Electric fields at L = 2.5 during geomagnetically disturbed conditions

Phef. SpnceSci.. Vol. 39, No. 9. pp. 1305-1320. Printed in Great Britain

ELECTRIC

00324633,91 $3.00+0.00 Pergamon Press plc

1991

FIELDS AT L = 2.5 DURING GEOMAGNETICALLY DISTURBED CONDITIONS J. M. SAXTON

Department

of Geology,

University

of Manchester,

Manchester

Ml3 9PL, U.K.

and A. J. SMITH British

Antarctic

Survey,

Madingley

Road,

Cambridge

CB3 OET, U.K.

(Camera-ready copy received in final form 29 July 1991) Abstract-We have used observations of the group time delay and Doppler shift of ducted VLF whistler mode signals propagating near L = 2.5 to deduce the azimuthal component of the plasmaspheric electric field during geomagnetically disturbed periods in June and July 1986. The whistler mode signals originated in the US Navy transmitters NAA and NSS and were recorded at Faraday, Antarctica. The average E, ~letwts LT curve for periods when I$ > 2+ has been compiled; when this is compared with the E, Z)EKWS LT curve for quiet times, it is found that the electric field is more eastwards from 18-22 LT and more westwards from 00-01 LT in disturbed times. This difference is consistent with published calculations of the penetration of the dawn-dusk electric field to L = 2.5. A variety of behaviour is evident when the data are examined on a case to case basis. Sometimes the dawn-dusk electric field becomes apparent during isolated intense substorms; this is attributed to increased penetration due to an increase in the aurora1 zone Pedersen conductivity. On one night the drifts seemed to be partly due

to the ionospheric

1

disturbance

dynamo

INTRODUCTION

Coupling of the terrestrial magnetic field to the solar wind is of funda.mental importance in controlling magnetospheric dynamics a.nd energy flow. The solar wind, flowing wit,h velocity v in magnetic field B, possesses, in the rest frame of the Earth, an electric field E = -vxB; this is mapped down along field lines into the polar cap where the electric field is aligned roughly from dawn to dusk (Stern, 1973; Lyons and Williams, 1984; Heppner and Maynard, 1987). The dawn-dusk electric field is thought to be partly shielded from the inner magnetosphere by the geomagnetic ring current (Wolf, 1975; Mauk and Zanetti, 1987). The portion that penetrates, however, may be detected by the effects it produces on the pla.sma drifts. Solar windmagnet,osphere coupling, and therefore magnetospheric convection, is highly variable; furbhermore, the magnetosphere has the capacity to store energy and rele
There have been two general approaches to experimental study of the penetration of the dawndusk field to low L-shells, both of which are used in this paper. One approach sorts the available observations according to a geomagnet,ic disturbance index, such as K,, and compiles average diurnal variations of electric field for different levels of disturbance. This approach was taken by, for example, Carpenter ei al. (1979) and Wand and Evans (1981). The observed electric field is viewed as the superposition of the quiet time electric field pattern, and a disturbed time field pattern, both of which are fixed in a Sun-Earth reference frame. Using this approach, therefore, the data are conveniently considered in local time. This view is a simplification in that the ‘disturbed’ component will vary with time, reflecting changes in the st.rength of the disturbance. Also, Kp has a time resolution of 3 hours, and this is longer than the time scales of many of the relevant individual phenomena (ring current shielding, substorms, convection of field lines over polar cap).

J.M. SaxtonandA. J.Smith

1306

An alternative approach studies the electric field at higher time resolution on a case by case basis, and interprets it in the context of other parameters with similar temporal resolution. These have included the solar wind magnetic field (IMF), t,he aurora1 electrojet indices (AE, AU, AL), local magnetograms, and sometimes electric fields at different latitudes (in particular electric fields at high latitudes which allow one to assess the strength of the twin cell convection pattern). 2

EXPERIMENTAL

METHOD

The electric fields described in this work were deduced from data from the VLF Doppler experiment at Faraday, Antarctica (65‘S, 64’W; LT = UT-4 c MLT). This experiment is designed to observe ducted whistler mode signals from the VLF transmitters NAA and NSS in the northeast U.S.A. that have propagated along field lines near L = 2.5. The group delay and Doppler shift of the ducted signal are recorded; the radial plasma drift and ea.st-west

electric field may be deduced from these quantities using the method described in Saxton and Smith (1989). Saxton and Smith (19S9) a,lsodescribed the plasmaspheric electric field and coupling fluses between the plasmasphere and the ionosphere, deduced during a geomagnetically quiet period in July 1986; this reference will hereafter be referred to as Paper I. Recently Rippeth et al. (1991) have published a computer model of the plasmasphere which predicts quiet time electric fields and coupling fluxes simihar to those of Paper I, and provides confirmation of the validity of Saxton and Smith’s method for deriving them from the whistler mode group delays and Doppler shifts. The measures of cross-L drift and electric field reported here refer to the equatorial plane. The installation at Faraday has been described by Strangeways and Thomson (1986) and some initial results by Smith et al. (1987). Use of the technique to infer variations in plasmaspheric equatorial electron density, under quiet and magnetic storm conditions, has been described by Clilverd et al. (1991) and Smith and Clilverd (1991).

