Aug 1, 1999 - substorms, ended quite differently for these two substorms: the pressure decreased until it approached the quiet time level in the course of the ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A8, PAGES 17,299-17,310, AUGUST 1, 1999
Responseof the midtail electric field to enhanced solar wind energy input R. Nakamura, 1,aL. F.Bargatze, a,3T. Mukai,4 T. Nagai, • K. B. Baker, 6 M. R. Hairston, 7 P.H. Reiff,8 A. A. Petrukovich, 9 M. Nozdrachev, 9 and O. A. Troshichev lø
Abstract. We studytheresponse of midtailplasmaandfieldparameters to enhanced solar wind electricfield input for two substormintervalson November22, 1995. The solar
windinputsignatures werequitedifferentfor thesetwo substorms, whichhadmajorPi2 onsetsat 1108 and 1502 UT The solarwind inputfor the 1108 UT substormhad a short timescale(•0.5 hour)electricfieldenhancement up to 1.5 mV/m, whereasthe solarwind electricfieldfor the 1502UT substorm continuously exceeded1.5 mV/m for •2 hours.In association with the expansion phaseonsetsof bothsubstorms, the electricfield fluctuation in the midtailplasmasheetcommenced.The electricfield disturbances lastedfor a time intervalcloseto the lengthof time, in whichthe solarwind electricfield wasenhanced.
Themidtailplasmasheetelectricfieldresponded well to theenhanced solarwindinput with a time delayof 45-80 min. The pressure decrease, whichstartedat the onsetof both substorms, endedquitedifferentlyfor thesetwo substorms: thepressure decreased until it approachedthe quiet time level in the courseof the 1108 UT substorm,whereasthe pressuredroppedbelowthe quiettime level duringthe expansionphaseof the 1502 UT
substorm andstayedat thislow leveluntillatein therecoveryphase.Usingpolarcap potentialdropandthepressure profilein themidtail,we estimatetherelationship between thedaysideandnightsidepotentialdrop.We suggest thatthe differencein thenightside flux transport ratecouldcontroltheconfiguration of the midtailandcouldexplainwhy thetail pressure responded differentlyduringtheexpansion andrecoveryphaseof thetwo substorms.
1. Introduction
gatzeet al., 1985] haveshownthat substormactivityresults of activitydrivendirectlyby the soBy comparingsolarwind parameters andauroralelectro- from the superposition lar wind and activity driven by the release(or unloading) jet indices,a numberof studies[e.g.,Bakeret al., 1983;Bar-
of energystored(or loaded)in the magnetotail.Evidence supporting the storageandreleaseof energyassociated with 1Max-Planck-Institut far extraterrestrische Physik,Garching, substorms was also obtained in the distant tail region based Germany. 2Solar-Terrestrial Environment Laboratory, Nagoya University,on systematicchangein thediameterof thetail andplasmoid observations[Baker et al., 1984].
Toyokawa,Japan.
3No.wat Department of Physics, Montana StateUniversity, In the midtail regiona statisticalstudyshowedthat the Bozeman. lobemagneticfield energydensitywasenhanced in associa4Institute of SpaceandAstronautical Science, Sagamihara,tionwith a southward turning of the IMF prior to a substorm Japan. onsetandthatthelobefieldenergystartedto recoverrapidly 5Earth andPlanetary Sciences, TokyoInstitute of Technology, at the expansiononset[Caan et al., 1973]. This analysis, Tokyo,Japan. however, wasperformedfor substormcaseswith the north6Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland. wardturningof theIMF Bz component andcouldtherefore 7Center forSpace Science, University ofTexasDallas,Richard- presentcharacteristics of only a subsetof substorms.In fact, son.
moredetailedeventanalysesshowedthatthe dissipationof 8Department of Space Physics andAstronomy, RiceUniversity, the accumulatedenergyobservedin the tail is not always
Houston, Texas.
9SpaceResearch Institute,RussianAcademyof Sciences, Moscow, Russia.
