"Substorms, plasmoids, flux ropes, and magnetotail flux loss on March

JOURNAL OF GEOPHYSICAL

SUBSTORMS,

RESEARCH, VOL. 94, NO. All, PAGES 15,135-15,152, NOVEMBER

PLASMOIDS, ON

FLUX

ROPES, AND MAGNETOTAIL

MARCH

25

1983:

CDAW

FLUX

1, 1989

LOSS

8

D. H. Fairfield,x D. N. Baker,x J. D. Craven,2 R. C. Elphic,a J. F. Fennell,4 L. A. Frank,2 I. G. Richardson,S,e H. J. Singer,? J. A. Slavin,x B. T. Tsurutani,s and R. D. Zwickl,9,xø

and producesa stormsuddencommencement (ssc) [e.g.Smithet al., 1986and references therein].

Abstract. During a 9-hour period following a storm sudden commencement on March 25, 1983, six spacecraft near geosynchronousorbit, one over the pole, and three in the magnetotail monitored a complex sequenceof magnetospheric variations. Magnetic field compressionsassociated with the sudden commencement were seen first by the nearEarth spacecraft and subsequently by the three downtail spacecraft with increasing time delays that were

consistent

with

the

tailward

movement

More generally, transfer of energy to the magnetosphereis regulated by the direction of the interplanetary magnetic field. In the presenceof a southward interplanetary field, magnetic flux is transferred from the dayside magnetosphereto the

geomagnetic tail [e.g., Baker et al., 1984b]. This

flux transfer moves the subsolar magnetopausecloser

to the Earth [Aubry et al., 1970; Fairfield, 1971; Holzer and Slavin, 19781while causingenhanced flaring of the tail boundary to encompassa larger diametermagnetotail[Maezawa,1975]. This flaring

of an

interplanetary-shock-associatedpressure enhancement. Ground magnetograms and synchronous orbit data are used to identify seven substorm intensifications during this geomagnetically active period. Six of these intensifications are clearly associated with tail

magnetotail is compressedby a larger component of solar wind pressure producing a stronger tail lobe

lobe field decreases ~18 RE behindthe Earth. Four

magneticfield with typically 1022ergs of stored

fieldincreases in thetail lobesat ~18and~30•RE

magnetotail is accompanied by a thinning of the

of these intensificationsare followed by both B_

and by the subsequentobservationof rapidly flowing

energy.

The developmentof this stressedhigh-field

near-Earth plasma sheet that separatesthe tail

plasma sheet plasma atISEE 3~110ER • down the plasma lobes. The northward field component sheet also decreases at this timewithin [e.g., the

tail. During two substorms where D was optically observingthe auroral oval, the area of the polar cap was observedto decreaseas the tail lobe field decreasedat 18 RF. All these observationsare consistent with

the sub•torm-associated

release of a

Fairfield, 1984] partic_ularly in the ta_ilregionwithin 20 R• of the Earth [Fairfield, 1986b] A small B

component m the inner magnetotail may make the tail more susceptible to magnetic reconnection via

plasmoidat a neutral line near 20 RE; however,the

the tearing mode instability [e.g., Schindlerand Birn, 1982] and the formationof a near-Earth

magnetic field thought to be characteristicof the passageof a plasmoid in the deep tail was not seen

neutral line. The energy released by magnetic reconnection at such a neutral line is thought by many workers to cause the onset of the expansive

classical north-south variation of the plasma sheet in every case.

phaseof a magnetospheric substorm[e.g., Baker et al., 1984b].

1. Introduction

On the earthward

side of such a near-Earth

neutral line, the reconnection model predicts the

Innumerablestudiesof the complicatedinteraction of the solar wind with the Earth's magnetosphere have led to an evolving and increasingly sophisticated model of energy transfer and dissipation. At times a sudden increase in solar wind pressure, such as that associated with an interplanetary shock, compressesthe magnetosphere

Cross-tail

XLaboratoryfor Extraterrestrial Physics,NASA 2University of Iowa, Iowa City. aUniversity of California at Los Angeles. 4AerospaceCorporation, Los Angeles. SImperial College, London. enow at NASA Goddard Space flight Center, Massachusetts.

Mexico.

lines in

On the tailward side of the neutral line, the neutral line model proposes that closed loops of magnetic flux are created in the plasma sheet and ejected down the tail once tail lobe fields begin to

reconnectat the neutral line [Hones,1979]. The tailward flowing plasma containing such flux loops is

known as a plasmoid[Hones,1979], and signatures

XøNow at NOAA ERL Space Environment

of such structures

Laboratory, Boulder, Colorado.

Copyright 1989 by the American Geophysical Union.

0148-0227 / 89/ 88JA-03763$05.00

field

al., 1985; Nagai, 1987].

6Jet Propulsion Laboratory, Pasadena, California. 9Los Alamos National Laboratory, Los Alamos,

Paper number 88JA03763

down

Rostoker et al., 1980; Lester et al., 1983; Singer et

?Air Force GeophysicsLaboratory, Hanscom Air

New

are diverted

substormonset [Clauer and McPherron,1974;

Greenbelt Maryland. Base

currents

the postmidnight magnetosphere where they flow westward through the auroral ionosphere and enhance the auroral electrojet. The currents then return to the magnetosphere in the premidnight sector. The integrated effect of this diverted current produces magnetic field perturbations at mid-latitude ground magnetic observatories and at the geosynchronousorbit, all of which are valuable diagnostics in determining times and longitudes of

Goddard Space Flight Center, Greenbelt Maryland.

Force

earthward movementof energeticplasma and the dipolarizationof the field in the inner magnetotail.

have

been

detected

in the

distant

tail by the ISEE 3 spacecraft[e.g., Hones et al., 1984; Baker et al., 1987 and referencestherein]. This sudden release of stored energy and magnetic flux will allow the remaining lobe field lines to expand into the newly available volume, thus

15,135

15,136

Fairfield et al.: Substorms,Plasmoids,Flux Ropes

decreasingboth the tail lobe field strength and the

UT.

