Changes in the distant tail configuration during geomagnetic storms

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. AS, PAGES 9587-9601, MAY 1, 1997

Changes in the distant tail configuration during geomagnetic storms R. Nakamura

and S. Kokubun

Solar-TerrestrialEnvironmentLaboratory,Nagoya University,Toyokawa,Japan

T. Mukai

and T. Yamamoto

Instituteof Spaceand AstronauticalScience,Sagamihara,Japan

Abstract. Changesin the structure of thedistanttail associated with geomagnetic stormsare studiedby usingplasmaandmagneticfield dataobtainedfromGeotail.Thirteenstormintervals between October 1993 and October 1994 are examined when the satellite was located in the distant

tail betweenX -- -83 RE andX-- -210 RE. Geotailobservedthemagnetosheath duringall storms includingthosewhenthe satellitewaslocatednearthenominaltail axis. Assumingtheflow directionin themagnetosheath to beparallelto themagnetotail axis,we estimated thedimension andtheflux of thetail for themagnetopause crossingevents.Systematic changesof the distant tail structure arefoundin association with thedevelopment of the storms.Beforethemainphase onsetof thestorms,whentheDst showspositiveexcursion,theenhancedsolarwindpressure reducestheaverageradiusof thedistanttail to ~23 RE ascomparedwith thequiettimevalue,~31 Re. Duringthe stormmainphasethedimensionof thetail is comparable to thequiettimevalue in spiteof thehighsolarwindpressure.This is attributedto theenhanced magneticflux in the tail in association with the southward interplanetary magneticfield (IMF) Bz. An averageenergy

of~5 x 1015J is stored alsoin thedistant tailduringthestormmainphase, whichis a comparablevalueto thatstoredin themidtailduringan intensesubstorm growthphasereportedin the previousstudies.Duringstormtime,whentheBz component of themagnetosheath field was largercompared with theBy component, theaveragetail crosssectionhasa north-south elongated ellipticalshape.The averagetail for quiettime,however,waselongatedtowardthedawn-dusk direction.By componentwaslargerthanBz componentin ourquiettime dataset. Theseobservationsimply thatthe anisotropyin themagnetosheath magneticfield pressurefor differentIMF orientationchangesthe shapeof the tail, whichwasalsoshownin globalMHD models.Changes in theaveragemagnetosheath parameters (density,speed,direction)observedduringthe stormsare foundto be consistent with solarwindfluctuations expectedfrom corotatinginteractionregions.

1. Introduction

The size and the shapeof the magnetosphere are controlled by the interplanetarymagneticfield (IMF) orientationand the solar wind [e.g., FairfieM, 1991 and referencestherein]. In the distant tail (X < -100 RE), where the flaring has ceased,the pressurebalanceholds betweenthe static and magneticpressure in the magnetosheathand the lobe pressure,the latter being predominantlythe lobe field pressure. The size of the distant tail then is determined by the magnetosheathpressure, which is approximatelyequivalent to the lobe field strength, and the amount of flux in the tail. A number of studies have discussed the effect of the IMF

on

the distant tail structure. The IMF Bz controls the amount of

flux transferred into the tail. During growth phase of substormsthe magnetotailat-•220 RE geocentricdistance was observedto grow diametrically in size, often by many Earth radii, owing to enhancementsin the flux [Baker et al., 1987].

On the basisof magnetotailobservations at 225 RE geocentric

Copyright1997by the AmericanGeophysical Union. Papernumber97JA00095. 0148-0227/97/97 JA-00095 $09.00

distanceduringa prolongednorthwardIMF Bz period,Fairfield [1993] suggestedthat the tail becomesnarrow at that distance owing to reconnectiontaking place predominantlyearthward of the satelliteand thereforeowing to a reductionof the flux in the distant tail.

Two

effects of IMF

orientation

in the Y-Z

plane are also consideredto alter the distanttail structureand have been supportedby the following observations:(1) tail twisting producedby field tension[Sibecket al., 1985a] and (2) flatteningof the tail producedby anisotropicfield pressure on the tail boundary[Sibecket al., 1986]. The average dimensionand the shape of the tail are still controversial. On the basis of magnetopausecrossings, a low-latitudemagnetotaildiameterof 60_+5RE was determined at IXI-- 130-225 R E [Slavin et al., 1985]. Tsurutani et al. [1984] obtaineda north-southwidth of 53-59 RE and a dawndusk width of about 45 RE beyond200 RE, which are consistent with an elongatedtail in the Z direction. This was criticized by Sibeck et al. [ 1986], who predicted a flattened tail mostly toward the dawn-duskdirectionwith a semimajoraxis length of 37.8 R E and a semiminoraxis length of 18.5 R E. Fairfield [ 1992] envisioneda radiusof 24 RE and a roundcross section of the distant tail by determining the relative frequency of observationsof the magnetosheathand magnetotail in the region where a nominal magnetotail is expected.