•øGeophysical Department, ArcticandAntarctic Institute, St. Petersburg,Russia.
Copyright1999by theAmericanGeophysical Union. Papernumber 1999JA900166. 0148-0227/99/1999JA900166509.00
the same [Fairfield et al., 1981]. That is, at sometimes the accumulatedenergyis rapidly dissipatedduring substorm, whereasat othertimesthe dissipationcan occurmore gradually during ongoingmagneticactivity. During the latter cases,the energysuppliedby the solarwind may even exceed that being dissipated,thus causingthe tail energy to increase.The factorcontrollingthesevariousresponsesof the tail is yet to be determined.
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NAKAMURA ET AL.' MIDTAIL RESPONSETO ENHANCED IMF INPUT
A numberof studieshave shownthat the high-speedflow burstscan carry most of the plasmatransportin the midtail region [e.g., Angelopouloset al., 1992; Baumjohann, 1993; Sergeevet al., 1996a]. The plasmasheetconvection is strongly inhomogeneousacrossthe tail [Nishida et al., 1995; Sergeevet al., 1996b]. Even duringperiodsof continuousexternaldriving (long intervalof southwardIMF), long-terminhomogeneityof the averageplasmasheetconvectionexists,and the mostintenseburstybulk flowstendto be confinedwithin a limited longitudinalsectorof the magnetotail (with scale size of convectionjet about 10-20 [Sergeevet al., 1996b; Yermolaevet al., 1999]. The combination
of data from
the International
10
Bz
lution of the magnetosphere for two sequentialsubstorms. The firstsubstormoccurredduringpersistently negativeIMF B z, whereasthe secondexpansionfollowed a northward turningof the IMF. It wasconcludedthatcontinuousloading during the first substormcreatedan inductiveelectricfield, which stabilizedthe tail and inhibitedfurtherdevelopment of a full-scale onset. The IMF conditionwas suggestedto be importantin explainingthe evolutionof the reconnection region after onsets. In this study,we examinethe temporalrelationshipsbetween solarwind andplasmasheetdisturbances for two substorm intervals on November 22, 1995, with different IMF
951122 (11) (111)
(1•
0
[nT] -10
2
Esw [mV/m]
1 o
3
Solar-
Terrestrial Physics (ISTP) satellitesand ground magnetic field dataprovidesan ideal datasetto studysubstormevolution and its relationshipto the interplanetary magneticfield (IMF)/solar wind. Pulkkinen et al. [1998] comparedevo-
(I)
2
(VxB)y [mV/m]
I o
102
ion-J3 loo -2
1o
o
H
[nT] -300 -600 ADL
ONW
UT
9:00
11:00
13:00
15:00
17:00
19:00
21:00
Bz input histories,to analyzeto what extentthe solarwind energyis transferredinto the midtail plasmasheetusingdata Figure 1. IMF Bz and solar wind electric field obtained by Wind, the dawn-duskcomponentof the -V x B convecfrom ISTP satellitesandground-based observations. tion electricfield and ion-• obtainedby Geotail, horizontal componentvariationsfrom groundmagneticfield stations 2. Observations locatedat high latitude, and high time resolutionmagnetic data from two night time low-latitude stations,ADL and 2.1. RelationshipsBetweenMidtail Electric Field and Solar Wind Input
ONW.