The magnitude and orientation of the

diameterof the tail lobe [Maezawa,1975; Baker et magneticfield are also shownwith the latitude and al., 1984a;1984b]. Sincemagneticfield lines longitudeanglesin GSM coordinates. The figure emanatingfrom the polar cap comprisethe magnetic containsa numberof intervalsof high (21 cm-8) field lines of the magnetotail lobes, the area of the

thermal electron densities when the field is

polar cap shoulddecreaseas reconnection converts open lobe field lines into closedplasmasheetfield

nontaillikeand the flow velocities(e.g., see Figures8 and 9) are near 500 km/s. These observations

lines. Although the above model is based on observations, and virtually all of the phenomena describedabove have been observedwith varying degreesof clarity, seldom have the majority of these phenomena been observed simultaneously during

indicate that the spacecraft was intermittently within the magnetosheath during this interval. Since such observations are not expected if the solar wind is flowing radially from the Sun, there are apparently unusual conditions prevailing at this time. If the tail is supposedto be circular with a radius of 25

individual events. Thework reported below utilizesR Eatthe 3 distance, it would require, ø a6dition to ISEE the nominal 4 ø tail aberration, a •n

the "event B" data in the large Coordinated Data

AnalysisWorkshop8 (CDAW 8) data baseto

illustrate the occurrence of most of these phenomena

during a series of substorms that occur on a single day. Section 2 describesthe locations of 10 spacecraftused in this study, and section 3 briefly reviews the geomagnetic activity. This activity is more fully discussedin the appendix. Section 4

emphasizes the near-tail (R • •00•• northward over the spacecrafton entry and ,... lOOO1 o 800-1

--,,

retreatingsouthwardagain on exit as is typical of

plasmoids, and (3) the south-north-south Bz field signaturewhich they interpretas the trailing half of

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133180 1

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one plasmoidfollowedby a secondplasmoid. Figure 9b illustratestwo intervalssubsequentto

0!

.--

-9

.........

/•

/.',,,

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.-/

' ...................

' .......

h_

0824 and 0902 UT which exhibit low magnetic field

MAGNITUDE

strengthand enhanced-temperature plasma characteristicof plasma sheetsor plasmoids. Examination of detailed plasma distributions

2O

..-.

indicatesthe spacecraftmay have briefly sampled the plasma sheet in the low-fieldregion at 0825 before returning to the boundary layer for ~3 more minutes. During the interval shownin Figure 9b the magnetotailapparentlymovednorthwardthen southwardcausingISEE 3 to pass from the

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'-[

10-•

,I

J

03

Bz

I

...-.180

northernlobeinto the plasmasheet(0828-0840 UT)

I

270 t I I I

--[

ß

AZIMUTH

-{90

and then into the southern hemisphere lobe

90-

J

(•)'• 45-

0843-.0902 UT I and finally back into the northern

obe(0914UT via the plasmasheet. Againthe

FLOWAZIMUTH

plasmavelocitiesderivedfrom electronand energetic ion data agree well while weak fields are being observedin the plasmasheetwhereasfree tailward streaming ions(highion velocities) and magnetosonic

I

-90 I 0700

• 0710

POLAR

I I 0720

0730

0740

0750

0800

ISEE3 (GSMCOORD)03-25-83

waves[Tsurutaniet al., 1985]are presentin the

plasma sheetboundary layers.Forthiseventa Fig.9a. High-resolution ISEE3 datafromthree strong tailward-directed electron heatfluxis present experiments. Twelve second resolution thermal both in the central region and the adjacent

electron data can be recognizedby its occurrencein

boundarylayers. The heat flux followsthe field directionquite faithfully as the field makesthe north-southhemispheretransition,but the velocities

temperature, and velocitymagnitudeand direction

are always tailwards. The intervals shown in Figure 9b have been

192-sblocksseparatedby 84-s gaps. Thermal electronheat flux magnitudeand direction,density, are shown in the upper panels. Energeticion density,bulk flow speedand directionare

superimposed with pointsat 64 s resolution.Lower panelsshowthe magneticfield magnitude, Bz and azimuthaland polaranglesin solar Scholer et al., [1984a]suggested that the spacecraft component, magnetospheric coordinates.Verticaldashedlines may haveremainedon openfield linesthroughout spana regionof plasmasheetplasmacharacterized this eventwhereasRichardsonand Cowley[1985]

studiedby Scholeret al. [1984a],Richardson and

Cowley [19851, andRichardson et al., [thisissue].

suggested thattheinterval following 0824UT may bylowfieldmagnitudes withvariable direction, high bea plasmoid. Closer examination [Richardson et energetic particle density, andhighthermal electron al.,thisissue] hasfailed to definitively confirm this density andheatflux. Theboundary layerregion event asa plasmoid because (1) it is notclearif adjacent to theplasma sheet is characterized tailward streaming energetic electrons decrease before primarily byfree-streaming energetic ionswhich lead entryintotheweak fieldregion at 0828, and(2) to thecalculation ofveryhightailward flowspeeds. thenorth-south variations in themagnetic field Also,elevated electron temperatures andheatflux characteristic of a plasmoid arerelatively weakasis continue intotheregion following theplasma sheet.

