ \ \ \ \ \\\ \

Fig. 10

I I I I

0200 0200 02/+0 0300 TIME j UT

Comparison of electric fields measured with the Stare auroral radar system and balloon-borne electrostatic probes.

1AO E. N i e l s e n and J . D . W h i t e h e a d

Balloon measurements. The spatial resolution of electric fields measured during a rocket flight is too small to match the spatial resolution of the Stare system. Thus, an element of uncertainty is introduced into the electric field comparisons. On the other hand, a balloon borne E-field detector has a field of view of (order of) 1OO km in diameter. It is therefore possible to average Stare data spatially to obtain a resolution comparable to that of the balloon experiment.

A balloon was launched as part of the SPARMO-79 campaign on June 7, 1979, in eastern Norway, and it drifted subsequently through the Stare field of view with a speed of about 100 km/h. The double-probe electric field measurements made on board of the balloon (the data are kindly supplied by Dr. K. Torkar, University of Graz, Austria) are shown in Figure 10 together with fields deduced from Stare observations. One sees clearly the reasonable agreement between the two sets of electric field measurements, but also that the Stare fields have a tendency at times to be somewhat larger than the balloon fields. The amplitude and angular difference between the two vector measurements has a r.m.s, value of 5 mV/m and 10 degrees, respectively. A correction for the drift of the balloon relative to the geometric field would increase the balloon field by about 1.5 mV/m in the southward direction, thereby increasing the ballon electric field amplitude and rotating it slightly clockwise, in better agreement with the Stare data. However, Parks [22] advises that balloon measurements of horizontal electric fields should not be interpreted to an accuracy of more than a few mV/m (I mV/m % 20 m/s), and the correction has therefore not been made.

77-02-15 19 06UT

i x., ~

Y=t

~ L O n T SOmVl m

/"

(

f

/ /

J ~

-"""i'i /% , ° .

,. ,'° ° .. ,

) / .J /

f ./ 1

f /. ~I¸ • ~" {I

/" I

\

[\..__

i

F i g . 11

77-02-15 19 36UT

Y.

lOOnT / • $OrnVl m . j / / / "

. ° ~-L ....... .

Comparison of electric fields measured with the Stare system and equivalent current vectors measured with the Scandinavian Magnetometer Array. The current vectors have their origin where the corresponding magnetic disturbance was recorded (courtesy of Baum~ohann et al [23]).

Radar Obserw]tions and Electric Fields 141

Magnetometer measurements. Ground based magnetometers respond to currents flowing in the ionosphere and also in the general case to currents that have been induced in the Earth or which flow in the magnetosphere. It is common to use magnetograms to deduce equivalent horizontal currents which, if they flowed in the ionosphere, would give rise to the observed magnetic effects on the ground. During near stationarity in time and for homogenous height integrated conductivities, the equivalent currents are a good estimate of the ionospheric Hall currents. For such geophysical conditions, Figure 11 shows two examples of a comparison between equivalent currents and Stare electric fields (Baumjohann et al [23]). Because the height integrated conductivities are not known, we can only compare the directions of the two sets of vectors. Since ideally E ~ J x B, it is clear that Stare gives a good estimate of the actual electric field direction.

VLF measurements. Very low frequency radio waves can be generated in the lower ionosphere by periodically modulating the conductivity, and thus the natural currents, using high power HF radio waves (Stubbe et al. [24]). In conjunction with the heating facility near Troms~, Norway, such waves have been generated and detected on the ground using two orthogonal receiving antennas so that the N-S and E-W components of the VLF signal could be reconstructed (Rietveld et al. [25]). Figure 12 shows the temporal variation of the component amplitudes and the phase of the VLF signal during a natural Pc5 pulsation event. The Stare N-S and E-W irregularity drift velocities for the same event are also shown in the figure. The scales of the velocities and the VLF signal amplitude are normalized for the best fit. The close agreement of the two data sets is in accordance with theoretical expectations. This implies that the Stare observations provide a good determination of the relative amplitude of the ionospheric electric field and of its direction.