0 0 ,w "ST : nT . -100’

’

’

10

1

I

.,.I....!...

1

20

30

20

30

*ST

nT -100

1 FIG. 1.

I _,,.

K,

AND

I..

10

Dst INDICES

FOR JUNE AND

JULY

1986.

Disturbed

3 1.1

Data

plasmaspheric

electric

1307

fields

Et.4vs

RESULTS

LT

0.4

studied

Data from eight nights in June/July 1986 were studied. This season (austral winter) was chosen because the whistler mode signals are often observed throughout 24 hours, on account of the high geographic latitude and the long periods of darkness at Faraday. The first nights studied (July 25-26 and 26-27) were selected solely according to their Kp index; subsequently, some IMF values and AE indices became available, and nights were chosen to make use of these. The eight nights studied were: June 3-4, 22-23, 24-25, 26-27, 27-28, 29-30, July 25-26, 26-27. Fig. 1 shows the KP and Dst indices for this period.

(dlsturbed

I

minus

qulet)

,

I

1 I

6

0.2 E 0.0

2 3 w

-0.2 -0.4

1 0

I 6

I 12

1 18

24

LT FIG.

THE

3.

QUIET

TIME

(DISTURBED The

gaps

be more

DIFFERENCE ELECTRIC -

THE DISTURBED

AT L =

2.5,

IN THE

AND SENSE

QUIET).

in the curve than

BETWEEN

FIELDS

are due to the requirement

5 points

contributing

t,o

that

there

bot.h the disturbed

and quiet time averages.

E* vs

LT

Kp

z

3.2

2+

0.4,

I

I

0.2 E

$

a

0.0

3 LJJ-0.2 -0.4

I 6

0

I 18

I 12

24

LT

o.4/

7

E,,vs

LT

,Kp

00

,

to,2+

Average

diurnal

netically

active

drift pattern times

E,

b

0.0

=i

3 w -0.2 -0.41

! 6

0

! 12

\ 18

24

LT AVERAGE

~+CJ. 2. FIELD

ON THE

LOCAL (a)

son

EQUATORIAL

FIELD

LINE

ELECTRIC

AS A FUNCTION

OF

TIME.

Derived

when

WESTWARD

FARADAY

I$ (lip

> <

by 2+. 2+).

averaging (b)

The

all quiet

available time

results

reference

for

periods

for compari-

geomag-

This was constructed by selecting all data points for periods when KP > 2+, sorting them into hourly bins and averaging them. Data were selected mainly from the days listed above. In addition, the period used to define the quiet time electric field (Paper I) contained some intervals when I$ > 2+ (amounting to about 4% of t.he time), and these data were also utilised. The average diurnal variation in E, is shown in Fig. 2a.; the quiet time reference is shown for comparison in Fig. 2b. The difference between these two curves (in the sense disturbed minus quiet) is shown in Fig. 3. The smooth curves in Figs. 2a and 2b are le=ast squares fits of the three term Fourier synthesis

0.2 E

for

= a,-, + 5a.cos

27rrqt - t,) 24

.

n=l

The corresponding coefficients a.re tabulated in Tables la and lb respectively. The diurnal coverage of the data used to compile Figs. 2a and 2b was, unfortunately, nonuniform, mainly since the measurements were more abundant near certain times of day than others; this point is discussed further in Paper I. Therefore, the curve in Fig. 3 is incomplete. However, at the three times of day when we have enough data to calculate the ‘disturbed minus quiet’ curve, namely in the evening, near midnight and near noon, the quiet and disturbed

J.M. SaxtonandA. J. Smith

1308

TABLEla. FOURIERCOEFFICIENTS FORTHEDISTURBEDTIME LEAST-SQUARES

n

FIT.

%I

(mVm-‘)

t,

(h LT)

3-hour time resolution, these curves do not tell us how (or indeed if) the electric field relates to individual substorms. We will therefore examine the drifts on individual nights. 3.3

0 1 2 3

-0.053 0.057 0.124 0.035

TABLE lb. FOURIERCOEFFICIENTS LEAST-SQUARES FIT.

n

0 1 2 3

14.5 2.0 3.3

FORTHEQUIET-TIME

%

1,

(mVm-‘)

(h LT)