15,144

Fairfield et al.: Substorms,Plasmoids,Flux Ropes

O-2 t HEAT FLUX -5

7.0

6.5

TEMPERATURE I

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0800

0810

0820

0830

0840

0850

0900

0910

0920

0930

ISLE 3 (GSM COORD) 03-25-83

Fig. 9b. A secondinterval of ISEE 3 plasma sheet plasma in the same format as Figure 9a. Tail motion causesISEE 3 to pass from the northern lobe through the plasma sheet into the south lobe at 0843 UT before returning back through the plasma sheet into the north lobe at 0914. High tailward flow velocities are detected by both particle instruments within the plasma sheet but the free-streaming,energeticions produce even higher velocities in the adjacent boundary layer regions.

apparent in Figure 9b.

One possibilityis that a

seriesof small plasmoidsoccurredas indicatedby north-southvariationsin the intervalsat 0825, 0828-0835,and 0835-0840UT. Certainly plasmoid

or plasma sheet-like plasma distributions were seen

duringtheseintervalsas wouldbe expected for any

spacecraft transiting from the northern to the southern

lobes.

Figure 9c illustrates another ISEE 3 encounter

with plasmasheet-like plasmathat followsthe 1023 UT substorm intensification by 10 min. The north-

angle is unlike the earlier event, however,perhaps because the spacecraft crossed the current sheet and measured fields with

directions characteristic

of the

southern hemisphere during the interval 1038-1046 UT. Note that during the field reversalsthe polar angle remained small so the field vector rotated in

the X-Y plane. Another directionchangeoccurred, so that at the peak in field strength at 1042 UT the field was oriented nearly dawnward in the cross-

tail direction. The generallytailwarddirected enhanced heat flux wasagainpresentin an

southfieldperturbation is unlikea classical plasmoidextended intervalthat included the boundary layers. during thisevent,but B andB_ arerathersimilar Theheatfluxfaithfully followed thefieldduringthe

tothe0720example ofFigure f•a. Theazimuthal fieldreversals to thesouthern hemisphere directions

Fairfield et al.: Substorms, Plasmoids, Flux Ropes

15,145

O-2 t HEAT FLUX 03

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HE•T FLUX AZIMUT

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ISEE 3 (GSM COORD) 03-25-83

o

'-" -4,5

•

' ................

1020 1030 1040 1050

1100 Fig. 9d. A fourth interval of ISEE 3 plasmasheet

ISEE 3 (GSM COOSO)03-25-83

plasma in thesameformatasFigure9a. Theinner

pair of vertical dashedlines denotethe plasmasheet region while the outer pair of lines separate Fig. 9c. A third intervalof ISEE 3 plasmasheet preceding and followingintervalswhenISEE 3 was plasmain the sameformat as Figure9a. Enhanced measuring high densities characteristic of the electrontemperatures, heat fluxes,and energeticion magnetosheath or magnetosphere boundarylayer.

densities againcharacterize the plasmasheet,but The north-south Bz transitionin the plasmasheet theyextendinto the adjacentboundary layerregions at 1323UT is associated with a B magnitude peak

where the field is more lobelike.

as is characteristic of many plasmoids.

exceptfor a 180ø reversalto keep it oriented tailward. Throughout these field rotations the plasmavelocityremainedtailward. This intervalis

proximity to the magnetosheathand the magnetosphericboundary layer, which was seen

somewhat different from the other intervals in that

the high velocitiesdeterminedfrom energeticions occurredwithin the plasma sheet near 1045 UT as well as in the adjacent boundary layer. Apparently

before 1306 and after 1341 UT, it displays many features of the earlier events and classical plasmoids. The field strength becomesquite weak at 1320 UT and a very brief excursion into the southern

hemisphereis suggested. After this interval the

the plasmais flowingfasterduringthis eventthan in other cases;such flow is consistentwith a short delaytime followingsubstorm onset(seebelow).

field remains near the X-Z plane and exhibits a

19 minutesafter the 1259 UT substormis shownin

than any value the adjacentlobe field ever attains.

large north then south excursioncharacteristicof a plasmoid. Associatedwith the north-southreversal

The final plasmasheet-like intervalthat occurred is a 30-nTpeakin B that is at least6 nT larger

Figure9d. Although thisintervaloccurred in close Neitherthe electron temperature or density shows a

15,146

Fairfield et al.: Substorms, Plasmoids, Flux Ropes TABLE Substorm

Plasma

Onset Time,

Sheet

Transit

Onset Time,

UT

3

Duration• min

L,

Plasma Plasma Plasmoid RE Velocity, km/s Sheet

rain

0713

to ISEE

Measured

Time,

UT

0650

2. Transit

Transit

Velocity, km/s

23

400-700

10

6

31

310

0750(0807) 0824

34(17)

800-1200

15

1.5

14

480

1259

lo 19

000-i400 300-600

18

42

270

1318

decreaseat the time of this field peak, nor does a detailed examination of the electron data reveal a departure from plasma sheet-like spectra. These facts indicate that the peak definitely occurs within

theplasma sheetandsuggest a lackof pressure

equilibrium at this time. Tailward convecting plasma is confined to the plasma sheet between the inner pair of vertical dashed lines. Higher energeticparticle velocities due to particle streaming occur in

10

trailing half of a 5 min plasmoid was detected, giving a 54-Rs plasmoid. For the last event we consider the p-lasmoidas the first 10 min of the interval where the classical north-south B_ is

observed. Thiseventyieldsa lengthof •2 RE.