Figure 1 shows an overview of solar wind conditions,
magnetospheric parameters,and groundmagneticactivity. ods with low ion-/3. The locationof Geotail for this date IMF B z and the solar wind electric field were obtained was XosM = -22 to -26/i•, Y•sM = -4 to 2/i•, and using data [Lepping et al., 1995; Ogilvie et al., 1995] ZGSM = --4 to --3/i•E. Superposed tracesof the horizonfrom Wind spacecraft.For this date, Wind was locatedat tal componentfrom groundmagneticfield are obtainedby XGS M =- 79- 76/i•E, YGSM : 47-49 RE, and ZOSM = usingdata from the high-latitudestations(640-74 ø in cor--14 to -1R•. The data from Wind are shifted in time rectedgeomagneticlatitudes). High time resolutionmagto accountfor solar wind propagationfrom Wind to the netic data from two night-time,low-latitudestations:Adegeomagnetic coorsubsolarmagnetopauselocation, i.e., to 10 R• upstream laide(ADL, 46.7øS,213.4øEin corrected from Earth. The solar wind electric field is calculated as dinates)andOnagawa(ONW, 31.8øN, 212.3øEin corrected
Esw - Vx(B} + B•/2)•/2sin4(O/2)[Bargatze, 1991], geomagneticcoordinates)[Yumotoet al.,
1992] are usedto
where0 = arctan(By/Bz). Thedawn-dusk component of monitorthe Pi2 activity. Two intervals of the enhanced solar wind electric field can the -V x B convectionelectric field and the ion-• are esbe identified, which cause two substorms. The onsets of timated using data from Geotail magneticfield experiment (MGF) [Kokubunet al., 1994] and low energyparticleex- major Pi2s associatedwith thesetwo substormsoccurredat periment(LEP) [Mukai et al., 1994]. Due to the large geo- 1108UT and 1502 UT, respectively.The solarwind electric metric factor of the LEP instrument,velocity momentscan field input for the 1108 UT substormincludeda shorttime
be obtained
also for tenuous ions with densities down to
interval(•0.3 hour) duringwhich the electricfield reached
0.01 cm-a This enabledus to investigate theion param- 1.5 mV/m, whereasthe solarwind electricfield signatures eters during the interval plotted in Figure 1 also for peri-
causingthe 1502UT substormwerecomposed of severalen-
NAKAMURA ET AL.' MIDTAIL RESPONSE TO ENHANCED IMF INPUT
951122
hancements,one of which continuouslyexceeded1.5 mV/m for •2 hours.Enhancedsolarwind inputintervalsareshown asshadedbarsin the secondpanelin Figure 1. Here the solid lines (I-V) indicate the start time of the solar wind sudden electric field enhancement(0.5 mV/10 min) with maximum value duringthe intervallargerthan 1.0 mV/m. The end of
WIND 2 -
Esw
GEOTAIL
transient
electric
field
enhance-
Ptot 0.3
ments up to about 3 mV/m, which is more than 10 times larger than the averageelectric field in the midtail during an active interval (0.25 mV/m) [Nishida et al., 1997]. Start times of the electric
maximum
field enhancements
more than 1.5 mV/m
in the midtail
are indicated
I
0.4INTERBALL ---
to stay weak (less than 0.2 mV/m). also observed
-
[mY/m]
the shaded bars shows the time when the electric field starts Geotail
17,301
[nPa] 0.2
with
with dashed
(VxB)y
lines (i-iv) in the lower four panels.To examinegeneralrelationshipsbetweenthe solar wind electricfield and magnetosphericelectric field, the shadedbars extendingfrom the dashedlines (i-iv) in the third panelare plottedwith the
[mY/m]
500
samelengthasthesolarwindinputs inthesecond panel(corresponding to I-V), respectively. Geotailwaslocatedmostlyin theplasmasheetbeforethe
-GEOTAIL ' GEoYML '.....',.....'•....
0 ,. ,.
[k•s]
'-:'t
-500
substorm expansion phaseandduringtherecovery phase. Geotailenteredthe lobeshortly(within10 min) afterthe substorm onsetandstayed inthelobeuntilthelateexpansion phase.Thesesignatures aretypicalfor plasmasheetthin-
I
GEOTAIL
ningobservations in themidtail[Bakeret al., 1996,andref-
.
erences therein].Thedawn-dusk component of themagnetospheric electricfieldhasmorehigh-frequency fluctuations thanthesolarwindelectric fieldpartlydueto theseabrupt changes in theplasmasheetthickness in association with the substorms anddueto flappingmotionsof thetail. Further-
•o• grdudd' ' ' ..... magnetosams
more,theplasmasheetappears to bestrongly turbulent, i.e., theflowisdominated byfluctuations thatareunpredictable, withrmsvelocities muchlargerthanthemeanvelocities and with fieldfluctuations comparable to the meanfieldvalues
[Borovsky etal., 1997].Nonetheless, therearesomestriking
I
o
H
[nT] -300-
.