9587

9588

NAKAMURA ET AL.: DISTANT TAIL CONFIGURATIONS DURING STORMS

Fairfield[ 1992]suggested thatthisrelativelysmallerradiusis consistent with an openmagnetotailwherefield linesare lost boththroughthemagnetopause andalsoby closingacrossthe equatorial currentsheet[Fairfield,1986].

Table

1.

Year

The dimensionof the tail is expectedto be highly time de-

Storm Events

Date

Minimum

Position ofGeotail, RE

DstIndex, nT

XGSM

YGSM

ZGSM

pendentowingto thedifferentpolarityof theIMF [Fairfield, 1986, 1993]. During storms,large magneticfield valuesin

1993

October

9

-91

- 139

15

9

1993

October

25

-82

- 191

- 11

9

the lobe were observed[Tsurutaniet al., 1995;Kokubunet al., 1996] in association with the enhancements in the solarwind

1993

November

3

- 119

-206

- 18

5

1993

November

18

-83

-210

-13

-3

pressure. The frequentobservations of the magnetosheath

1993

December

1

-113

-200

-9

-9

from the satelliteslocatedin the nominal tail during active Kp

1993

December

7

-95

-187

-8

-11

1994 1994 1994 1994 1994 1994

January11 February5 April 17 May 1 July 14 September 25

1994

October 2

or Dst intervals[Fairfield, 1993;Nakarnuraet al., 1996] also indicate that the tail orientation is fluctuating with respect to the nominal orientation of the tail. In addition to the external

conditions,an internaleffect could be also importantat times of substormsin associationwith the passageof a plasmoid [Baker et al., 1987]. While the effect of the substormcauses variationsof the boundaryof the magnetotailpredominantly at the timescaleof tens of minutes,the flapping of the tail

-42 - 126 -201 -79 - 53 -73 -93

-91 -86 - 197 -200 - 120 - 170 -162

15 34 23 - 13 19 22 5

1 13 9 1 -2 5 6

causes variations at the timescale of hours [Shodhan et al., 1996; Kokubun et al., 1996].

at This paperstudieschanges in the distanttail configuration tail mission. Geotail frequentlycrossedthe magnetopause associatedwith storms,when both the solar wind and the IMF

are variable,by usingGeotaildatacollectedduringits distant

-lOO

-50 11/3/93

.............................................. _

.......................................... *..

nominal tail

different storm phases. We obtain a descriptionof the systematic responseof the distant tail in relation to storm evolution based on statistical studies of the plasma and magnetic field parameters obtained from the magnetopausecrossing events during 13 storm intervals which occurred between October 1993 and October 1994. Descriptionsof eight storms during this period when Geotail observed large lobe field eventsare given by Kokubunet al. [1996]. Sincethe method usedin this studyallows us to determinethe lobe field strength and the dimension of the tail as independentparameters,we can discussflux variationsand the causeof the changesin tail dimensionduring storms. We show how the effects discussed above alter the size and the shapeof the distanttail.

.........................

50

x

x

x

loo

-50

-100

2.

-150

-200

-250

-300

XGSM -lOO

Observations

This studyis basedon magneticfield and plasmadata from Geotail. Detailed descriptionson the magneticfield measurements(MGF) and the low-energyparticleexperiment(LEP) on Geotail were given by Kokubun et al. [1994] and by Mukai et al. [ 1994], respectively. Figure 1 shows locations of the Geotail satellite during storm intervals between October

-50

•

• 0 N

11/3/93

x x•ee 0 ................• ...., ......•-....................* ......................... ß ß ee x X

.................

•(.............................................

50

•oo

,

o

,

,

m I

-•o

,

,

,

•

I

•

-• oo

m m m I

I

-• •o

m m m I

s

-•oo

t

I

,

I

-•o

,

i

m

-•oo

XGSM

Figure ]. Locationof the O•omi] satelliteduHn[ sto• inregals between October ]993 and Octob• ]994. •

squares

codespondto O•otai] locationswhen the satellite observed the ma[netosphereas well as the ma[netosheath,while the c•oss•sam fo• sto•s when the satellite stayedin the ma[n•tosh•ath. • nominallocationof the tail (•adius30 RE, aberration 4ø) is also indicated.

1993 and October

1994 when

the Dst disturbancewas strongerthan-40 nT. The squarescorrespond to Geotail locations when the satellite observed the magnetosphere as well as the magnetosheath, while the crossesare for stormswhen the satellite stayedin the magnetosheath. The nominal location of the tail beyond 100 RE (radius30 RE, aberration4ø) is alsoindicatedin Figure 1. The fact that Geotail observedthe magnetosheathin all intervals indicates that the structure of the distant tail significantly changes. To study these changes,we examine plasma and magnetic field parameters for the magnetopausecrossing events during 13 storm intervals in which Geotail observed the distanttail (eventsplottedwith squaresin Figure 1). When the Dst deflectionis strongerthan -40 nT in its peak, we identify it as stormactivity. The date of the main phaseonset,the minimum Dst value, and Geotail positionfor thesestorm intervalsare listed in Table 1. We showan exampleof a distant tail observation during a storm which commenced on November 3, 1993, in section 2.1 and presentstatisticalresults in section 2.2.