Note that the portions of the plasma sheet intervals following the plasmoids in Figures 9c and 9d have the predominantly southward fields characteristicof the postplasmoidplasma sheet discussedby

the adjacentboundarylayers,and tailward heat flux

Richardsonand Cowley [1985]. Subsequent

occurs in the trailing boundary layer. Although the heat flux is somewhat weaker in the plasma sheetlike region compared to earlier events, the strongest

plasmoidsmay follow those discussedabove in Figures 9a and 9b. Finally we assume that the plasmoidsare

intervalof heat flux at 1328 UT is associated with

releasedfrom 20 Re at the time of substormonset.

earthwarddirection. This event appearsto be the

lengthby the transit time in column3 (and

clearestexampleof a plasmoid on this day (seealso Richardson et al. [thisissue]).

accounting for the unseenearlierarrivalbefore0713 and1033UT) we obtainan average transitvelocity

a cooler, denser plasmaandit is directed in the

Sinceplasmoidsare one possibleinterpretationof

the eventsof Figures9a-9d,it is importantto estimate

their

sizes and

their

arrival

times

at

ISEE

Dividing the distance 90 RE minusthe plasmoid shownin column8. In all casesthis average

transitvelocityis lessthan the measured "final" velocity. This differencecan be interpreted as

3 under the plasmoid assumption. The first three

evidence for acceleration of the plasmoid from zero

columns

velocity so that the averagevalue that is calculated

of Table

2 list

the

substorm

onset

time

associatedwith the four plasma sheet intervals, the onset time of the low-field region containing plasma sheet-like plasma at ISEE 3, and the difference between these times. Following the 0750 UT onset we list the alternative 0807 UT time, discussed above and in the appendix, whose use is necessary to obtain

reasonable

velocities.

The

measured

plasma velocities are displayed in column 4, which

gives a range of velocitieswhich roughly bracket the velocitiesfrom the plasma and energeticion instruments.Column 5 lists the duration of the low-

field, higher-temperature plasmasheetinterval, and column 6 the duration of the subinterval judged a

is less than the final velocity that is measured. Another

characteristic

of at least three of the

four plasmasheetevents(Figures9a,9c and 9d) is a field magnitude peak within the weak field region. This feature is not a previously reported feature of

plasmoidsand is contrary to the simple plasmoid model with an "O" type neutral line at the center of a flux loop, at least if the spacecraftis passing

anywhere near this weak field region. However, examinationof both publishedand unpublished examplesof plasmoidsreveals that such a field peak is in fact a feature of the majority of plasmoids

[e.g.,Bakeret al., 1987, Figures4, 5 and 6;

possibleplasmoidon the basisof the Bz signature.

Richardsonet al., 1987. Figures 3 and 4; see also

Richardsonet al. [this issue],that the first of two

peak is invariablyassociated with any north-south

For the event in Figure 9a we assume, following

plasmoids arrived 3 min before the spacecraft moved into the weak field region and hence the leading half was not detected by the spacecraft. It is then concluded that the plasmoid duration is 6 min. Multiplying this duration by the midpoint velocity

from column 4 gives a plasmoidlength of 31 RE as is shown

in column

7.

The

0824

UT

event

in

Figure 9b involves a high measured velocity and is complicated by the transverse motion of the tail that moves the spacecraft from the north to south

Slavinet al., thisissue].In theseexamples the

B_& transitiono in the manner illustrated especially clearly by Figure 9d.

Two explanationsof this high field peak seem possible. Slavin et al., [this issue]proposean explanationthat is essentiallythat usedto explain

traveling compression regions [Slavin et al., 1984].

This explanation states that the portion of the closedplasmoid field loop north or south of the "O"

brief north-southBz variationat 0825 UT with

line and closest to the lobe, will be compressed between the enhanced pressure of the adjacent lobe field and the plasmoid. This compressionis less important near the ends of the plasmoid, so that a weak field region will be seen on either side of the peak. This explanation implies that the peak will

followedRichardsonet al., [this.issue]and selected the interval four times as long (0828-0834)and obtaineda lengthfour timesas long (56 RE). For

but it will have maximumamplitudeif it p•sses near the northern or southernboundariesof the plasmoid. As mentionedabove,however,the

lobes.

This

event

is the most difficult

to reconcile

with the plasmoid model. However, we can obtain a plasmoidof length 14 Rœ if we focus on a very duration 90 s.

Alternatively, we might have

the 1033 UT event we again assumethat only the

be absent if the spacecraftpassesnear the "O" line

thermal electron data at the time of the 1323 UT

Fairfield et al.: Substorms, Plasmoids, Flux Ropes

peak do not reveal any decrease in plasma pressure as one might expect if plasma were being squeezed out of the region.

An alternative explanationof the field magnitude

peakis that of a flux rope. In this casea strong axial field at the center of the flux rope would provide a natural explanation of the peak if the spacecraft moves directly through the structure.

This in fact was Sibecket al.'s [1984]original

15,147

substorm onset. This observation further supports the occurrence of reconnection that converts open tail lobe field lines to closed plasma sheet field lines

during substorms.

It was furthernotedthat the tail lobefield strengthat 20 Rœtendedto decreasein proportion to the polar cap-flux decreasefollowingthe 1259 UT onset so that little change in the tail radius

near20 RE wasrequired to accommodate the time

varyingopenflux. The changingfield strengthat evidence[Richardsonet al., this issue]now suggests 20 RE, however, requireda substantial changein that such uniform motion did not occur. It seems tail flaring angle;an extremelyflared tail was presentbeforeonsetthat is consistent with a closethat the flux rope model, and in particular a explanation for the 1033 UT event, although recent

theoretically interesting force-free equilibrium model with constant pitchof the field [e.g.,Burlaga,1988], mightwell explainboththe observed magnitude and angular variations.The problem with this explanation is that sucha flux ropewouldbe approximately oriented alongthe tail axisin Figures 9a and9d, andit is difficultto reconcile suchan

in subsolar magnetopause, whereas a lessflared magnetotail waspresentduringrecovery that must be accompanied by a moredistantsubsolar magnetopause. Thisconclusion is consistent with tail fluxreturning to the dayside duringthe substorm. The constancy of the tail radiusnear20 RE suggests that thisdistance is nearthe "pivotpoint"

plasmoid model. Another feature not readily reconciled with the

will reach a maximum at more tailward locations before onset and decrease at such locations during the substorm. Earthward of this pivot point the

classicalplasmoidpicture is the existenceof strong heat flux in the plasmoidlikeregions, if indeed the measurementsrepresent a true heat flux. Closed flux loops might support a bidirectionalelectron anisotropy,but such loopswould be detachedfrom any heat source,which would seemto precludea strong heat flux. In fact a closeexaminationof

changeswill be in the oppositesense. To create suchboundarychangesfor this event a neutral line wouldprobablycreateclosedflux in the vicinityof

followingthe 0750 UT{or 0807 UT} substorm, althougha slightlylargerchangein polar cap area suggests a decrease in the tail radiusat X•--20RE.

such a bidirectional anisotropy along with the

at X'•-110RE resemble thoseassociated with

orientationwith the observedmotion and time delays that are more easily explained by the

electron distributions sometimes reveals evidence for evidence for a heat flux.

