Magnetometer and incoherent scatter observations of an intense Ps 6 pulsation event

Journal of Amospheric and Terresrrial Printed 1x1Great Britain.

Physics,

Vol. SO, Nos 4/S, pp. 351.-361, 1988.

fX!i-9169/88 $3.00+ .X-I Pergamon Press plc

Magnetometer and incoherent scatter observations of an intense Ps6 pulsation event S. BUCHERT, W. BAUMJOHA~N, G. HAERENDFJL Max-Planck-Institut

fiir Physik und Astrophysik, Institut fiir extraterrestrische Physik, Garching bei Miinchen, F.R.G. c.

LA

Hoz

EISCAT Scientific Association, Ramtjordbotn,

Norway

and H. LOHR Institut fur Geophysik und Meteorologie der TU Braunschweig, Braunschweig, F.R.G. (Receivedfor publication 18 November 1987)

Abstract-In the morning sector of 21 April 19X.5,during the recovery phase of a geomagnetic storm, a Ps6 pulsation event was recorded by the EISCAT magnetometer cross in northern Scandinavia. Simul~neously, the EISCAT incoherent scatter radar measured E- and F-region plasma parameters with a latjtudinal scanning program. Electric fields and height-integrated Hall and Pedersen conductivities are derived. Two-dimensional patterns of these quantities are constructed for one Ps 6 period. The conductance patterns closely resemble the typical aurora1 forms of eastward drifting R bands with low and high conductances at the northern and southern edges of the scanned area, respectively. From the equatorward region a tongue of high ionization extends poleward into the dark area. The location of the maximum southward current is slightly displaced towards the west from the centre of the conductance tongue. The east-west disturbance electric field points towards the tongue; the north-south fields are enhanced outside and reduced inside the high conductance region. As has been previously suggested, the observations can be explained with a model which superposes currents caused by conductance variations and electric fields. Both effects need to be taken into account for this event. The current structures move within a few degrees in the direction of the b~kground E x B drift, but their speed is about 15% lower than the average Fregion piasma drift. 1. INTRODUCTION Ps6 pulsations of the magnetic field on the Earth’s surface have been described by SAITO (1978). They are quasi-periodic pulses with periods in the range lO_ 40min which are strongest in the Y component of the magnetic field, while the X component remains relatively undisturbed. They usually occur in the morning hours around 0500 MLT during the recovery phase of substorms (ROSTOKER and BARICHELLO, 1980). During the last few years the three-dimensional current structure and the electric field associated with magnetic Ps6 pulsations have been studied in considerable detail using improved measurements made by ground-based magnetometer networks, coherent radars, riometers, baltoons, all-sky cameras and low-altitude satellites. The connection between eastward drifting Q bands (AKASOFU, 1974) and Ps6 pulsations has been firmly established (KAWASAKI and ROSTOKER, 1979 ; BAUMJOHANN, 1979 ; ANDRE and

BAUMJOHANN,1982). An essentially stationary current system travels eastward with the R bands and so creates ‘pulsations’ seen at ground-based magnetometers. Two different models for the ionospheric current systems associated with the pulsations have been suggested. ROSTOKER and APPS (1981) used the cosmic noise absorption measured by riometers to infer the conductivity pattern of the structures. They presented 4 events, where the southward current maximized in phase with the riometer deflections when the structure was passing overhead in the eastward direction. Therefore, they interpreted this current as a southward Pedersen current driven by the background electric field in north-south oriented structures of enhanced conductance. GUSTAF~SON et nt. (1981), studying one pulsation event, found the maximum southward current west of the maximum absorption. Electric field detectors on a balloon recorded a mainly east-west oriented disturbance field, with the vectors pointing towards the absorption region. Hence, they