-0.025 0.099 0.079 0.026

17.7 5.1 6.0

_

curves are different. In the evening (18-22 LT) the plasma drifts outwards in disturbed times and inwards in quiet times, whilst for 00-01 LT active times are characterised by inward drifts and quiet times by outward drifts. The curves appear fairly similar later in the second half of the night, taking into account the greater scatter of points and the larger error bars. Both curves show outward drifts for 07-11 LT (morning, after both ends of the Faraday field line become sunlit). The two curves differ again near noon (lo-13 LT); however, the four points for the disturbed curve come from only one day (27th July), and since, even on quiet days E, at a given LT can vary by up to f0.1 mVm_’ from day to day, this difference is not, regarded as significant. At the other times where the curves differ, each is an average of several days’ results. Fig. 2a suggests the following hypothesis or ‘rule’:

‘In disturbed times on the Faraday field line, the plasma drifts outwards in the late afternoon and evening, and inwards later in the night.’ These graphs clearly indicate that the drifts are different for quiet (K, 5 2+) and disturbed (K, > 2+) times. However, since I$ has only a

D~iftftson individual nights

Presentation of results In Figs. 4-11, E, has been plotted on the same time axis as the Faraday (Argentine Islands) magnetograms, the AU and AL indices, the D,t index and hourly IMF B, and B, values. From top to bottom, the graphs show, where available, (i) Sunrise and sunset times at 300 km altitude at Faraday (lower bar) and the Faraday conjugate (upper bar); the white sections of the bars indicate times of daylight. (ii) E, deduced from the Faraday VLF Doppler data. The large symbols indicate hourly averages of E, deduced using the method described in Paper I. This method makes use of the rates of change of group and phase times and tries to deduce both E, (from the radial plasma drift) and the plasmasphereionosphere coupling flux ‘P, and is referred to hereafter as the ‘full’ method. The small symbols make use of a simplified method in which the flux is assumed to be zero and E,,, is deduced from the Doppler shift alone (allowance is made for the contribution to Af and t, from transionospheric propagation, as described in Paper I). Using this method, an estimate of the electric field may be obtained every 15 minutes for each transmitter. The Doppler shift alone may be used to give a reasonable indication of the electric field because of the following. If we look at the quiet time reference for fluxes and electric fields (Paper I), we see that a typical daytime filling corresponds to a maximum flux of 3x 1012 me2s-‘; whilst E, typically ranges up to 0.2 mVm_’ in magnitude. At L = 2.5, 3~10’~ rn-‘s-l and 0.2 mVm_’ each correspond to Doppler shifts of 100 mHz. The magnitude of the Doppler shift from @ is therefore comparable to that from E,. However, over a range of a few ~10’~ m-2s-1, the flux changes fairly slowly (except following sunrise); the electric field (and especially disturbed time electric fields) can change more rapidly. Therefore, we are justified in as-

1309

Disturbedplasmasphericelectricfields

form (paper chart) were digitised before plotting on the figures shown here, and in doing so the finest detail (e.g. micropulsations) was lost. This should not affect the visibility of substorms.

cribing changes in Doppler shift with time constants of only an hour or two and of reasonable magnitude (> 100 mHz) to E, rather than 0. One sees from the graphs that the electric fields deduced by the two methods generally agree fairly well. Where there is a discrepancy, it is generally an offset having a period greater than a few hours. (iii)

The magnetoFaraday magnetograms. grams, which were originally in analogue

1986

(iv) Provisional

AU and AL indices.

(v) The Dst index. (vi) Hourly values of IMF B, and B,, obtained from NSSDC (National Space Science Data Centre, Maryland, USA).

June

03-04

1.0

0.5 Ew

;

0.0 rl”/m

-0.5

-1.0

H

D

z

12

1s

150 nT 1l(Y ]50nT.

500 RU 0

nT

AL -500 -1000 0 Dst nT

BY nT

Bz

-20 -40 -60 10 5 0 -5 -10 10 5 0 -5 -10

UT ttLT

12 8 FIG. 4. DEDUCED WESTWARD ELECTRIC FIELD, FARADAY MAGNETOGRAMS, AU AND AL INDICES AND IMF DATA FOR 3-4 JUNE 1986.

12

J. M. Saxton andA. J. Smith

1310

These plots form Figs. 4-11. On all plots except June 03-04, the time axis runs from 18 UT on one day to 18 UT on the next. On June 03-04, the time axis runs from 12 UT to 12 UT. Description

The E, data during the substorm, full method, are more reliable.

from the

1986 June 22-23 (Fig.

5) One substorm is clearly apparent in the AU/AL indices; it is also visible in the Faraday- magnetogram _ D component. This substorm starts at 0130 UT, just before Faraday conjugate sunset, and appears to be associated with a positive excursion of E, (in the opposite sense to the ‘rule’ devised from average disturbed time drifts for this time of day).

of nights

1

1986 June 03-04 (Fig. 4) There is one isolated substorm near 21 UT, clearly visible on both Faraday H component and the AL index (AL corresponds to the morning electrojet). The substorm is not apparent in the E,,, results. The plasma drifts are-possiblyunusual before the substorm, but data were not available using the full method for E,.