A true heat flux is

probablymore compatiblewith the flux rope explanation. 7. Summary and Conclusions

aroundwhich the boundary"rotates";the tail radius

20R•. Similar polarcapfluxchanges occurred In many respectsthe observationsfrom ISEE 3

plasmoids, and thereforeare consistent with the

near-Earth neutral line model of substorms. The

four principalgroundonsetsthat were associated both with major tail lobe reconfigurations and with the largesttail energylossesare also thosefour events where ISEE 3 detected distinct weak

Multiple spacecraft data sets of March 25, 1983, have been used to confirm many characteristics of magnetotail dynamics that have been discovered by earlier studies. At the time of a ssc on March 25, a magnetic field compression was observed in

magneticfield regionscontaininghot plasmasheet plasmaflowingrapidly in the tailwarddirection. The simplenorth-south Bz signature characteristic of a singleplasmoidwas clearlypresentonly for the

convected pressure increase associated withan interplanetary shockmovingawayfromthe Sun. Two spacecraft in the tail lobesnearX'•-20and

mightconstitute an individual plasmoid.The plasmasheetbehavior andsurrounding boundary layerphenomena are characteristic of plasmoids, yet

last of four events, although multiple plasmoids

succession byfourdifferent spacecraft at increasingcould perhaps explain several others. Thearrival distances in thetailward direction. Thedelay times times arecompatible withtheplasmoid model once were compatible withtheexpected delay times fora appropriate decisions aremade about exactly what -30RE detected magnetic fieldchanges typical of

substorms. The tail lobefieldstrength decreased in

theyaretypical of almost anyplasma sheet entry andhencecannotbe considered uniquesupportfor

association with six of sevensubstormonsetsor intensifications as determined by ground

plasmoids [SeealsoTsurutaniet al., 1987]. The heatflux withinthe plasmoids seemsto argue

on the ground, a tail magneticfield reconfiguration

suspect.

at 20 and30 RE wasindicated by an increase in Bz duringthe tensof minutes following the

On the otherhand,fluxropesmayprovide an alternative explanation for at leastsomeof the ISEE

of substorm onsets and{21moremagnetic flux

explained by plasmoids, but the fieldsignatures

magnetometers andgeosynchronous spacecraft. againstthe plasmoid picture,yet interpretation of Duringthe fourlargestintensifications as determinedthesemeasurements as a true heatflux maybe

substorm onset. Theseobservations confirmthat {1} 3 events. It is not immediately apparenthowto the tail typicallylosesenergybeginning at the time reconcile this modelwith otherfeaturesmoreeasily closesacrossthe equatorialplane after a substorm, as wouldbe expectedif formerlyopenfield lines had becomeclosedby reconnection. The changing amountof openflux threadingthe polar cap during substormswas determinedby measuringthe size of the dark polar cap on spacecraftimagesof the aurora. During the two substormsimagedby spacecraft,the polar cap flux decreased by 30 and 36% duringthe hour following

appearconsistent with that expectedfrom a spacecraft movingthroughsucha structure. In an attempt to reconcilethe variouspointsof view we suggest the followingconsiderations. First, the standardplasmoidpicture may be basicallycorrect,but the plasmoidmay be of limited extent in the Y directionas was found in the three-dimensional simulationsof Birn and Hones [1981]. The edgesof the plasmoidmight appear

15,148

Fairfieldet al.' Substorms, Plasmoids, Flux Ropes

different from what is seen nearer the center.

The

0650 06231

time relation with substormsmight still be present,

'

but the different and unknown structures on the

edges might explain those events that deviate from the standard

1259

I

I

•

250 NT I

I

,,I

model.

0951 0923 1023

0750

A second possibility is that the substorm onset may on occasion be associated with an increase in

the reconnection rate at a preexisting near-Earth neutral line. In this case no plasmoid is formed but the plasma sheet tailward of the neutral line

RC

! i

expands over the spacecraft, as discussedby

Richardsonet al., [this issue].This suggestionis

I

supported by the slight preference for southward fields in the 0824 and 1033 UT plasma sheet encounters. Additional field loops within such a region could explain further complexity. A third possibility is that at substorm onset a plasmoidlike structure is released that carries fieldaligned currents and is similar to a flux rope.

Hughesand Sibeck[1987]have suggestedsuch a

i I i

'I 1

off and

convect

downstream

at

the

time

I I

I

SD

model whereby a three-dimensional flux rope extending across the tail is created by near-Earth reconnection in the presence of a cross-tail field component. At least one end of this flux rope may break

I

I

,

of

substorm onset. In the limit of no B_ component, J• . the model reverts to the simple two-d•mens•onal

I

plasmoid,closed-field-loop picture. Birn et al. [1988] have quantitatively extended the qualitative model of

I

Hughesand Sibeck[1087] and emphasizedthe

I

i

i

I I

I I

I i

I I

I I

I I

I

I

I

I

I I I

I I

complexity of this three-dimensional configuration with

B.#0.