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attributed the magnetic field variations to clockwise and counterclockwise Hall current loops around eastwest oriented pairs of field-aligned currents. Modelling the three-dimensional structure under the assumption of homogeneous ionospheric conductances, they found good agreement between the theoretical and measured equivalent currents. NIELSEN and SOFKO (1982), using the STARE radar, observed an electric field pattern in rough agreement with that inferred by GUSTAFSSONet al. (1981), but a detailed comparison with the magnetic field deflections indicated also the presence of spatial conductivity gradients. So far, the most advanced model has been proposed by OPGENOORTH et al. (1983). They assumed spatially varying conductances and used idealized background and disturbance electric fields corresponding to the values measured by the STARE radar to calculate the current structure and found good agreement with the observed magnetic variations. ANDRE?and BAUMJOHANN(1982) compared the drift vector of the equivalent current structure obtained with a magnetometer array and the aurora1 forms with the background electric field measured by STARE. They found the eastward motion of the omega bands and associated current structures to be E x B drift within the accuracy of the measurements. RAJARAM et al. (1986) analysed the phase and group velocities and the wavenumbers of 27 Ps6 events. Within the errors of their analysis they found the two velocities to be equal, indicating only a small dispersion of the pulses when drifting eastward. Projecting the wavelengths and phase velocities into the magnetotail they found a local time dependence, in agreement with the predictions of the model of ROSTOKER and SAMSON (1984). As a source mechanism for the Ps6 disturbances these authors proposed the Kelvin-Helmholtz instability occurring at the interface between the low latitude boundary layer and the central plasma sheet. LYONS and WALTERSCHEID (1985) instead proposed the occurrence of the Kelvin-Helmholtz instability in the neutral atmosphere. A neutral jet stream could form within an aurora1 band generating large wind shears because the ion drag on the neutrals in the E-layer would be stronger in aurora1 regions with high electron densities. LYONS and FENNELL (1986) presented the electron precipitation obtained from the DMSP-F6 satellite passing over a visible rZ band. In addition to the hard precipitation within the diffuse aurora on the morningside, the spectra indicated acceleration by a parallel potential drop of < 1 keV above the poleward extending aurora1 tongue. In this work we present observations of an intense

Ps6 pulsation event with the EISCAT UHF incoherent scatter radar and the EISCAT magnetometer cross. Simultaneous measurements of both the conductances and the electric fields allow us to reconstruct the two-dimensional picture of a Ps 6 pulse. We will give an estimate of the Pedersen and Hall current densities and compare their relative contributions to the magnetic variations recorded for this event. The magnetometer data provide an accurate estimate of the eastward motion of the structures, which is compared with the E x B drift of the background field measured by the radar. The study closes with some remarks about the relevance of the proposed generation mechanisms for this event.

2. INSTRUMENTATIONAND

DATA PROCESSING

The incoherent scatter technique in general has been described. for example, by EVANS (1969), the EISCAT facility by FOLKESTADet al. (1983). On 21 April 1985 the EISCAT tristatic UHF radar was operated with a latitudinal scanning program of the Max-PlanckInstitut fiir extraterrestrische Physik to compare ionospheric observations with measurements made by the AMPTEjIRM satellite located in the magnetotail. Each scan lasted 27.5 min and consisted of 16 positions. The integration time for each position was typically 100s. The altitudes of the antenna intersection volumes were switched during each scan in a favourable pattern, minimizing the time used for antenna motions, with 5 positions at 165 km and 11 positions at 250 km. The F-region ion drifts V,, are derived from the Doppler shifts of the radar spectra. For both altitudes it is assumed that the ion motion is a pure Hall drift. The perpendicular electric fields E, are then calculated using E, = -V,,

x B,

(1)

where B is the Earth’s magnetic field. The electron densities N, of the E-region were obtained from the power profiles of a short pulse with high spatial resolution (3 km). Using theoretical ionneutral collision rates (SCHUNK and WALKER, 1973) and neutral densities from an atmospheric model (CIRA, 1972), the height-integrated conductivities are derived. BREKKE et al. (1974) used different collision rates and neutral densities. They found that the height-integrated conductivities depend little on these model values. The scattered power of the radar wave is approximately proportional to N,/( 1+ TJT,), where T, and T, are the electron and ion temperatures, respectively. If both the electron and ion gases are coupled sufficiently strongly to the neutral gas, T,. and