-600
UT
9:00
.....
I
.....
11:00
13:00
similarities betweentheprofileof theelectricfieldin thesolar windandthatobservedin thetail, i.e., theinternalelectric Figure 2.
tricfielddisturbance withrespect to thesolarwindranges
Solarwind electricfieldandtotalpressure obtainedat Geotai!(solidline), magneticpressure at Interball (thickline),nominalvalueof thelobefieldstrength obtained usingthe solarwind value[FairfieldandJones,1996],convertedto pressure(dottedline), the dawn-dusk component of the-V x/3 electricfield,Vx,/3z, andion-/•obtained by Geotail,andgroundmagnetogram datain theperiod900-
between45 and 80 min. It shouldbe notedthat the starttime
1300 UT on November 22, 1995. Vertical lines show the
field disturbanceshave a durationtime similar to that of the
solarwindelectricfieldenhancements (seethe shadedbars in thirdpanelsshowingthetimescale of thesolarwindelectricfieldenhancements). Thedelaytimeof theinternalelec-
andtheendtimeof thesemagnetospheric electricfielden- start times of the electric field enhancements in the same forhancement intervals correspond favorably withplasma sheet mat as in Figure1. observations. Theobserved characteristics (Figure1) show thatat leastfor thelong-term variations, theplasmasheet electricfieldresponds consistently to thesolarwinddriver. 2.2. Midtail Disturbances DuringSubstorms
IMF [Fairfieldand Jones, 1996], the dawn-duskcomponentof the -V x /3 electricfield, Vx,/3z, and ion-• ob-
2.2.1. 1108 UT substorm. Figure2 showssolarwind tained by Geotail. Interball was locatedat XGSM : --25 electricfield,tailparameters, andground magnetic fieldbe- to -28/t•, YG$M = 5-0/•E, and Z•SM = 11- 10/t• tween0900 and1300UT. Thetail parameters shownarethe
andstayedmostlyin thelobesothatthemagneticfieldpres-
totalpressure obtained at Geotail(solidline),themagnetic sureis equivalentto the total pressure.Data from Interball pressureat Interball (thick line), and the nominal value for
magnetic field measurement[Nozdrachevet al., 1995] are
thetotalpressure in thetail, Pref (dotted line),whichis convertedto thosefor theX locationof Geotailusinga scalobtained usingthesolarwinddynamic pressure valueand inglawforthemagnetotail lobemagnetic field/3 • /.-1.46
17,302
NAKAMURA ET AL.' MIDTAIL RESPONSETO ENHANCED IMF INPUT
in association with [FairfieldandJones,1996],wherer is theradialdistance a cold componentshowingconvection fromEarthandB is the magneticfieldmagnitude. In association
with
the enhancement
in the solar wind
electricfield from 1000 UT, the total pressurein the midtail both at Geotail and Interball increased from • 1010 UT,
the flow enhancements[Nagai et al., 1998]. The sourcere-
gionis expected to belocatedtailwardof thesatellitefor the third intensification. The location of the source is therefore shifted farther tailward for the later intensifications. Such
which is a typical signatureof the growthphase. The en- tailward shift of the sourceregioncould,in reality,be the of newacceleration regionsat moredistantsites hancementin total pressureduring the growthphaseof a appearance in the tail [Angelopoulos et al., 1996]. After theseintensifisubstormwasexplainedby the magneticflux enhancements cations, all the midtail parameters (flow,electricfield,presin the midtail region[Rybal'chenkoand Sergeev,1985]. The sure) return to their quiet level values. It shouldbenotedthat pressure in thelobe(Interball)agreeswell withthatobserved these tail disturbances took place for a timeintervalequalin in the plasmasheet(Geotail),as is expectedfrom the presduration to the period that the solar wind inputwaselevated. surebalance.The brief dip of the Interballpressureat 0943 2.2.2. 1502 UT substorm. Midtail, solar wind, and UT corresponds to a brief plasmasheetencounter, in which ground parameters during the 1502 UT substorm areshown magneticfieldpressurealonecannotrepresent thetotalpressure. Flow enhancementsin Vx with valuesof severalhundredsof km/s were observedduring the growthphasestart-
in Figure3 in the sameformatasFigure2. The solarwind inputwasmuchstronger, andits durationwaslongerthan
ing from 1037 UT, mainly in the componentparallelto the that for the 1108 UT substorm.The totalpressurestartedto field (not shown). One interpretationof suchparallelflows increase in association with the solar wind electric field enduringthe growthphaseis the intensificationof reconnection at the distant X-line.
