Two spacecraft observation of plasma sheet ... - Wiley Online Library

GEOPHYSICAL RESEARCH LETTERS, VOL. 26, NO.2, PAGES 177-180, JANUARY 15, 1999

Two spacecraft observation of plasma sheet convection jet during continuous external driving Yu. I. Yermolaev;V. A. Sergeev,L. M. Zelenyi, A. A. Petrukovich, J.-A. Sauvaud, a T. Mukai,4 S. Kokubun Abstract.

During -•18 hours of continuousactivity caused

by long interval of southwardIMF, the INTERBALL/Tail Probe and GEOTAIL spacecraftmonitored the plasma sheet in dusk and dawn sectorsof the midtail plasma sheet at 2530 RE. While bursty bulk flows were persistently observed at both spacecraft,there was a systematic differencebetween the hourly averagesof the Earthward flux transport. Dur-

the measurements at comparabledistances(r -• 25-30 RE) in dawn and dusk parts of the plasma sheet with a crosstail separation •,20 RE, thus allowing to study a large scale behaviour

and cross-tail

structure

of convection.

Observations

ing a 3-hourintervalthe averageflux transportrate (Ey) on On Decernber 22,1996 the plasma sheet parameters was GEOTAIL (dawn plasma sheet) was •,3 times larger than monitoredby two spacecraft,INTERBALL/Tail Probe and both the averageEv on INTERBALL (duskplasmasheet), GEOTAIL (Figure 1). Most of the time both spacecraft and the cross-tailEv0 expected for the existing IMF conditions. This implies that the most intense bursty bulk flows tend to be confined within a limited longitudinal sector of

stayed near the neutral sheet at distances 20-30 RE. GEOTAIL was in the dawn plasma sheet, its GSM coordinates at

substormonsetswere [-28.5,-10.8,-1.5]RE at 13 UT (footpoint at •-01.9 h MLT) and [-26.3,-13.8,-6.0] at 22 UT age convectionjet feature. (•,03.7 h MLT). For INTERBALL the coordinateswere [23.4,12.2,-2.4](footpointat •,21.6 h MLT) and [-20.2,•0.9,Introduction 2.0] (-•21.9 h MLT), respectively.The verticallinesmarked 1, 2, 3 correspondto the substormonsets(at 1251, •,1625 The traditional picture of laminar Earthward convecand 2200 UT) inferredfrom groundobservations of magnetic tion in the plasma sheet under the homogeneouslarge-scale variations(seethe top panelof Figure 2) and Pi2 pulsations.

the magnetotail(X < -30 RE) formingthe observedaver-

dawn-duskelectric field has been challengedby recent obser-

vations.It appeared(e.g. Angelopoulos, 1992;Baumjohann, 1993)that the main transportof mass,energyand magnetic flux in the midtail plasma sheet is realized via short lived

high-speed plasmaflows(burstybulk flows,BBFs). The origin and spatial/temporalcharacteristicsof these transient features in the plasma sheet are not yet understood. An important, yet unknown property of convection features is theft cross-tail

extent at different

time scales. To this

end simultaneousmeasurementswith at least two spacecraft

During the most interesting time period between substorms 1 and 3 the INTERBALL

and GEOTAIL

were at similar

ra-

dial distances(•25 and -,•30RE) half way from the tail axis to the flank but at the dusk and dawn sides,respectively.

From GEOTAIL we used the magnetic field (MGF, [Kokubunet. al., 1994]) and plasma (LEP, [Mukai et al., 1994]) data at 12 s resolution. From INTERBALL/Tail Probe we used magnetic field and plasma observations. The moments

of distribution

functions

from the CORALL

ion in-

strumentcovering0.05-25 keV energyrange (Yermolaevet are needed.At the small (•01 min) time scale,BBFs have al., 1997)wereavailableat the spacecraftspinresolution;we a smallsize (lessthan -•1/10 of tail diameter,e.g. Sergeev use only the Vz flow component(along the spacecraftspin et al., 1996, Angelopoulos et al., 1997). However,there are axes)which, on the one hand, is the rnostimportant charindications( Sergeevand Lennartsson,1988, Nishida et al. acteristics of magnetotail convection and, on the one hand, 1995) that eventhe time-averaged(•01 h) convectionis also inhomogeneous. In this paper we present a rare casein which simultaneous