An intense Ps 6 pulsation T, are close to the neutral temperature. We assume a constant ratio of T,/T, in our data analysis. However, recent measurements indicate that in the presence of high electric fields, T, can be considerably elevated above T, in the height range 1055115 km (WICKWAR et al., 1981 ; ICARASHI and SCHLEGEL, 1987). This would result in an underestimation of N, in our analysis and consequently in an underestimation of the Hall and Pedersen conductivities oH and crp. The maximum value of o,, is typically found at altitudes between 100 and 110 km, that of op between 120 and 130 km. Therefore, the height-integrated Hall conductivity C, might be considerably underestimated due to this effect, while the Pedcrsen conductance Xp should be less seriously affected. NIELSEN et al. (1987) estimated Hall conductances from STARE and magnetometer measurements and compared them with those obtained by EISCAT with the procedure described above. They found a discrepancy of a factor of 1.5 for electric fields of 75 mV m ’ and suggest that this effect is the most likely explanation. The power profiles are averaged in time over 20s. The main contributions to the conductances come from a layer of typically 20 km thickness. Depending on the antenna elevation in Tromso, the height-integrated conductivities are therefore horizontally averaged values. For the lowest elevation (worst case) the average is taken over a horizontal range of about 38 km. In Fig. 1 the geometry of the scan is shown. The locations of the conductivity measurements are on the line connecting the locations of the electric field measurements, but are closer to the Tromso site due to the lower altitude of this measurement. The EISCAT magnetometer cross, a joint project of Scandinavian and German research groups, has been described by L~~HKet al. (1984). Five of the seven stations are aligned to a magnetic meridian and two are located on the flanks, as shown in Fig. 1. The instruments continuously monitor all three components of the magnetic field. The digitized outputs are averaged over 20 s. No optical data were available for this night. The daylight was already too intense. 3. OBSERVATIONS On the night of 20/21 April 1985 the K,, index reached a maximum of 8 + Several aurora1 substorms were recorded by magnetometers in northern Scandinavia, the largest causing a negative bay in the X component of the geomagnetic field of about 1600 nT. In Fig. 2 the Y components of the seven stations of the EISCAT magnetometer cross are shown between

event

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Fig. 1. Locations of the EISCAT instruments on a geographic grid. The radar sites in Tromso, Kiruna and Sodankyll are marked with symbols for the antenna dishes, the seven magnetometer stations with t and the locations of the electric field measurements during the radar scan with * (250 km altitude) and x (165 km altitude).

0300 and 0430UT. Five positive deflections can be seen, where at least the three in the middle, at 0320, 0341 and 0356 UT, respectively, are clearly identified as Ps 6 type disturbances. Cross-correlating the Y curves of the most northern stations, the cross correlation coefficient reaches values of 0.96, however, when using stations south of KAU, it drops below 0.5. A detailed calculation reveals that the pulses arrive simultaneously approximately along a geographic meridian. Therefore we conclude that the southern edge of the structures is located somewhere between MU0 and KAU, that they are north-south aligned and that they extend northward at least beyond SOR. The Z components of the EISCAT chain stations are shown in Fig. 3. Their variations agree with the picture of mainly southward directed equivalent current structures passing overhead from west to east. The X components show much smaller periodic features, which correlate with the Y and Z components. In Fig. 4 the Y components of the eastern and western stations of the cross are plotted together with the conductances measured by the EISCAT radar. The form of the Y component curves slightly changes between KIL and KEV, however, the central peak is

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Y components of the geomagnetic field at the seven EISCAT magnetometer stations showing at least three Ps 6 pulses between 03 15 and 0400 UT.

EISCAT-MAGNETOMETER CROSS 1985-04-21

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Fig. 3. The % components of the geomagnetic field at the seven stations of the EISCAT magnetometer cross.

two lower panels show the Y components of the geomagnetic field at the western (KIL) and eastern (KEV) stations of the EISCAT magnetometer cross, The two panels above show the height-integrated Pedersen (Z,) and Hall (Z,) conductivities in Siemens measured by the EISCAT radar. In the top panel the geographic latitudes of the antenna beam at 110 km altitude are indicated. These are approximately the latitudes of the conductance measurements.

well defined for all three pulses. Observed from a reference frame that is moving with the structures, they would appear stationary to some degree. The time delay of the maxima of the Y components between KIL and KEV clearly indicates the drift of the structures. Knowing also the direction of the drift, we can calculate the full drift vector from the crosscorrelation function, The conductivities are measured on longitudes close to that of KIL. The conductance curves exhibit similar variations as the Y component curves, their maxima roughly coinciding with the Y component maxima in KIL for the first and third of the significant pulses. For the second pulse the measurement of the conductivities happens to be south of KAU, where the southern edge of the structure is located. Indeed, the conductance curves show a wide peak with no well-defined maximum at that pulse. If we assume that the cosmic noise absorption measured by riometers monitors the Hall conductance patterns, at least the first and the third pulse are similar to the events presented by ROSTOKER and APPS (1981) with respect to the relative phase between the maximum currents and maximum conductances. The drift velocities for the ions in the F-region, including 2-3 antenna scans before and after the pulsation event, are shown in Fig. 5. The background ion drift is calculated from running averages over