75O

S00

[nVs] -250

-50~ -'/SO

5OO

-2S0 -SOC

9~

Comparison between 97SHz signal H from Heat~J nnd electron drift

velocity F--) from STARE for 16 Oct 1981

i i i

i m i

• " ° " " . - , , ~S " •

o~o ' o~zo

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• ° . . • • . . °% . •

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021 Ol/* 007

0 ipT] -Q07

-0% -021

01~

Q07 0

~" -007 -014

TIME JUT]

,t 0~50

Bw IpT]

Fig. 12 The VLF amplitude are plotted with the negative amplitude showing the 180 ° p h a s e r e v e r s a l . Super imposed a r e shown t h e S t a r e e l e c t r o n d r i f t v e l o c i t i e s ob - t a i n e d by a v e r a g i n g t h e v a l u e s f rom g r i d p o i n t s c e n t e r e d on t h e h e a t i n g t r a n s - m i t t e r c o o r d i n a t e s . The bo t tom p a n e l shows t h e a n t i c l o c k w i s e a n g l e w i t h r e s p e c t t o e a s t of t h e p l a n e o f t h e S t a r e e l e c t r o n d r i f t v e l o c i t y [ t a n - 1 (VN/VE)] and t h e a n t i c l o c k w i s e a n g l e w i t h r e s p e c t to n o r t h of t h e ma jo r a x i s of t ~ e -VLF magnetic polarization ellipse [ tan -I (Bw/BN) ] (courtesy of Rietveld et al [25])

Eiscat measurements. The European Incoherent Scatter System (Eiscat) is located within the Stare field of view, thereby allowing a direct comparison between Stare electric fields and fields calculated from incoherent scatter measurements. Electric fields deduced from such measurements are believed to be very good approximations of the real fields in the ionosphere. In the comparison we have used all available measurements. Thus, we compare electric drift velocity estimates made at many different geographic locations, and during a variety of geophysical conditions, both in the eastward and westward electrojet regions, during which the irregularitiy drift velocities showed considerable fluctuations in time and space.

142 E, Nielsen and J.D. Whitehead

m I l T

Fig. 1 3

VELOCITY AMPLITUDE~ MS -T

, , , 1 6 0 0

i - -

~1200-- u O J lad >

IJ_ 800

> -

c12

~ 400

A i.JJ

" " 300 I - - -

>.-

O . _ I

" ' 2 0 0

i1

> -

F -

~-- 100

t a J

Or"

F-

o

- 0 0 0

0 0 0 0

o o

o o o

o /ooo o 13.

~ ° ] I l I

o

o

4.00 800 1200 1600 ELECTRON DRIFT VELOCITY (EISCAT)

I I I I

VELOCITY ANGLE DE6 I

O0 0 o

0 0 ~ / 0 0

b

I I I

100 200 300

ELECTRON DRIFT VELOCITY (EISCAT)

A comparison of the amplitude (a) and angle (b) of the irregularity drift velocity measured with Stare and the electron drift velocity measured with Eiscat.

Radar Observatiol~s and Electric Fields 143

Thus, the data set is not ideal for an evaluation of the Stare system as an electric field detector. However, as a preliminary effort the comparison is interesting. Figure 13a shows the relationship between the amplitudes of the Stare irregularity drift velocity and the electron drift velocity measured with Eiscat. The curve represents the relationship expected from the fluid theory. There is a reasonably good agreement (proportionality) between the data sets up to about 800 ms -I. For these data points the rms elevation from the curve is about 1OO ms-1 Only few data points are available for large velocities, but they all indicate the irregularity drift velocity is underestimating the electron drift velocity. That is just the trend predicted by the kinetic theory (see Figure I). However, more observations are needed before a final conclusion can be made. Figure |3b shows good agreement between the direction of the irregularity drift velocity and of the electron drift velocity. The rms deviation of the data points from the curve is 15 degrees.

General geophysical results. The above direct comparisons of Stare with electric fields obtained by other methods are strong evidence that the irregularity drift velocity is closely related to the horizontal electi~ic field in the ionosphere.

Data from the Stare system have also been used to study a variety of geophysical phenomena. These studies have led to new insights. They have also tended to confirm the conclusions of earlier studies or the predictions of well founded theories (Walker et al [26] ; Nielsen and Greenwald [27,28] ; Greenwald et al [29] ; Zi and Nielsen [30]; Villain [31] ; Poulter et al [32] ; Poulter and Nielsen [33] ; Poulter [34] ; Rietveld et ai,[25] ). These results give further support for the deduction of electric fields from Stare observations.