measurements from the INTERBALL/Tail Probe and the GEOTAIL spacecraftwere available in the mid-tail plasma sheet for many hours under conditionsof prolonged southward IMF. A unique feature is that two spacecraft provided

is lessaffected by the time variations during the 2 min spin period of the spacecraft. The density and temperature of plasma sheet electrons was derived from the ELECTRON

instrument($auvaudet al., 1997). The observed variability of the plasma flows is due to both the temporal variations and spatial effectsas the spacecraft crossesdifferent parts of the flapping plasma sheet or even exits to the lobes. A favourable situation for probing

1SpaceResearchInstitute, RussianAcademy of Sciences, the inner part of the plasma sheet (IPS, here, defined by Moscow_,Russia

2Institute of Physics,St.PeterburgState Univesity, St.Petersburg, Russia

the conditionthat the plasma/• parametershouldbe >0.5) occuredafter 1330 UT (after the plasmasheetexpansionassociatedwith the first substorm)whenINTERBALL stayed

SCentred'EtudeSpatiMedesRayonnements, Toulouse,France in the IPS for more that 8 hours, as indicated by low values 4ISAS,Sagamihara, Japan of the Bx magneticcomponentin the Figure i (left). GEO5STEL, NagoyaUniversity,Japan

Papernumber1998GL900118.

TAIL also spent considerable time near the neutral sheet, but the Bx behaviour was more variable. Therefore, to avoid spatial effects and also to facilitate data comparison of both satellites we used 2 min data from GEOTAIL and we plot-

0094-8276/99/1998GL900118505.00

ted in Figure 1 (right) only those2 min averagedparameter

Copyright 1999bytheAmerican Geophysical Union.

177

178

YERMOLAEV ET AL.' CONVECTION JET IN THE PLASMA SHEET

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Figure 1. Surveyof plasmasheetparameters measured in the duskplasmasheet(INTERBALL, left panels)and in the dawnplasmasheet(GEOTAIL, right panels).The tail lobemagneticpressure at X -- -30Rzc (P30)computedfor given solarwind conditions(accordingto Fairfieldand Jones•1996)is shownfor referenceon the bottompanels.The onsetsoF three substorms are shownby verticallines1, 2 and 3. Note that only data in the inner plasmasheet(under,.•>0.5) •re included

for GEOTAIL.

valuesat GEOTAIL which were measuredin the IPS. (It is necessaryto note that we used the plasma density from the ELECTRON instrument and ion temperature from the CORALL instrument to compute the total pressureon IN-

level at the bottom plots in Figure 1. During this time interval the strongest flows were observedat both spacecraft, but they were higher at the GEOTAIL. These features are potentially very important and they will be discussedbellow.

TERBALL.) Variationsof the total (plasmaplus magnetic)pressure in the tail (Figure 1, bottom panels)showa systematicsub-

show some similarities

storm responsesimilar at both spacecraft. In each substorm

plasma density value after the first substorm was about 0.1

The plasmas measured by GEOTAIL and INTERBALL as well

as some differences.

The

were someeventone can recognizethe increaseof total pressure(load- cm-3 on both spacecraft.The temperatures ing) preceedingthe onset. At the sametime one couldalso what low for the disturbed conditions,the proton tempersee that during more than a half of the time interval between the substorm onsetsthere were no rapid changesof total pressure. At these times the plasma convectionwas also strong and variable. A remarkable feature is the low total pressurebetween 14 and 19 UT, which is smaller than the averagelobe magneticfield pressure(Pso)computedaf-

ature changed from •2 keV to •4 kev during the studied time interval on GEOTAIL where the strongestflows were observed. On INTERBALL the proton temperature was somewhat higher, changing between •3 and •8 keV. This differencecould be prescribedeither to the azimuthal gra-

dient (drift due to the ion accelerationin the electric field ter Fairfield and Jones,(1996) and shownas the reference from dawn to dusk) or enhancedlocal betatron acceleration

YERMOLAEV ET AL.' CONVECTION JET IN THE PLASMA SHEET

December

22

, 1996

the lobemagneticfield (and tail pressure)with the external driving (slowaverageincrease)as well as with the internal substormeffects(temporaldropouts).