An intense Ps 6 pulsation

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Fig. 5. Ion drift velocity vectors between 0220 and 0508 UT. A vector towards the right indicates a vector towards magnetic east, which is l-3” away from geographic east for the scan positions. Two different heights of measurement, 165 (thin arrows) and 250 km (thick arrows), are plotted together in this figure without making any projections. The estimated errors change between the individual arrows because of variable signal-to-noise ratio. They are about 100-400m s-i in each direction. The time interval of the Ps 6

pulsation event is marked. k 27Smin around each vector. Therefore it represents a spatial average over the latitudes covered by the antenna scan (66.5-73.2”) and a temporal average over at least 2 scan cycles. The direction ofthis average ion drift before and during the pulsations (0220~1OUT) is eastward within 3”. Also, the motion of the current structures inferred from the magnetometer cross is directed eastward. Both directions are equal within a few degrees. They are not aligned to the morning sector aurora1 oval over Scandinavia (cf. BAUMJOHANN and KAMIDE, 1981), in contrast to the event studied by OPGENOORTH et ai.(1983). Only after the event, from 0410UT on, is the background ion drift vector directed north of east by up to 20”. About half an hour after the last pulse at 0430UT the field strength drops to smaller values and the individual vector directions become more irregular. In Fig. 6 the magnitudes of ion drift and structure

Fig. 6. Running averages (2 27.5 min) of the drift measured by the EISCAT radar (full motion of the current structures obtained from tive deflections in the magnetic Y components

eastward ion line) and the the live posi{dashed line).

motion are compared. They are obtained from running averages of the plasma velocities and crosscorrelation functions of the magnetometer Y components, respectively. For the three Ps6 pulses we note a systematic difference, the current structures being about 200m SC’ slower than the ionosphe~c plasma. The radar measurements have small statistical errors, because they are averages over many positions. A check has been made for the magnetometer data using the Y components recorded at ALT and KEV. The curves from these stations have a more similar shape than the curves from KIL and KEV, but their east-west separation is less favourable. The results were the same within less than 5% error. Therefore, the difference between these two drifts seems to be significant. At the third Ps6 pulse the scan happens to pass through the center of the magnetic disturbance and all velocity fits converged. The magnetic east-west and northhsouth components of the electric fields are plotted in Fig. 7. The north-south field is greatly enhanced at times of low conductivities and reduced at times of high conductivities. The total electric field strength reaches 190mV mm’ at 0350 UT. (This value is consistent with the observation of enhanced ion temperatures in the F-region due to Joule heating. The values are in rough agreement with the theory. Work on these aspects of the event is under progress and will be published later.) Between the first and second pulse also very large field strengths, up to 150 mV m- ‘, are observed. Because the structures drift eastward, the electric field at the eastern ridge of the peak

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is observed before the passage and that at the western edge after. Accordingly, the pulses are accompanied by east-west electric fields pointing towards the region of enhanced conductivities of roughly 60 mV mm’ magnitude, as is obvious from Fig. 7. In order to construct a two-dimensional picture of one period, we assume that the three periods consist of stationary patterns drifting eastward and that they are similar to each other except for a variable longitudinal extent. The time of each conductance and ion drift velocity measurement then indicates its longitudinal location. We use the change of the northsouth ion velocity component from southward to northward between the individual pulses to separate them. This shear can be identified clearly in all cases and is predicted by models for Ps 6 disturbances (see below). Using the drift velocities of the current patterns from Fig. 6 and the time separation of the Y component maxima at KIL and KEV, apparent wavelengths of 1100, 1700 and 1000 km for the pulses are obtained. The drift velocity vectors of the current structures are subtracted from the ion velocity vectors and their bases are projected to a common altitude of I 10 km. The conductance measurements are averaged to the same time resolution as the velocity measurements. We now scale each pulse to a reference pulse of 1000 km length and merge both the conductance

01.