9589

NAKAMURA ET AL.: DISTANT TAIL CONFIGURATIONSDURING STORMS

-200

magnetosheath

magnetosphere

::':' '""

.

::::•:::::•_::::: :::.':'::::i:':

: :

ß"2-' "z." ' ß'.'.' .', .... .' '.•.': '

,

ß

:',.: :: :::::::::::::::::::::::::::::::::::::

(nT) :::. : ': '

DATE XGSM YGSM ZGSM

11/2 -205.0 -17.4 6.4

11/3 -205.9 -17.7 5.9

11/4 -206.8 -18.0 5.4

:::•::::;•.g;:::::::.';::::•: : :::i•:ii:•.:ii::i::"!::':':" i:•" !

11/5 -207.5 -18.2 4.8

; : ' ß

11/6 -208.1 -18.3 4.3

ß

Figure 2. Dst indexand 1-minaverages of densityN, dynamicpressure Pd, andmagnetic fieldB obtained by Geotail for November 2-5, 1993. The vertical lines show the times when the satelliteencounteredthe magnetopause.

count that the solar wind density and therefore the magnetosheathdensity significantlychangeduring storms,the folFigure2 showstheDst indexand1-minaverages of density, lowing criteria are used to identify the magnetosheath:high

2.1. November

3, 1993, Storm

dynamicpressure, andmagneticfield magnitudeobtainedby

density(N > 1 (/cm3)),low temperature (T < 400eV), anti-

Geotail for November2-5, 1993. During this period, Geotail was locatedat a downtaildistanceof XGSM ---205 ~-208 RE, YGSM -- -17 ~ -18 RE andZGSM = 4 ~ 6 RE. If we assumean

sunwardflow (Vx l(nPa)) as compared with the magnetosphere. The boundariesselectedfor the analysisthen had characteristicsof tangentialdiscontinuitiesin general. The density and velocity valuesin the magnetosheath are similar to thosestudiedby Shodhanet al. [1996]. The vertical lines show the times when Geotail encounteredthe magnetopauseand the two designated regions, magnetosheathand magnetosphere,are also shown in Figure 2. Note that although the satellite stayed near the nominal boundaryof the tail, the probabilityof an encounterwith the magnetopausesignificantlychangesduring the courseof the storm. The satellite is predominantlyin the magnetosheath, entering the magnetosphereon short timescales,except for the

intervals

2100

UT

on November

2 to

1800

UT

on

9590

NAKAMURA

ET AL.: DISTANT TAIL CONFIGURATIONS

November 3 and 1600-2200 UT on November 5, when the

sheathand magnetosphere values. The smoothtransitionbetween the magnetosheath and magnetosphere pressuresindicatesthat pressurebalanceholds, in general,betweentheses two regions. Here we have not includedthe electronpressure to obtain pressurebalance. If we assumeequal electronand protonpressure,the magnitudeof the lobe field estimatedfrom the pressurebecomes~10 nT, even during quiet times where the expectedlobe field was ~9.2 nT [Slavin et al., 1985]. It is therefore anticipated that the electron pressuredoes not changesignificantly,at least in the vicinity of the magnetopause. The result obtained in this study with respectto stormevolutionis not seriouslyaffectedwhetherwe use only the protonpressureor whetherwe includethe electronpressure with a value of 1-2 timesthe protonpressure.We plan to in-

satellite stayedmainly in the magnetosphere.As will be discussedbelow, these variations are closely related to changes in orientationand size of the tail during the storm. The magnetic field magnitude,both in the magnetosphere and the magnetosheath,starts to increaseduring the interval of Dst enhancement and hasa maximumvalue duringthe interval of decreasingDst. Large field magnitudein the lobe is a typical stormfeature [Nakamura et al., 1996; Kokubunet al., 1996]. Enhanced field in the distant tail is attributed to the enhancedpressurein the magnetosheath.The dynamic pressure in the magnetosheathin Figure 2 also showsa significant increase associatedwith a strong lobe field suggestingenhanced solar wind pressure. This suggestsan increasein the magnetosheathstatic and magnetic pressure,which was predicted to be ~4% of the dynamic pressurein the solar wind basedon MHD calculations[e.g., Ogino et al., 1994a]. Figure 3 presentsGeotail measurementsin the magnetosheath and will be used to infer the solar wind condition.

DURING STORMS

clude the electron moment in future studies to examine the

pressurebalancein the distanttail boundaryin more detail. The dotted lines in the flow angle plots correspondto the nominaldirectionof solarwind flow by takinga 4ø aberration into account.

The

ThroughoutNovember2, before the storm,the flow speed was 350-450 km/s, and the number density was low (N