1986 1’

v nv1

1986 June 24-25 (Fig. 6) This night is anomalous.

June

It appears

22-23

‘I:

D

2

1% nT

500 RU 0

nT FIL -500 -1000

0 tlst nT

-20 -40 -60 10 5

BY nT

0 -5 -10 10 5

Bz

0 -5 -to Ul nLT

FIG.

5.

As FIG.4,

BUT

FOR

22-23

JUNE 198G.

very quiet in the magne-

Disturbedplasmasphericelectricfields tometer records, yet has drifts more characteristic of a disturbed period. E, is negative near 23 UT and very large and negative near 03 UT. The drifts are more reliable near 00 UT, however (full method). D,, is very flat. It is most unlikely that the large negative E,,, is a product of our assumption that QI = 0, since the Doppler shift was negative at this time, and, if this were to be interpreted as a ff ux and not electric fkId, would imply a net ~~~~r~ flux (plasmasphere filling) of about 1~10’~ m-2s-1. Typic& daytime fluxes, when the plasmasphere is filling, areonly around 3 x 10r2 rn-%sql, and we

n

Rst nl

-20 -40 -60 10 5

h nT

0 -5 -10 10 5

8-r

0 -5

1311

would actually expect the pl~m~phere to be emptying near local midnight (Paper I>. 1986 June 28-27 (Fig. 7) Substorm activity on AL commenced sharply at 03 UT. & suggests it is associated with ring current injection. The substorm is associated with a negative excursion in E, lasting about 2 hours. Whilst E, would go negative near 03 UT in quiet times anyway, the short cluration of the negative excursion suggests we may be seeing a substorm signature in the ele&ric field. Unfort~ately, E, is only available from

J. M. Saxton and A. J. Smith

1312

the quick method (E, results from the full method are fragmentary and poor, showing much scatter). The strong negative excursion of E,,, near 08 UT can be dismissed as a sunrise effect.

The plasma drift was characteristically outward in the evening (prior to 00 LT = 04 UT). The E, data were good; the quick method gives high time resolution (15 minutes) and may be calibrated by the full method with hourly resolution.

1986 June 27-28 (Fig. 8) This was the most active night studied. D,, suggests ring cur-

E, changed briefly and dramatically to positive during 04 05 UT. This coincided precisely with a spike in AL and a disturbance in the ma.gnet,ograms.

rent injection in the evening, until 01 UT, and perhaps again near 04 UT. The latter might be associated with the well defined substorm visible on AL (morning electrojet). B, remained southward until about 07 UT.

With the exception of the spike nea.r 04 UT, excursions in E, are not well correlated with events in the magnetograms. In connection

1986

June

26-27

1.0

D

1

10’

2

] 50 nT

500 NJ nT

0

RL -500 -1000 0 List nT

-20 -40 -60 10 5

BY n-r

0 -5 -10 10 5

Bz

0 -5 -10 UT rlLT

,

0

18

14

b 2

20 FIG.

7.

As FIG. 4,

BUT

FOR

12 8

26-27 JUNE 1986.

Itl 14

with this, however, note that (i) the 04 UT feature is particularly large, in its effect on both AL and the plasma drift and (ii) partic&r enhancements in IS, f), a, AD or At are not particuiarly weft correlated with each other. 1988 June 29-30 (Fig. 9) Good E, data, using the full method, are available, in the evening (19-91 UT). On this night, the drifts were correlated with magnetic disturbances, and follow the ‘rule’. Elowe5rer,the Dst index suggests that ring current injection was occurring prior to the appearance of the substorm signature on the magne-

t 500 Rii

a

nf

-1uou

u ost

-2*

nT

-40 -66 10

5 BY

0 -5

tograms. There is some suggestion that the drifts are fnstest when the magnetic disturbance is greatest, near 23-89 UT, after which the outward drifts die away Tk second &XtO~M, nenr 0%04 UT, ma37 ako be associated with outwards drifts, aIthough the &vi&ion from quiet time drifts is not as marked in this case. X986; July 25-2(i (Fig. 10) Aumrd electrojet indices west! not a-&fable for July. Faraday magn~togr~s are avaifsble, however, and show significant substorrn activity on this night. The drifts seem to be outwa.rd prior to loclzr midnight (22-03 UT), in ac-

x3x4 cordance with the ‘rule’. Also, there is some indication of their changing to inwards (E, becoming more positive) around 04 UT ffocal ~dnjght). The negative E, point at 21 UT is slightly puzzling, since the magnetic disturbance has only just started. The ring current grew rapidly (14 nT per hour) from 22-24 UT; however, this time is excluded in the results from the full method. 1986 July 26-21 (Fig. 13) There U’eFt5 three periods of substorm activity on this night. About 2 hours of substorm activity near 00 UT is fairly well correlated, tempors& with a change to outward drifts, The data

are poor for the substorm near W-06 UT, and whilst the data might suggest inward drifts eorraspondmg to the substorm, they are hardly convincing. The substorm near 9-11 UT is associated with rapid inward drifts (E, positive). Both the rapid inward drifts and the substorm activity on the F’araday magnetograms are unusual at this time of day.