Birn

et al. even discuss how

This

model

would

seem

able

1:3

MARCH 2õ, 1983

common closed topologmal charactenstms of a twodimensional plasmoid disappear with B. #0. They describe the three-d•menmonal plasmmd as a flux rope that may be initially connected to the Earth but is still surrounded by a complicated set of intermingled flux tubes that may connect to the interplanetary magnetic field, to the Earth, or to both.

11

the

to reconcile

many of the plasmoid-flux rope ambiguities described above.

Fig. 10a. Bx componentmagnetograms from the

AFGL longitudinal chain of observatories.

The first substorm intensification following the sudden

commencement

0023

UT.

the onsetof By perturbations at the samestations 0650 I

0623I

data will be used to determine the onset or

at

subauroralAFGL magnetometer chain (not shown),

Appendix: SubstormActivity on March 25, 1983 In this section,ground magnetogramsand geosynchronous magneticfield and energeticparticle

occurred

Indicators are the onset of Pi 2 pulsations at the

Nw

0951

0750

0923

I•

1023

1259

-'•

I

intensificationtimes of geomagneticsubstorms occurring between 0500 and 1400 UT on March 25

1983. Figure10 displays the B• andBy

i i

RC

i i

I

componentsfor the (Air Force Geophysics Laboratory)stationswhich coveran approximately 00ø longitude range near 54ø geomagneticlatitude. The station St Johns to the east supplementsthis

chain. Figure11 displays Bx andBycomponents

(or the equivalentH and D componentsas the case may be) from a seriesof auroral zone stations spanning the Alaska-Canadian sector. Figure 12 illustrates energeticelectron and proton fluxes, magnetic field magnitude B, and field latitude angle 8 from five different geosynchronous spacecraft. The mid-latitude magnetogramsof Figure 3 reveal the rapid developmentof a modestmagnetic storm main phase after the ssc of 0544 UT.

The

decreasing magnitudeof the Bx componentis

\

I

250 NT

quadrant where such decreasesare particularly pronounced. This main phase begins developing without any clear substormonset, although ongoing

magnetic activity at auroral latitudes is indicated by

the Leirvogurmagnetogram(not shown).

I I

particularly evident at Guam due to the low latitude of this station and its location in the dusk

'V, 9

MARCH

I

II 25

, 11

I

1983

Fig.10b. SameasFigure 10afortheBy

magnetogramcomponents.

•

13

Fairfieldet al.' Substorms, Plasmoids, Flux Ropes 0650

0951

0750

O623

0923 I 1023

•

1259

0951

0623 I ;

i I

15,149

0750

i

0923 1023

i

Bz

I

1259 •_

• ?'

I

•

I

I

I

i

I

I

I

i I

I'-•

•

I

I ME I

• I '

I.

I•f I I

I

GW

i

•

5

I

•

I

I

I

I

I

II

i

9

i

I

I

11

MABCH 25 198•

Fig. 11c. Same as Figure 11a for the Bz

I 11

magnetogram

13

components.

MARCH 25 1983

(Figure10b), and similar B, perturbations at the

Fig. 11a. Bx component magnetograms fromauroral U.S.observatories (Figure3). The B. perturbations

zone stations.

changed sign between the AFGL stations Rapid City and Mount Clemens, with the station in between, Camp Douglas, showing a positive-to-negative D change delayed about 4 min from the onset. This

•51

0623•

0750

,,

0923 I I I

1023

1259

behavior

I

dy

I ,,

locates

the

central

meridian

of the

current

wedge at Camp Douglas near 320ø geomagnetic longitude or very near local midnight. This is consistent with

both the data from GOES

2 near

the same meridian IFiõure 12) which showedan increase in the 8 angle at 0624 UT and the data

from 1081-025Bwhich detectedelectron and proton increases3 min later. GOES 5 and spacecraft 1982-019A I

500iNT

I

I

I

¾F_.I•

FC

I

I II

!

iI I

I

wedge. Thelatitude oftheauroral electrojet

is indicative

of a westward

movement

of the

associated with this current wedge was north of

Glen: Glenlea(59.5ø) but well southof Churchill(68.8ø) and and Lynn Lake (67.3ø) sinceno significanteffects were were seen at the latter location. The wedge was

I II .,,•,._A, •, •

I

to

positive B. perturbation over the following 30 min

i

iI

I

Stations

which

•

•._

confi• confined time.

At

GW • I•

saw no efTects.

I

I ill!

at 0130 LT

the west of the onset {Glenlea(not shown),Rapid City {Figure 10b), Tucson,Boulder,and Lompoc {Figure 3)) exhibited a reversalof this initially

I

II

to no more than

0650

UT

a further

2 or 3 hours of local substorm

intensification

was indicatedby positiveB. at Sudbury(Figure

•

i

I

10b)andFredricksburg (not yshown) withsimilar,

slightly delayed perturbations at Mount Clemens, Camp Douglas, and Rapid City. Farther to the east San Juan saw no significant D perturbation but experienced a positive H perturbation at 0654 UT

MARCH

25

1983

(not shown). Pi 2 pulsationsat the AFGL chain tend to confirm the 0650 UT onset time, although

Fig. 11b. Sameas Figure11afor the By

magnetogram

components.

ongoing pulsation activity tends to obscure the new intensification. Evidence for the auroral electrojet associated with these perturbations comes from a

15,150

Fairfield et al.: Substorms, Plasmoids, Flux Ropes

Glemens(Figure lob), and Boulder(Figure 3). Positive H bays were seen at Rapid City, Newport,

1982-019A

3045 KeV

(Figure 10a), Tucson,and Boulderand slightly later at Lompoc(Figure 3). The auroral electrojetwas

10"

electrons

200

Ao t

I I

•u

apparently near Meanook judging from a large sharp

negative .Bx perturbation at 0750that was

.100

II

,.-,----'--"'"GOES5

1982-019A 10••

accompanied by little Bz perturbation. These

observations place the onset near 310ø longitude or very near local midnight. A field dipolarization was detected by GOES 2 at 0753 UT near 0100 LT but not at GOES 5 near 0300 LT. At 2300 LT, intense fluxes of energetic electrons at 1981-025B

145-160 KeY•! !i "'!•!.•"n'"'•'• •r ,t,ons 1981-025B 106 I lO". I I I +200 protons

10ß ß

3045KeV

.