At substormonset,1108 UT, maximumpressurewasobserved.Afterward,thepressure decreased graduallyat Interball until • 1240 UT, whereasthis gradualdecreasewas accompaniedby sharpdecreaseandrecoveryat Geotail. The
951122
Es w
o
smoothprofileof the nominalvalueP•.•f indicatesthatsolar wind pressurevariationscannotaccountfor thesepressurechanges.Discrepancies in pressurebetweenhigh latitude (Interball) and low latitude (Geotail) were reported by Petrukovichet al. [1999] duringthe substormexpansion
phase,and were considered to be dueto nonstationary effectsand partial compensation by the magnetictensionof
0.3
Ptot 0.2 [nPa] 0.1
the curved lobe field lines. In fact, ground magnetogram intensificationsare observed(bottompanel in Figure 2) in associationwith thesetransientpressuredepletions.Except (VxB)y
for thesetemporaldiscrepancies betweenlow latitudeand high latitudein association with substormintensifications, the pressureat Geotail and Interballtendsto decreaseand recoverback to the quiet level by 1250 UT, when the westward electrojetactivityreturnedto the quietlevel. Over the long timescale,the pressurebalancebetweenthe lobe and the plasmasheetholds,as canbe seenin Figure2.
hanced. These observations indicate that Geotail was located
at the tailward side of the reconnectionregion accordingto the near-Earthneutral line model [e.g., Baker at al., 1996, and referencestherein]. Plasma sheet thinning occurred, and Geotail exitedfrom the plasmasheetat 1119 UT. The
0.0 4
2
[mV/m]
0 8OO
Vx
0
[km/s] -800
In association with the first Pi2 onset, enhancementin the
plasmasheettailwardflow,mainlyperpendicular to themagnetic field, was observedaccompanied by negativeB z so that the dawn-to-duskcomponentof the electricfield is en-
2
[mY/m] l
5
Bz [nTl
0 -5
102
ion-[I l00 -2
10
next enhancement in the electric field was associated with
0
H
the electrojetintensification at 1140 UT. The Z component of the magneticfield stayednegative,which suggests that Geotail was likely to be located at the tailward side of the
[nT] -300
reconnectionregion. The third intensificationof the electric field was accompanied by a fastearthwardflow,mainlypar-
UT
-600
13:00
15:00
17:00
19:00
21:00
allel to the magneticfield, and positiveB z enhancements.Figure 3. Same as Figure 2, exceptfor the period 1300Ions around1210 UT showedcounterstreaming featuresand 2100 UT on November 22, 1995.