and ion velocity measurements from all pulses to a single plot. The magnitudes of the velocity components are not scaled, only the bases of the vectors. Also, the secondary conductance peak at 0350UT before the arrival of the third pulse is omitted, because it might be related to the obviously non-stationary small peak in the Y component which is not yet present at KIL but is already seen at the more eastern stations. The maximum Y deflection is chosen as the origin of the coordinate system. The result is shown in Fig. 8. Fortunately, the antennas scan the patterns in such a way that the locations of the conductance measurements are pretty well uniformly distributed in Fig. 8. Of course, some interpolation needed to be done. due to the long scan cycle. The 30 conductance measurements were divided into 4 groups. Within each group they could be connected by a line running approximately from east to west through the plot. Equidistant interpolated values were obtained using cubic splines. Also, some extrapolation was necessary at the upper right and lower left corners of the figure, which did not affect the overall picture very much. The electric field pattern is in general agreement with the observations by GUSTAFSS~N et al. (1981), ANDRE and BAUMJOHANN(1982), NIELSENand SOFKO (1982) and OPGENOORTH et al. (1983). However, except for the balloon measurements by GUSTAFSSON et uI. (1981) along a line somewhere through the structure, the electric fields inside the tongues could not be observed previously, probably because the irregularities scattering the STARE radar waves are not excited there. The magnitudes of the magnetic disturbances are comparable to the event presented by NIELSEN and SOFKO (1982). Spectral shifts corresponding to about 2000 m s- ’ were recorded by the STARE radar, but according to the works by NIELSEN and SCHLEGEL (1983, 1985) the Doppler velocity measured by STARE is less in magnitude than the E x B drift for high electric fields. This would imply that these authors have underestimated the electric fields and that their magnitudes were also exceeding 1OOmV mm’ around a Ps6 pulse of similar magnetic field strength. We summarize the observations as follows. -

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The magnetic variations on the ground suggest regions of southward directed current structures that drift eastward with an average velocity of about 1200m s-‘. The structures arc approximately stationary in the appropriate reference frame. The ‘conductivity tongue’ is about 400 km wide for a pulsation wavelength of 1000 km and it extends roughly 400 km poleward from a region of enhanced ionization.

An intense Ps 6 pulsation event Pedersen 400

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The conductances show large gradients and vary by about one order of magnitude. Their maximum is shifted from the maximum southward current by 100-200 km to the east, which corresponds to 2-3 min time difference for an observatory on the ground. The average electric field is directed southward and is enhanced at the time of the pulsations, On average, the plasma drifts eastward faster than the current structures by 200 m s-l. The east-west disturbance electric fields point towards the regions of enhanced conductances/ currents. Their magnitude is about + 60mV mm’, the maximum value being located just ouside the region of enhanced conductance There is a small eastward electric field of about 20 mV m ’ during the passage of the pulse. The north-south electric field is greatly enhanced

in the regions of low conductances and reduced in the regions of high conductances.

4.

DISCUSSlON

The aurora1 zone current systems, including those associated with R bands, have been reviewed by BAUMJOHANN (1983). In Fig. 9 the background and the two systems which have been suggested to be responsible for the Ps6 type disturban~s are drawn schematically. For the discussion of these systems we have plotted in Pig. 10 the theoretical latitude and longitude profiles of the magnetic Y component caused by a horizontal southward current of finite extent. This is a good approximation to a real current system, if the current is closed by horizontal return currents flowing sufficiently far away. If, on the other hand, the horizontal current is closed by field-aligned

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Fig. 9. The idealized background situation and the two systems which have been suggested to explain Ps 6 disturbances are drawn schematically. An approximate distance scale for this event is given. In System 1 in the first pulse the disturbance Hall currents are closed by field-aligned currents at

the edges, in the second pulse the situation for a complete Cowling channel is drawn. In System 2 the vectors for the Pedersen currents also represent the disturbance electric fields. currents at the northern and southern edges of the sheet, these currents decrease the ground magnetic effect of the horizontal current. That case is also plotted in Fig. 10. The background electric field in the morning sector is directed southward, resulting in a westward Hall current and a southward Pedersen current closed by field-aligned currents at the poleward and equatorward edges of the aurora1 belt. If homogeneous background Hall and Pedersen conductances are assumed, only the Hall current produces a negative deviation of the X component; the effects of the Pedersen and field-aligned currents cancel each other (FUKUSHIMA, 1969). One disturbance current system has been suggested and modelled by KAWASAKI and ROSTOKER (1979). We call it System 1. It consists of regions of enhanced conductivities drifting eastward. The conductivity enhancements are associated with the poleward auroral tongues of the eastward drifting s2 bands, which is justified from previous observations (ANDRE and BAUMJOHANN, 1982; OPGENOORTH et al., 1983). An additional southward Pedersen current is driven by

current

or

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Fig. 10. Latitude (upper panel) and longitude (middle panel) profiles of the Y component due to the current system drawn in the lower panel. The profiles are through the center of the system, the total current is OSMA. Rectangular current distributions are assumed. The dashed lines are the contribution of the horizontal current only, the full lines indicate the Y component caused by all currents.