Some features of these nights are summa&red in Table 2. We see that a study of data on individual nights provides some support for the Lruke),espe-

-

FIG. 9. As Fro. 4, BUT FOR 29-30 JUNE 1986.

1315

Disturbedplasmasphericelectric fields

magnetic activity has been reported by a number of authors. We will first consider the average electric field for ‘disturbed times’; that is, for times selected according to whether I$ or AE is above a certain threshold. Of particular interest to us are the published results from Siple (L = 4.2), Millstone Hill (L = 3.2) and Saint Santin (L = 1.8). The reader should be aware, however, of the limitations of this approach discussed in Section 1. Fig. 12 shows the results of Carpenter et al. (1979) from Siple (SI), Wand and Evans (1981) from Millstone Hill (MH) and the Faraday results (FA). Note that ‘disturbed’ conditions are

cially prior to local midnight. However, during active times, we cannot extend the rule to specific enhancements in disturbance on the magnetograms. We should remember that even during quiet times the electric field at a given local time may vary from day to day, and some of the ‘scatter’ in Table 2 may reflect this, rather than variations in the level of magnetic disturbance. 4 4.1

Average

DISCUSSION

disturbed time

dTiftS

The variation of the plasmaspheric electric field depending on the presence or otherwise of geo1986

Julu

25-26

1.0 0.5 Eu 0.0 n”/m

-0.5 -1.0

IS

6

0

12

18

H

D

z

RU

-r

150 nT

500 0

nT RL

-500 -1000

J

0 nst

nT

-20

I

-40 -60 10 5

.

3Y

0 . . L , -5 flT -10 IO 5 32 0 -5 L,,,,,,,,,,,,,,,,,,,,,,,I -to UT (8 flLT 14

FIG. 10. DAY

. 1

0 20

DEDUCED

MAGNETOGRAMS,

1886. No AU

6 2

WESTWARD AND

IMF

12 a

ELECTRIC FIELD, FARADATA FOR 25-26 JULY

or AL indices were available for this night.

18 14

1316

J. M. SaxtonandA. J. Smith

defined differently for each data set; details are given in Table 3. The curve for Siple is a nine term harmonic fit to the data. A number of features are common to more than one curve. In the evening, the electric field is negative (outwards drift) at all stations. The field reverses after local midnight, giving inward drifts, at both Faraday and Siple. These inward drifts appear to persist longer at Siple than at Faraday. There is less agreement between the three curves in the daytime, especially shortly after local noon. Where the published res&s referred to the ionosphere, they have been mapped to the equator assuming field lines to be equipotentials.

A number of other observations of plasmaspheric electric fields have been published. Blanc (1983) described his observations of equinox drifts at Saint Santin (L = 1.8). He sorted the available observations according to IC, and compared those for I& > 2+ with those for I& 5 2+. In contrast to the results from Faraday, no clear effect was found on the east-west field (radial plasma drift), although there was a marked effeet on the radial electric field (azimuthal drift), the drift being consistently westwards throughout the day. There was, however, a large scatter in the east-west electric field, of magnitude 2050 ms-‘; mapped into the equatorial plane this corresponds to about 0.4-0.8 mVm-‘.

1.0 0.5 Ew 0.0

n”/n

s

-0. -1.0

H

D

I1 10'

2

AU

500 0

I-IT RL

-500 -1000 0

ost nl

-20 -40 -60 IO 5

BY

0 -5

nT

-10 10 s

BZ

0 -5 -10 UT llLT

18

0

14

20

FIG.

11. As FIG. 10.

12 a

6 2

BUT

FOR 26-27

JULY

1986.

l8 14

Disturbed plasmaspheric TABLE 2. SUMMARY

Night (1986)

June 03-04 22-23 24-25 26-27 27-28 29-30 July 25-26 26-27

OF PLASMA DRIFTS ON GEOMAGNETICALLY

‘Rule’ obeyed before 00 LT after 00 LT

_

NO NO NO maybe YES

not clear NO not clear partly

YES YES YES

1317

electric fields ACTIVE NIGHTS.