I I

j

i, I

increased even more at 0759 UT, and the field angle

I

I

•,v,, I !"P•..l•11.,

Bin, I

computedfrom the electronanisotropy(not shown) [Bakeret al., 1981] also increased. At 0300 LT,

II,

,J__

• •l'.•

spacecraft 1982-019A detected a brief electron increase at 0800 UT that was superimposed on a decreasingtrend. Further variations of magnetograms in the subsequent half hour suggest the possibility of

GOES 2

further

intensifications.

A time

of about

0807

UT

seemssignificant,however,basedon a Bx increase

at Boulderand Tucson,largeBy c.hanges at

1981-025B 10 •• 145-160

I

KeV

protons

Scatha10 • ...--"•'•

I

104-189 KeV protons

Scatha

numerous stations, and a decrease m energetic particles at spacecraft 1982-019A at ~0300 LT followed by an increase at 0813 UT. It is, however, difficult to distinguish spatial motion of current systems from time changes in intensity during this very complicated interval. A rather

minor

intensification

occurred

at

a time

which is tentatively placed at 0923 UT. The event was apparently centered near 0100 LT as deduced

from positiveB, at Victoria (not shown)and Newport and n•gative Bv values at Rapid City, and

I

stations to the east along with positive H bays at Boulder, Tucson, Lompoc, Rapid City and Newport. Little evidence was seen for the auroral electrojet,

although therewasa largenegative By perturbation

Scatha

ß I

47-66 KeY

electrons 1 ß '

5

at Barrow. GOES 2 (0230 LT) saw a small

I ! ,r,/v• I ,•

dipolarization beginning at 0923, and spacecraft

I 6

7

8

9

10

11

12

13

4

HOURS

March 25, 1983

Fig, Z2, Energetic electron and proton intensities aud magnetic field magnitude and latitude angie from five different synchronous orbit spacecraft, ¾ertica]

dashed

lines

indicate

substorm

onset

times

deduced from these data aud ground magnetograms,

1981-025B saw a small increase in energetic protons at 0928 UT. SCATHA at 1930 LT saw a particle increase and a small dipolarization, but this may be unrelated to the event near midnight at such an early local time. Another intensification was marked by positive B perturbations at Mount Clemens and Camp

D•uglasat 0951UT and at RapidCity, Boulder,

and Tucson

further

to the west

a few minutes

later.

NegativeB. occurredat San Juan (not shown)and

Sudbury; tl•esenegative perturbations weremore

gradual but occurred at an earlier time of about 0946 UT. Positive bays occurred at Fredricksburg

and San Juan (not shown)and Sudburyat 0951 negativeperturbationat Great Whale River (Figure

UT.

This event therefore appeared to be centered

0650. The wedgeapparently wascentered near10ø longitude or at least2 hourspastlocalmidnight.

0400LT. Thesegroundobservations aresupported by the satellite data. GOES5 near0500LT

This interpretation is consistent with an observed dipolarization at GOES 5 that began slowly near 0650 UT when this spacecraft was near the local time of the onset and an increase in energetic

detected a theta increase at 0946 UT, but GOES 2 near 0300 LT observed only a small increase near

this time.

electrons

Spacecraft1982-019A detected a large increasein electronsand ions but not until 0959 UT (0500

11a)andevenSt Johns (Figure 10a)beginning near near350ø,whichplaces it at theunusual location of

and

ions

at

1982-019A

at

0655

UT

at

a

location only 7 ø west of GOES 5. This event had little effect on particles measured by 1981-025B in the region just before midnight. The data are missing at GOES 2. Pi 2 pulsations and magnetogram component changes indicated that the next important intensification

occurred

at 0750

UT.

Positive

B.

perturbations occurred at Newport(Figure10b)•nd Lompoc(Figure 3), and negativeB. perturbations occurred at Rapid City, Camp Douglas, Mount

Spacecraft 1981-025B near 0100 LT

observed electron and ion increases at 0949 UT.

A very significant intensificationoccurredat 1023 UT.

Most mid-latitude

stations within

North

AmericadetectednegativeBy perturbationswith the

stations more to the west exhibiting the reversal from the earlier positive perturbation at a somewhat later time.

The likely local time of the current

wedgecenter would appear to be near the west coast of North America or near 0200 LT.

This

Fairfield et al.: Substorms, Plasmoids, Flux Ropes

event,in contrastto the intensifications duringthe previous2 hours,produced very largenegativebays

at the stations above 84ø geomagneticlatitude. Onset times were College 1027 UT, Yellowknife 1025

UT, Churchill1024 UT. There was a small dipolarization at SCATHA 12130LTI at 1019UT.

GOES 2 near 0300 LT detected high levels of fluctuationsbut no clear dipolarization,and GOES 5 had moved too far to the east to see any effects.

The 1981-025Benergeticelectronintensityincreased

15,151

Hones,Jr., D. J. McComas,and R. D. Zwickl, Correlateddynamicalchangesin the near-Earth and distant magnetotailregions:ISEE 3, Geophlrs.Res., 89, 3855-3864,1984a.

Baker,D. N., S.-I.-.l•kasofu, W. Baumjohann, J. W.