NAKAMURAET AL.: MIDTAIL RESPONSE TO ENHANCEDIMF INPUT
17,303
flow hancementand reachedthe maximum near the major onset the innerplasmasheet.The starttime of perpendicular of thesubstorm. Duringtheperiod1340-1445UT, several enhancements(more than 300 km/s) are markedby vertical earthward flow enhancements were observed. These earth- lines. It can be seenthat theseperpendicularflow enhance-
wardflowsweremainlyfield-aligned flows(notshown)and ments, which contributed to the increases in the dawn-towere associatedwith entries into the lobe or plasma sheet dusk electric field, were associatedwith enhancements in the bounday layer(PSBL).Again,thefeaturecouldbe related latitudeangleanddecreasesin pressure(bothplasmaandtoto the enhancementof the distantneutralline. In particu- tal pressure)and in density.Theseperpendicularflows have of plasma-depleted flux tubes,or "bubbles" lar, the earthwardflow at 1400-1410UT wasaccompanied characteristics by northward flowenhancement withan appreciable com- [Chen and Wolf, 1993; Sergeevet al., 1996a], which have ponentperpendicular to thefield(notshown).The pressure been consideredto explain some fast flows in the plasma decreased duringtheperiodindicatinga decrease in thelo- sheet. Whereasthesehigh-speedflows characterizedsome calenergydensity.Thesesignatures suggest thatthesatellite plasmasheetintervals,therewere alsoplasmasheetperiods couldbe locatedcloserto the sourceregionduringtheearth- withouthigh-speedearthwardflows. During the time inter-
wardflow at 1400-1410UT thanduringtheotherearthward val 1624-1626 UT, near the current sheet, ions were almost flows.An alternativeexplanation for thisearthwardflow is isotropic,convectingslowly(lessthan200 km/s)dawnward localactivationbeforethemajorsubstorm onset,i.e., a pseu- [Nagai et al., 1998]. The plasma sheetduring the lowdobreakup, whichhasbeenobserved to occurat higherlati- pressureperiod thereforeconsistsof localized and/or trantudes(moretailward)thanthemajoronset[Nakamuraet al., sientbubblesconvectingearthwardwith a high speedand slowly convectingplasmas. 1994]. Equivalentcurrentsystemsduringthe 1502UT substorm The majoronsetat 1502UT seenin the groundmagneare shownin Figure 5. To avoidthe effectfrom the 1108 togramscoincided with the dawn-to-dusk magnetospheric electric field enhancementand the suddendecreasefollowed UT substorm,we showa differentialcurrentsystem,where thecomponent valuesobserved at 1430UT. The by animmediaterecoveryin thepressure. Thesesignatures,we subtract initially in the premidnight as well as tailward flows, and negativeB z, followed by westwardelectrojetdeveloped to a globalnightside electrojetpatsatelliteentryintothelobe,areall similarto thoseobserved sectorandthenexpanded for theonsetof the 1108UT substorm.During theexpansion tern as can be seenat 1520 UT and 1550 UT. During the re-
electrojet in thedusksectorand phase,the totalpressure decreased significantly to a level coveryphase,theeastward lowerthanthe quietlevel. Prolonged activityof fastearth- westwardelectrojetin the morningsectorremainedstrong ward flows continuedduring the recoveryphase,when the evenafter the westwardelectrojetat the midnightsectorreplasma sheetreappeared,and Bz turned positive. Similar covered,as can be seenat 1620 UT and 1720 UT. During
to the1108UT substorm case,theGeotailobservations for theprolonged low-pressure periodin themidtail,theequivapattern consisted of a globaltwo-cellconvection the 1502 UT substormcould be interpretedas a near-Earth lentcurrent DP2,withoutsignificant contribution fromthesubsourceregion,earthwardof the satellite,at the substormon- pattern, set and subsequentdetectionof a more tailward locatedaccelerationregion during the recoveryphase. In contrastto the 1108 UT substormcase,however,the pressurestayedat a depressedlevel for a prolongedinterval and recoveredto the quiet level only after the decreaseof the electrojetactivity. Interballhad a datagapbetween1426 and 1822 UT, so that comparisonbetweenthe two spacecraftcan be made only nearthe endof the recoveryphasefor the 1502 UT substorm.Nonetheless,therecoveryof thepressurefrom its low level to the quietlevel wasdetectedalsoin the lobeby Interball, which suggeststhat this long-termpressuredepression
stormcurrentwedge.