Drift anomalies correlated Faraday H AU

YES NO

NO YES magnetometer NO NO

not clear

YES

YES

maybe YES

NO YES

_ _

The results of Andrews (1980) from the VLF Doppler experirnent in New Zeala.nd (L = 2.3), cont,ain one observation for KP = 4- in the evening; namely a radial drift of 133 ms-’ inferred from 0940-1130 UT (= 2140-2230 LT) on 21 January 1976. This outwards drift is in agreement with the Faraday results, at a similar L-shell. Maynard et al. (1983) have described their observations of plasmaspheric electric fields from L = 2-6 with the ISEEl satellite. They have compiled their data to yield average drifts at difat L 21 2--3, ferent levels of KP. Unfortunately, the data coverage is rather incomplete and the measurements at these L-shells suffer larger experimental uncertainty due to the motion of the satellite. However, their results do show indications of outward drifts pre-midnight and inward drifts post-midnight. There is t,hus general agreement between our average diurnal variation in E,,, and that, deduced by previous workers, with the exception of the electric field observed at Saint Santin. The Srlint Santin data set also has the lowest L-shell of those considered here. There have been several recent theoretical studies of the low L-shell, disturbed time, electric field. These fall into two categories, each looking at a different aspect of the field. Hare1 et al. (1981) and Senior and Blanc (1984) considered the penetration of the dawn-dusk electric field, taking into account ring current shielding and ionospheric conductivities (we will refer to their models as the ‘Rice’ and ‘S-B’ models respectively). In both models, the main input is

NO YES

with AL

NO YES very quiet YES NO (except spike) YES _

the polar cap potential drop. Guiding centre motions were used to calculate current divergence at the inner edge of the ring current,, which in turn gave the region-2 Birkeland currents. Blanc and Richmond (1980) considered the ionospheric disturbance dynamo, ca.used by winds created by Joule heating in the aurora1 zone, and calculated the result.ing ionospheric electric fields. We will compare our results to the polar cap electric field penetration models first. The Rice model was applied to a ‘substorm type event’ observed on 19 September 1976; the avera.ge azimuthal electric field at L = 4 over the duration of this event was calculated and compared with the results of Carpenter et al. (1979) (Spiro et al., 1981). Senior and Blanc computed the response of the inner magnetosphere to a step increase in the polar cap potential drop of 50 kV, and compared the initial and steady state response to observations at Millstone Hill (Wand and Evans, 1981). The Rice model results agreed well with those of Carpenter (with both the model and observed fields being eastward for 18-23 LT and westward for 23-08 LT); the S-B results (especially the final state azimuthal component) showed general agreement with the Millstone Hill results. Further, both sets of model predictions are roughly consistent between themselves and with our observations. Since periods of geomagnetic activity are associated with enhanced polar cap potential drop (Kivelson, 1976), the simplest interpretation of our data is to say that the additional electric field seen in disturbed times (Fig. 3) is the signature of the dawn-dusk electric field at L = 2.5, and that the degree of shielding

J. M. Saxton and A. J. Smith

1318

ST

MH

FFI

18

6

0

24

:: FIG. 12. COMPARISON OF E, versus LT AT DIFFERENT OBSERVATORIES. The average ‘disturbed time’ westward electric field at L = 2.5 deduced in this work (FA, as Fig. 2a), compared with results from Millstone Hill (MH, from Wand and Evans (1981)) and Siple (SI, from Carpenter et al., 1979)) See Table 2 for further details of stations and techniques, and the text for discussion.

approximately constant (i.e. the fraction of the dawn-dusk field that penetrates to L = 2.5 is approximately constant with time}. Encouraging though this is, it is essential to realise that Fig. 3 may be contaminated with the effects of the ionospheric disturbance dynamo. In their modelling study of this effect, Blanc and Richmond (1980) considered the winds generated due to two levels of Joule heating in the auroral zone, one (case 1) corresponding to a severe storm, and the other (case 2) to quiet or very mildly disturbed conditions. They considered both an axially symmetric wind system, and remains

the effects of constraining the winds to the day or nightside, and calculated the ionospheric electrosta.tic field at L = 1.8. They found that the north-south component, of the ionospheric electrostatic field was generally larger than the azimuthal component. (For both the wind system and ionospheric conductivity zonally symmetric, the electrostatic potential would be independent of LT and thus the azimuthal electric field zero; in practice, we expect the conductivities at least to be LT dependent, due to day-night asymmctry). Blanc and Richmond’s case 1 simulation, with both globally symmetric and dayside only winds, produced a eastward (ionospheric) field of 2 mVm_’ during the night and a westward field of similar magnitude during the day. Nightside winds produced much smaller fields. Their case 2 simulation produced fields an order of magnitude smaller. If we assume that the fields are roughly constant in magnitude from 45- 50” latitudr in the ionosphere, then the Blanc-Richmond results suggest that the ionospheric disturbance dynamo is capable of producing a field of N 0.5 mVm_’ in the equatorial plane near L = 2.5 in severely disturbed times (those corresponding to BlancRichmond case 1). In conditions with less Joule heating (as is probably appropriate for the data being studied here), the electric field would be smaller, and possibly insignificant. However, the azimuthal field shown in Fig. 3 changes sign near local midnight, which the field predicted by the disturbance dynamo model does not. This strongly suggests that the ionospheric disturbance dynamo is not a major contributor to the azimuthal electric field at Faraday in the data being studied here. (However, the BlancRichmond model predicted larger radial than az-

TABLE 3. SOURCES OF ELECTRIC FIELD DATA PRESENTED

IN FIG. 12.