Bieber,D. H. Fairfield,E. W. Hones,Jr., B. H. Mauk, R. L. McPherron,and T. E. Moore, Substormsin the magnetosphere, chapter 8, Solar TerrestrialPhysics- Presentand Future, NASA Publ. 1120, pp. 8-1 to 8-55, NASA Washington

D.C. 1984b. at 1027UT 10130LTI, and the inferredfield angle Baker,D. N., R. C. Anderson, R. D. Zwickl,and J. computed from the electrondistributions showeda A. Slavin,Averageplasmaand magneticfield largeincrease at the sametime. Energeticelectrons variations in the distant magnetotail associated at spacecraft 1982-019Aalsoshowedan increaseat

1030 UT even though this spacecrafthad almost

reached the dawn meridian.

with near-Earth substormeffects,J. Geophlrs. Res., 92, 71-81 1987.

By 1200 UT the SCATHA spacecraft was approaching the midnightmeridian. SCATHA saw a classical stretching of the field [Bakeret al., 1981] and disappearance of energetic particlesfollowedby

Bam•', S.•., R. •. Anderson, J. R. Asbridge, D. N.

Thislargeonsetproduced negative, mid-latitude B,

Birn,J., and E. W. Hones,Jr., Three-dimensional

anabruptdipolarization andparticle returnat 1259.

valuesthroughoutthe North Americansectorand•s far westas Honolulu (not shown). A largepositive

H at GuamwithnoB• perturbation marked this longitude (2200LT) as nearthe onsetmeridian.

Largenegative H baysareseenat College and Canadian observatories to the east. The other

synchronous orbit spacecraft had movedwell to the eastby this time and saw only delayedparticle increases.

In conclusion, we have identifiedseventimesof significant substormonsetsor intensifications as are markedon the figures. It shouldbe appreciated, however, that duringthis complexintervalof

ongoing activitythereare probably othertimesof

increases in field-alignedcurrentsystemswith lesser

magnitudes and smallerspatialscales that can affect individualobservingpointsboth on the groundand in space. We feel that the seveneventsare the

most reasonablechoicesfor comparisonswith data in the deeper magnetotail. Acknowledzments. Portions of this work were

done at the Jet PropulsionLaboratory,California Institute of Technologyunder contractwith NASA. At the University of Iowa this researchwas

Baker,W. C. Feldman,J. T. Gosling,E. W. Hones,Jr., D. J. McComas, and R. D. Zwickl, Plasmaregimesin the deepgeomagnetic tail:

ISEE3, Geophlrs. Res.Lett.,10,912-915, 1983. computermodelingof dynamicreconnection in the geomagnetic tail, J. Geophlrs. Res.,86, 6802-6808, 1981.

Burlaga,L. F., Magneticcloudsand force-freefields

with constant a, J. Geophlrs. Res.,9•3,7217-7224, 1988.

Christon,S. P., D. G. Mitchell, D. J. Williams, L. A. Frank, C. Y. Huang, and T. E. Eastman,

Energy spectraof plasmasheet ions and electrons from ~50 eV/e to ~1 MeV duringplasma temperaturetransitions,J. Geophlrs.Res., 9•3, 2562-2572, 1988.

Clauer,R. C., and R. L. McPherron,Mappingthe local time-

universal time development of

magnetospheric substormsusingmid-latitude magneticobservatories, J. Geophlrs.Res., 7.•9, 2811-2820, 1974.

Fairfield D. H., Averageand unusuallocationsof the Earth's magnetopauseand bow shock,J• Geophlrs.Res., 76, 6700-6716,1971.

Fairfield,D. H., Gr•al aspects of the Earth's magnetopause, Magnetospheric BoundaryLayers, Eur. SpaceA•:encySpec.Publ., ESA SP-148, 5-13, 1979.

Fairfield,D. H., Magnetotailenergystorageand the variability of the magnetotailcurrent sheet,in supported in part by NASA undergrantsNAGS-483 Mal•neticReconnection in Spaceand Laboratory and NGL-16-001-002 and by the Office of Navel Plasmas,168-177,Geophlrs.Mono•r. Ser. vol 30, Research undergrant N00014-85-K-0404.The work editedby E. W. Hones,Jr., AGU, Washington at Aerospace wassupported in part by the U.S. Air

ForceSystems Com__mand's SpaceDivisionunder D.C., 1984. contract F04701-85-C-0086. The workat UCLAwas Fairfield,D. H., Timevariations of the distant

supported by NASAgrantNAS5-28448. I.G.R.

acknowledges an SERC Research Assistantship.We alsowant to acknowledge the help of the staff of the NSSDC and particularlyH. K. Hills.

The Editor thanks W. Baurnjohannand D. G. Mitchell for their assistancein evaluatingthis paper. References

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D. N. Baker, NASA Goddard Space Flight Center, Code 690, Greenbelt MD 20771. J. D. Craven and L. A. Frank, Department of Physics, University of Iowa, Iowa City, IA, 52242. R. C. Elphic, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024.

D. H. Fairfield, NASA Goddard Space Flight Center, Code 695, Greenbelt MD 20771. J. F. Fennell, Aerospace Corporation, M2-259, P. O. Box 92957, Los Angeles, CA 91009. I. G. Richardson, NASA Goddard Space Flight Center, Code 661, Greenbelt MD 20771.

H. J. Singer,AFGL/PHG, HanscomAir Force Base, MA 01731.

J. A. Slavin, NASA Goddard Space Flight Center, Code 696, Greenbelt MD 20771. B. T. Tsurutani, Jet Propulsion Laboratory, MS 169-506, 4800 Oak Grove Drive, Pasadena, CA 911O9.

1t. D. Zwickl, Los Alamos National Laboratory, Mail Stop D 438, Los Alamos NM 87545.

Slavin, J. A., B. T. Tsurutani, E. J. Smith, D. E. Jones, and D. G. Sibeck, Average configuration

of the distant (