400

200

179

.

"• -200 x

Our view of this interval as controlled by continuous .strong external driving is supported both by the observa-

"" -400

tions of the gradual developmentof the ring current (with

c

0

-600

maximal Dst depression •,-40 nT reached at •-23 UT, not

-800

shownhere) and considerableelectrojet activity in the auroral zone(Figure 2).

9O

• 60

Convection jet feature in the plasma sheet.

"

Cross-To•lPotentlo

The bottom panel of Figure 2 presents the half-hour av-

0

eragesVx x Bz in the inner part of the plasma sheet (the averageswere plotted only if the measurements in the IPS occupied at least 10 minutes of entire 30 min averaging time

interval). These values give a good proxy of the Earthward magneticflux transport [V x B]u at the GEOTAIL position. In steady state, the total average Earthward flux

transport (E u integratedacrossthe tail) shouldcorrespond to the cross-tailpotential drop (I), and for (I) - 80 kV (computed after Boyle et al., 1997,seea middlepanelin Figure 2) and tail radius25 Rz (computedusingthe $hue et al., 1997 formula)the expectedaverageflux transport rate shouldbe

UT

Figure

E0•-0.25mV/m. TheINTERBALL-based estimate ofE•T

2. Ground, solar wind and plasma sheet activity

on December 22, 1996. From top to the bottom: - superimposed magnetogramsof 11 ground stations used in derivation of standard AE index; - time variations of the expected cross-tail potential differencecomputed from the solar wind

gives an average Vx x Bz value of the same order as E0.

At GEOTAIL, during three hours (14 to 17 UT) the flux

transport rateE• T was-•3 timeslargerthanonINTERBALL. Note (Figure 1) that most of this time the total tail

pressure at both spacecraft was nearly steady and it was considerably reduced as compared to the average value of parametersaccordingto Boyle et al., 1997, (shifted in time the tail lobe pressure. The observation of stronger convecto X = -30Rr distance);- comparisonof 30 rain average tion in the dawn plasma sheet is consistentwith the auroral valuesof the Earthwardmagneticflux transport (V• x Bz) in the inner plasmasheet(under fl > 0.5) as measuredon zone magnetometer data. Between 13 and 17 UT the deepINTERBALL and GEOTAIL spacecraft. For reference we est negative magnetic bay which forms a lower envelope in

plot in Figure2 (.top)camefrom Barrow alsoshowthe amplitudeof expected(homogeneous) cross- the magnetogram station (•, 69 ø CGLat, magneticmidnight at •,12.5 UT). tail electric field with the cross-tail potential 80 kV and tail diameter

This confirms that the strongest auroral electrojet currents occuredin the high-latitude portion of postmidnight auroral

50

oval.

becausethe electron temperature on INTERBALL is changing in parallel with the proton one favouring the latter ex-planation. The flows in the IPS region were exclusively in the Earthward direction at both spacecraftwhich is true for high speedflows observedduring the loading and unloading

Our results give a direct evidence of strong long-term inhomogencity of the averaged plasma sheet convection, and that the convectionjet occupied the dawn half of plasma sheet. The upper estimate of the possiblecross-tail width of the convectionjet in the plasma sheet Ay might be found from right cross-tail potential drop E0 and experimental val-

phases(seee.g. GEOTAIL data for the substorm2), aswell

uesE• •": 3 x E• : 3 x E0asAy ,• (E0/EycT) xDT:

as during the episodesof steady total pressure. Concluding the observationsectionwe would like to comment briefly on the solar wind and magnetospheric condi-

1/3 x 50Rz •- 15Rz. It should be noted that the differenceof the averageV• x Bz values on GEOTAIL

tions

a time and, after 17 UT (followingthe secondsubstorm),

which

will

be studied

in more

detail

elsewhere.

Ac-

IT

and INTERBALL

decreased with

cordingto the measurements of the WIND spacecraft(posi- both observed values became comparable to the expected tioned at X •, 80Rz and Y