Code

Station

L

Technique

‘disturbed' definedby

Reference

SI

Siple

4.3

whistlers

AE > 200 nT (34 days, 1963-75)

Carpenter et al. 1979

MH

Millstone Hill

3.2

incoherent scatter radar

I<, 2 30 (May 1976-Nov 1977)

Wand and Evans (1981)

FA

Faraday

2.5

whistler mode signals * 18 days, June/July 1986

I$

this work

2 3-

1319

Disturbedplasmasphericelectricfields imuthal fields, so that disturbance dynamo effects might have been noticeable had we been able to study the radial component of the electric field). 4.2

Electric

field on individual

days

Much variety in behaviour is evident when the data are examined on a case by case basis. However, if we regard the electric field in Fig. 3 as the signature of the dawn-dusk electric field at L = 2.5, then it is possible to discern some order in our observations. We note in particular: 1. Some isolated well-defined substorms occur simultaneously with enhancements in the dawn-dusk field signature at L = 2.5. Examples are June 23 01 UT, June 28 04 UT and July 27 10 UT. 2. On other occasions E, agrees in sign with the disturbed time field shown in Fig. 2a and the magnetograms or AU/AL are generally disturbed but there is no clear correlation between enhancements in the disturbances in the electric field and in the magnetograms. Examples are June 27-28 1903 UT, June 29 19-24 UT and July 25-26 21-01 UT. Most of these periods are associated with ring current growth (& becoming more negative). In some cases it is not easy to distinguish between a general enhancement in AU or AL (or disturbance on the magnetograms), possibly indicative of increased high latitude convection, and a substorm or succession of substorms. Some substorms (e.g. July 27 at 10 UT) are themselves associated with ring current growth. The above two observations are characteristic of most of the data. An exception occurs on June 03 at 21 UT when a substorm is not associated with any noticeable disturbance in the electric field. Also, the night of June 24-25 is unusual, as mentioned previously. A possible cause of the appearance of the dawn-dusk electric field during substorms is an increase in the aurora1 zone Pedersen conductivity due to precipitation. It is this conductivity that limits the DC shielding effect of the ring current. A sudden increase in this conductivity would discharge some of the polarisation charges produced by the ring current (which are responsible for cancelling the dawn-dusk electric field in the inner magnetosphere). A sudden increase

of electric fields is seen at substorm onset in the Rice model; it is attributed to a sudden enhancement in auroral zone conductivity (Spiro et al., 1981) and is particularly marked in the night. This would seem to be the most natural explanation for excursions in E, such as June 28 at 04 UT and July 27 at 10 UT, in view of their excellent temporal correlation with substorms registered on the magnetograms. The unusual drifts on June 24-25 might possibly be explained by the ionospheric disturbance dynamo. The azimuthal electric field from about 23-04 UT (19-24 LT) differs from the average quiet .day reference field (Fig. 2b) by an eastwards offset of about 0.2-0.3 mVm-‘. This is in the correct sense for the disturbance dynamo, and the earlier part of June 24 was fairly active, with ICp reaching 3+. The dawn-dusk field is also observed at Faraday during times of ring current growth and/or increased AU/AL indices. This is harder to understand. The increased electrojet (Hall) currents would suggest increased convection and therefore an increased polar cap potential drop. The ring current growth also suggests sunward motion of hot plasma due to increased magnetospheric convection. It is therefore tempting to suggest the dawn-dusk field is observed at Faraday under such conditions simply because the polar cap potential drop is increased. It is not immediately obvious, however, to what extent variations in ring current shielding are important. Ring current growth implies the simple S-B model is not applicable. Dawn-dusk electric field penetration does occur in the Rice simulation (which started with no pre-existing ring current, and included injection of hot plasma from the plasma sheet). _

5

CONCLUSIONS

1. When the availa,ble measures of E, are sorted according to Ii,, and average E,-LT curves compiled, the nighttime electric field is seen to be clearly different for geomagnetically quiet and disturbed conditions. The average disturbed time field, relative to the quiet time reference, (I$ < 2+) is more eastwards prior to local midnight and more westward after local midnight. The most straightforward explanation of this is that during disturbed times, the magnitude of the dawn-dusk electric field increases,