SUMMARY

Experimental studies of some assumptions underlying the design of the Stare/Sabre systems have been carried out. It is encouraging that the cosine-relationshlp provides a systematic method for combining observed radial Doppler velocities. Although the radar auroral power spectra are not always symmetric, the detected asymmetries are small. Thus, the average Doppler velocity is not always as good a measure of the drift velocity component associated with the electric field as one could hope. Nevertheless, discrepancies between the irregularity drift velocity calculated from the single double-pulse observations and the one~etermined from the more detailed power spectral observations are small, no more than 1OO ms . Further research into these relationships are needed. Comparisons of electric fields estimated from Stare measurements with simultaneously obtained E-field observations from a variety of other detectors, tend to confirm the ability of Stare to measure horizontal ionospheric electric fields.

ACKNOWLEDGEMENT

The Stare radars are operated by the Max-Planck-lnstitut fHr Aeronomie in cooperation with the Finnish Meteorological Institute and ELAB, The Technical University of Trondheim. The Sabre system is operated jointly by the Max-Planck-lnstitut f~r Aeronomie and Leicester University in cooperation with Uppsala Ionospheric Observatory.

REFERENCES

i. E. Nielsen, accepted for publication in J. Geophys. Res. (1982). 2. R.S. Unwln and M. Gadsen, Nature 180, 1469, 1957. 3. O. Buneman, Phys. Rev. Letters iO, 285 (1963). 4. D.T. Farley, J. Geophys. Res. 68, 6083 (1963). 5. C.G. Reid, J. Geophys. Res. 73, 1627 (1968). 6. A. Roglster and N. D'Angelo, J. Geophys. Res. 75, 3879 (1970). 7. W.D. Rummler, Bell Telephone Laboratories Report, No. MM68-4121-15, November 7, 1968. 8. R.N. Sudan, J. Akinrimisl and D.T. Farley, J. Geophys. Res. 78, 240 (1973). 9. R.A. Greenwald, J. Geophys. Res. 79, 4807 (1974).

10. D. Andre, submitted for publication (1982). ii. K. Schlegel and E. Nielsen, MPAE-W-04-81-12, (1981). 12. K. Schlegel and J.P. St. Maurice, J. Geophys. Res. 86, 1447, (1981). 13. J.P. St. Maurice, K. Schlegel, and P.M. Banks, J. Geophys. Res. 86, 1453 (1981). 14. R.A. Greenwald, W. Weiss, E. Nielsen and N.R. Thomson, Radio Science i_33, 1021 (1978). 15. T.B. Jones and E. Nielsen, EOS (1982). 16. E. Nielsen, J.D. Whitehead, T.B. Jones and A.A. Hedberg, Manuscript in preparation (1982) 17. J.D. Whitehead, H.M. Ierkic and E. Nielsen, submitted for publication, (1982). 18. R.S. Unwin and P.V. Johnston, J. Geophys. Res. 86, 5733 (1981). 19. J.D. Whitehead, MPAE-P-04-82-04, (1982). 20. L.J. Cahill Jr., R.A. Greenwald and E. Nielsen, Geophys. Res. Letters 5, 687 (1978). 21. L.J. Zanetti Jr., R.L. Arnoldy, L.J. Cahill Jr., D.A. Behm and R.A. Gr~enwald,

Space Science Instrument 5, 183 (1980). 22. C.G. Parks, J. Geophys. Res. 81, 168 (1976). 23. W. Baumjohann, J. Untiedt and R.A. Greenwald, J. Geophys. Res. 85, 1963 (1980).


24. P. Stubbe, H. Kopka, and R.L. Dowden, J. Geophys. Res. 86, 9073 (1981). 25. M. Rietveld, H. Kopka, E. Nielsen and P. Stubbe, submitted for publication (1981) 26. A.D.M. Walker, R.A. Greenwald, W.F. Stuart and C.A. Green, J. Geophys. Res. 844,

3373 (1979). 27. E. Nielson and R.A. Greenwald, J. Geophys. Res. 844, 4189 (1978). 28. E. Nielson and R.A. Greenwald, J. Geophys. Res. 8__3, 5645 (1978). 29. R.A. Greenwald, T.A. Potemra and N.A. Saflekos, J. Geophys. Res. 85, 563 (1980). 30. Zi, M., and E. Nielsen, Polar Upper Atmosphere, Reidel, Dordrecht, 293 (1981). 31. J.P. Villain, J. Geophys. Res. 8/7, 129 (1982). 32. E.M. Poulter, E. Nielsen and T.A. Potemra, accepted in J. Geophys. Res. (1981). 33. E.M. Poulter and E. Nielsen, accepted in J. Geophys. Res. (1982). 34. E.M. Poulter, submitted to J. Geophys. Res. (1981).

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Radar auroral observations and ionospheric electric fields