2.3. Day-Night Relationships
The closerelationshipsbetweenthe timescalesof the solar wind electric field enhancements and that of the midtail
electricfield indicatethat for a timescalelonger than • !0 min, which is longer than the transientchangein the tail due to the substormonsetprocessor flow bursts,the midtail parameterscan be controlledby solarwind conditions. On the otherhand,the midtail pressureresponseto the solar is not confined at low latitudes. Rather, it is related to a more wind input was quite differentbetweenthe two substorms, as shownin section2.2. In this sectionwe try to estimate global changein the tail configuration. the relationshipsbetweenthe solar wind energy input and Figure 4 showsthe magneticfield and the plasmaflow datafrom Geotailduringthelow-pressure periodof the 1502 the disturbanceof the internalplasmaparametersand magon the basisof midtail, inUT substorm.ShownaretheBx component,thelatitudean- neticfieldsmore quantitatively, Here we usethe gle of the magneticfield, the X componentof the velocity, terplanetary,andionosphericobservations. thatthechangein thetail magnetic flux,OF/ the Y componentof the electricfield, the pressureand the relationship density.The X componentin the flow perpendicularto the is obtainedas a balancebetweenthe magneticflux stripped field is shownasa shadedprofilein thethird panel.The solid from the dayside (and addedto the tail) and the flux transline in the pressureplots showsthe total pressurewhile the portedawayfrom the midtailregion.The formertakesplace process,whereasthe latter is dottedline givestheplasmapressure.High-speedearthward via the daysidereconnection processes in the distant flow with the main componentparallelto the magneticfield controlledby nightsidereconnection was observedat the outer plasmasheet. On the other hand, tail as well as in the near-Earthtail. The relationshipcan be flows perpendicularto the magneticfield were detectedat written as
17,304
NAKAMURAETAL.:MIDTAILRESPONSE TOENHANCED IMF INPUT
951122
GEOTAIL
10
gX
-10
[nT]
-20
-30 60
0B
30 0
-30 900
600
VX [km/s]
300 0
-300 2
(VxB)y 0 [mY/m]
-2 -4
0.2
P
[nPa]
0.1 0.0
0.4
N
[/cc]
UT
0.2
16:00
17:00
18:00
Figure4. Magneticfieldandplasmaflowsobserved duringthelow-pressure period.ShownareBx, thelatitudeangleof themagnetic field,theX component of thevelocity, theY component of theelectric field,thepressure, andthedensity.TheX component of theflowperpendicular to thefieldis shownas a shaded profilein thethirdpanel.Thesolidlinein thepressure plotshows thetotalpressure, whilethe dottedlinegivestheplasmapressure. Thestarttimeof perpendicular flowenhancements (morethan300 km/s)aremarkedby verticallines.Thesolidverticallinesaretheevents whenall theconditions for the bubble (see text) are fulfilled.
OF/Or - A(I)a - A(I),•. A(I)a is the daysidepotentialdrop, whichis equivalentto the rate at which flux is addedinto the midtailfrom thedayside. A(I),• is the nightsideeffectivepotentialdrop,which is equivalentto the rate at whichflux is lostfrom the mid-
tail dueto nightside reconnection. The term"effective" is usedhere,sincetheflux transferis contributed by theinduc-
tive(nonpotential)electric field.Fluxbalance estimation has beendiscussed previously usingpolarcapsize,lobefieldob-
servations, orflux erosion atthe dayside magnetosphere. We shouldpointout that we useadditionalplasmasheetinformation,which givesus a clue to the nightsideflux transport
process.The flux estimates includebothopenandclosed fluxesin thetail. Figures6a and6b showtheresultof theestimated flux balancesduringthe 1108 UT and 1502 UT substorms, respectively.Shownaredaysidepotentialdrop,midtailmagneticflux,nightside effectivepotentialdrop,nightside elec-
NAKAMURA ET AL.' MIDTAIL RESPONSETO ENHANCED IMF INPUT
951122 I
15:20
,
GROUND
.,..-r-
I
i
i
BASED
i
'i
I
i
STATIONS
i
15:50
-.
i
17,305
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i
i
i
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0 MLT
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300 nT
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16:20
.
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