J. M. Saxton and A. J. Smith

1320

whilst the degree of shielding remains approximately constant. The ionospheric disturbance dynamo does not appear to make a significant contribution to the average electric field in the data studied here. However, this approach does not tell us how the electric field depends on triggered energy flow (substorms), or direct solar wind magnetosphere coupling (polar cap potential drop). Some discrete substorms are associated with an enhanced signature of the dawn-dusk field. This is attributed to increased penetration of the dawn-dusk field due to increased auroral zone Pedersen conductivity. The signature of the dawn-dusk field is also increased at Faraday at other times, characterised by increased aur0ra.l electrojet activity and ring current growth. Whilst it is tempting to attribute this simply to DC penetration of an increased polar cap electric field, the extent to which possible variability of ring current shielding is important is not clear. On one magnetically very quiet evening (June 24-25) unusual drifts were observed. These followed a magnetically disturbed day and the azimuthal electric field is consistent with a. combination of the normal quiet time field and the ionospheric disturbance dynamo electric field as described by Blanc and Richmond (1980).

. .

Aclcnowledgements-We itre grateful to Roy Moffett, Alan Rodger, Michael Rycroft and Michel Blanc for helpful discussions. Alan Rodger also supplied t,he Faraday magnetograms. The Faraday VLF Doppler receivers were built by Peter Hughes, Neil Thomson and Chris Howe, and Mark Clilverd operated the experiment al, Faraday in 1986/7. This work was performed whilst one of us (JMS) was a postgraduate student in the University of Sheffield, and he acknowledges support from the Science and Engineering Research Council in t,he form of a pa&graduate studentship.

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Blanc, M. (1983) Magnetospheric convection efrects at. mid-latitudes 1. Saint-Santin observations. J. geophys. Res. 88, 211. Blanc, M. and Richmond, A. D (198U) The ionospheric disturbance dynamo. J. geophys. Res. 85, 1669. Carpenter, D. L., Park, C. G. and Miller, T. R. (1979) A model of substorm electric fields in the plasmasphere based on whistler data. J. geophys. Res. 84, 6559. Clilverd, M. A., Smith, A. J. and Thomson, N. R. (1991) The annual variation in quiet time plasmaspheric electron density, determined from whistler mode group delays. Planet. Space Sci. 39, 1059. Harel, M., Wolf, R. A., Reiff, P. II., Spiro, R. W., Burke, W. J.. Rich, F. J. and Smiddy, M. (1981) Quantitative simulation of a magnetospheric substorm 1. Model logic and overview. J. geophys. Res. 86, 2217. Kivelson, M. G. (1976) Magnetospheric electric fields and their variation with geomagnetic activity. Rev. Geophys. Space Phys. 14, 189. Lyons, L. R. and Williams, D. J. (1984) Quantitative aspects of magnetospheric physics, D. Reidel Publishing Company, Dordrecht/Boston/Lancaster. Mauk, B.H. and Zanetti, L.J. (1987) Magnetospheric electric fields and current,s. Reviews of geophysics 25, 541. Maynard, N. C., Aggson, T. L. and Heppner, J. P. (1983) The plasmaspheric electric field as measured by ISEEl. J. geophys. Res. 88, 3981. Rippeth, Y., Moffett, R. J. and Bailey, G. J. (1991) Model plasmasphere calculations for L-values near 2.5 at the longitude of Argentine Islands, Antarctica. J. afmos. ferr. Phys. (in press). Saxton, J. M. and Smith, A. 3. (1989) Plasmaspheric electric fields and plasmasphere-ionosphere coupling fluxes at L = 2.5. Planet. Space Sci. 37, 283 (Paper I). Senior, C. and Blanc, M. (1984) On the control of magnetospheric convection by the spatial distribution of ioncr spheric conductivities. J. geophys. Res. 89, 261. Smith, A. J., Yearby, K. H., Bullough, K., Saxton, J. M., Strangeways, H. J. and Thomson, N. R. (1987) Whistler mode signals from VLF transmitters, observed at Faraday, Antarctica. Memoirs of National Instilwtc of Polar Research Special Issue 48, 183. Smith, A. J. and Clilverd M. A. (1991) Magnetic storm effects on the mid-latitude plasmasphere. Planet. Space Sci. 39, 1069. Spiro, R. W., Harel, M., Wolf, R. A. and Reiff, P. H. (1981) Quantitative simulation of a magnetospheric substorm 3. Plasmaspheric electric fields and evolution of the plasmapause. J. geophys. Res. 86, 2261. Stern, D. P. (1973) A st.udy of the electric field in an open magnetosphere model. J. geophys. Res. 78, 7292. Strangeways, H. J. and Thomson, N. R. (1986) 24 kHz MSK Doppler receiver at Faraday, Antarctica. IERE Publ. 68, 41. Wand, R. H. and Evans, J. V. (1981) Seasonal and magnetic activity variations of ionospheric electric fields over Millstone Hill. J. geophys. Res. 86, 103. Wolf, R. A. (1975) Ionosphere-magnetosphere coupling. Space Sci. Rev. 17, 537.