the background electric field in the areas of enhanced conductances. It must be closed by additional fieldaligned currents at the northern and southern edges of the structure. This current produces a positive peak in the Y component while the structure passes eastward overhead. KAWASAKI and ROSTOKER(1979) did not exclude the possibility of an additional Hall current due to a turning of the electric field direction from south towards southeast inside the structure. For their events, however, they favoured the conductivity effect as the primary mechanism. We estimate first the Pedersen currents and then the Hall currents of their model for the parameters of the pulse at 0356 UT. The southward electric field is reduced to an average value of about 30mV m-’ at the time of high conductances. An estimate of the Pedersen current density using Z,Z 18 S gives 0.5 A mm’. Using a longitudinal extent of the horizontal currents of 400 km gives a total southward Pedersen current in this pulse of roughly 0.2 MA. Note that in Fig. 10 the Ycomponent profiles are plotted for a total current of 0.5 MA. For the time scales involved we can assume that l&15% of the measured magnetic Y variations are due to

An intense Ps 6 pulsation event induced currents in the Earth. Clearly the model Y variations are weaker than the measured ones. In the presence of conductivity gradients an eastward electric field component inside a Ps6 tongue responsible for southward disturbance Hall currents can be explained by the Cowling effect (3os~~~, 1975). Additional westward Hall currents are driven by the background electric field in the high conductance region. They can be closed by secondary field-aligned currents at the western and eastern edges of the structure. This secondary system would be equivalent to the primary one rotated by 90” and is drawn schematically into the first pulse of System 1. It would produce negative variations in the Xcomponent when passing overhead, however, the effects of the fieldaligned currents tend to smear them out ; compare the latitude profile in Fig. 10, which is the longitude profile for the secondary system. A complete Cowling channel occurs when the excess Hall current is cancelled by a Pedersen current due to an eastward polarization electric field. This electric field would also drive a strong southward Hall current that would add to the primary Pedersen current. The situation is drawn schematically into the second pulse of System 1. Indeed inspection of Fig. 5 reveals an average value of the eastward electric field of about 20mV mm’ during the passage of the structure. This could be explained by a weak Cowling effect. Using C, x 60 S (see Fig. 4) a secondary total Hall current of roughly another OSMA is added to the Pedersen current. Therefore the Hall currents from System 1 should contribute an amplitude of about 250 nT (cf. Fig. 10). Still the observed magnetic deflections are larger by a factor of at least 3. Now we will discuss the system first proposed by GUSTAFS~~Net al. (1981), and here labelled System 2. In the original version the authors neglected conductivity gradients. In Fig. 9 we extend their line-type Birkeland currents to current sheets to emphasize the north-south extent of the system [as also done by BAUMJOHANN (1983) and OPGENOORTHet al. (1983)] and draw the situation for a train of two pulses. Eastwest oriented disturbance electric fields drive Pedersen currents away from downward to upward fieldaligned current sheets. The latter would coincide with the visible aurora1 forms. These currents woufd not produce any magnetic variations on the ground for homogeneous conductances. However, the accompanying Hall current loops around the Birkeland sheets would result in a strong southward current west and a northward one east of the aurora. In such a system the Y component maximizes before the arrival of the signatures of electron precipitation, since the structure passes eastward. In this event the main

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southward current flows somewhere between the location of the highest eastward electric field and the high conductance region. Certainly the Hall current pattern of this system is distorted by the horizontal conductivity gradients. One might use XHz 30 S, an eastward electric field of about 30mV m-’ (both values are half the peak values) and an edge width of 200 km, where Hall currents of 0.9 A mm’ are flowing. This yields a total southward Hall current of 0.2 MA at the western edge of the tongue. This current is closed by corresponding northward currents at the eastern edge of the tongue. Therefore, at a ground magnetometer only contributions from the horizontal currents are observed. These are stronger than the combined effects of horizontal and field-aligned closure currents, as indicated by the dashed lines in Fig. 10. Still, the addition of all currents of Systems 1 and 2, which are inferred from the radar measurements of electric fields and conductances, seem to yield magnetic disturbance fields that are too small by a factor of 1.5-2. However, considering the uncertainties involved in our estimates, they are marginally consistent with the observations by the magnetometers. We cannot exclude the possibility that our derived current densities are biased due to possible underestimation of the Hall conductivities. At times of high electric fields the conductance values are low and quite possibly underestimated, but in the regions of low conductances only small currents are flowing according to the magnetometers. In order to explain the large magnetic variations observed on the ground, an underestimate of the Hall conductances at times where the electric field strength is only about 40mV rn--’ would be required. That disagrees with expectations. Owing to the uncertainties in the data and the complicated scenario it is not possible to study the problem in a more quantitative way. Nevertheless, the observations demonstrate that a combination of Systems 1 and 2 indeed seems to be necessary to model the currents responsible for Ps6 pulsations. This has been suggested by OPGENOOR~H ef al. (1983). They assumed Hall and Pedersen conductance variations of only 25% along the east-west direction through the model and could reproduce the observed magnetic disturbances quantitatively. They concluded that in their event System 2 was the more important one and found a smaller cont~bution from System 1. Here the relative conductance variations are larger than the relative electric field variations (AX/I: > AEIE) and consequently the currents seem to Row mainly in the areas of high conductances, which is in agreement with the features of System 1. In System 1 the Hall currents due to a weak Cowling

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effect are probably more important than the Pedersen currents. However, it is difficult to explain the magnitudes of the magnetic field variations with System 1 alone, and the presence of the electric fields predicted by System 2 is clearly measured. Thus we might say, for this event, that the contributions of both Systems 1 and 2 are comparable. Now we consider for this event the generation mechanism proposed by LYONS and WALTERSCHEID (1985). The observed phase velocities of the ‘waves’ are more than 100% of the average E x B drift before the onset of the pulsations. Waves due to a KelvinHelmholtz instability in the thermosphere, as suggested by these authors, would have phase velocities of less than 50% of the average E xB drifts. In addition, the phase velocities recorded here significantly exceed the largest values attained in their model. So far magnetospheric signatures of the event could not be found. The AMPTE/IRM satellite was located at around 15 RE, close to local midnight in the magnetotail. No clear feature that could be related to the pulsations was detected. This is probably due to the large longitudinal separation of the satellite from the location of the ground-based instruments.

5. CONCLUSIONS AND SUMMARY In this work we have presented the most complete observation of a Ps 6 pulsation event so far. To the best of our knowledge, for the first time absolute conductance values over a whole Ps6 structure and electric fields inside a Ps 6 tongue were obtained. The currents inferred from the incoherent scatter radar are qualitatively in agreement with the ground magnetometer variations, but probably a discrepancy exists concerning their magnitudes. The widely used procedure for obtaining height-integrated Hall and

Pedersen conductivities from the radar power profiles needs to be investigated and possibly corrected for high electric field strengths. The observations can be explained by the ionospheric current and electric field model suggested by OPCENOORTHet a/. (1983), which includes the effects of both electric fields and conductance variations. This is not the case if either of the previously suggested systems is used alone. In principle Ps 6 current/electric field patterns should always consist of a superposition of Systems 1 and 2. Depending on the ratios of AC/C and AE/E an observer might find either of the systems the dominant one for an individual event. The pulsations presented here obviously cannot be explained as signatures of a neutral atmosphere wave generated by wind shears. The observations neither support nor disprove a generation mechanism involving magnetospheric processes such as a KelvinHelmholtz instability occurring at the boundary of the central plasma sheet (ROSTOKER and SAMSON, 1984). In the light of the observed large conductance gradients we may speculate that ionosphere-magnetosphere coupling plays an essential role in this event and needs to be taken into account in future, more theoretically oriented, studies. Acknowle&ements-The EISCAT Scientific Association is supported by the Max-Planck-Gesellschaft (F.R.G.), Centre National de la Recherche Scientifiaue (France). Science and Engineering Research Council &.I;.), N&ges Almenritenskaplige Forskningsrid (Norway), Naturvetenskapliga Forskingsrldet (Sweden) and the Suomen Akatemia (Finland). We thank the EISCAT staff for their efforts and technical and operational assistance. The EISCAT Magnetometer Cross is a joint enterprise of the Finnish Meteorological Institute, the Sodankylg Geophysical Observatory and the Technical University of Braunschweig. The German part of the project is funded by grants from the Deutsche Forschungsgemeinschaft. The work of W. B. was financially supported by a Heisenberg Fellowship from the Deutsche Forschungsgemeinschaft.

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