Neutral line model of substorms: Past results and present view

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JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 101, NO. A6, PAGES 12,975-13,010, JUNE 1, 1996
Neutral line model of substorms:Past resultsand presentview
D. N. Baker,• T. I. Pulkkinen,
2 V. Angelopoulos,
3 W. Baumjohann,
4
and R. L. McPherron •
Abstract. The near-Earthneutralline (NENL) modelof magnetospheric
substorms
is reviewed.
The observed
phenomenology
of substorms
is discussed
includingtherole of couplingwith the
solarwind andinterplanetary
magneticfield,thegrowthphasesequence,
theexpansion
phase(and
onset),andtherecoveryphase.New observations
andmodelingresultsareputintothecontextof
thepriormodelframework.Significantissuesandconcerns
abouttheshortcomings
of theNENL
modelareaddressed.
Suchissuesasionosphere-tail
coupling,large-scale
mapping,onsettriggering, andobservational
timingarediscussed.
It is concluded
thattheNENL modelis evolvingand
beingimprovedsoasto includenewobservations
andtheoretical
insights.More workis clearly
requiredin orderto incorporate
fully thecompletesetof ionospheric,
near-tail,midtail,anddeeptail featuresof substorms.
Nonetheless,
the NENL modelstill seemsto providethe bestavailableframeworkfor orderingthecomplex,globalmanifestations
of substorms.
1. Introduction
that a viscous
interaction
between
the Earth's field
and the
solar wind allows closed field lines of the magnetosphereto
Geomagneticactivity is the generic term usedto define be dragged antisunwardcreating an internal convection sysvariationsin the Earth'smagneticfield causedby sourcesex- tem [Axford and Hines, 1961]. The second explanation was
ternalto the Earth. Many differenttypes of activity have that magnetic reconnection at the subsolarpoint allows the
beenidentifiedincludingthe daily variation,the stormtime interplanetarymagneticfield (IMF) to connectto the Earth's
variation,bay disturbances
(substorms),
suddenimpulsesand field. Flow of the solar wind aroundthe Earth drags newly
commencements,DPY or cusp currents, NBZ (northward in- opened field lines antisunward,also creating an internal
terplanetary
field)currents,anda wide variety of magnetic convection system [Dungey, 1961]. Both processesappear
pulsations.
Thesevariationsare causedby fluctuations
in dif- to be involved in the generation of geomagnetic activity
ferentcurrentsystemswithin the ionosphereand magneto- with the reconnection process being much more important
sphere.The fluctuations
are controlledby changes
in the so- for substormsthan the viscous interaction [Cowley, 1982].
lar wind,interplanetary
magneticfield, and/orthe geometri- As the solar wind impinges on the magnetosphere,changcal relation of the sun and Earth [Russelland McPherron, ing its velocity or density and, especially, when the IMF
1973a]. In this review we will be mainly concernedwith changes magnitudeor direction, the magnetosphereundermagnetospheric
substorms,
the physicalprocessresponsible goes substantialalterations. The size of the magnetosphere
for bay disturbances.
is modified, strongionosphericcurrentsmay be established,
Very early studiesof solar-terrestrial
physicsestablished and significant configurational changes can be seen
that geomagnetic
activity is correlated
with the sunspotcy- throughoutthe magnetotail. The substorminitial phase is a
cle (l 1-year period), the position of the Earth in its orbit slow processin which energy is stored in the tail by inaroundthe Sun(6-monthperiod),andthe solarrotationpe- creasingthe magneticflux contentof the magnetotail lobes.
riod (27 days) [Chapmanand Bartels, 1962]. The reasonsfor This phase is typically associatedwith southwardIMF, thus
these correlations was not confirmed until the solar wind was
suggestingmagnetic reconnectionat the front side of the
discovered
at the beginningof the spaceage, and it was magnetosphere.This leads to the additionof magnetic flux
demonstrated
that geomagnetic
activity is caused
by the in- to the lobes of the magnetotail. The next step in the subteractionof the terrestrialmagneticfield with the solar wind storm cycle is the rapid releaseof this stored energy. Evi[Snyderet al., 1963].Two physicalexplanationsfor the cor- denceindicatesthat a large fraction of the plasmapopulating
relationweresuggested
in 1961. The first explanationwas the plasmasheetregion is ejectedtailwardas a "plasmoid"
which propagates
tailwardat speedsof the orderof hundreds
1Laboratory
forAtmospheric
andSpace
Physics,
University
ofColo- of kilometersper second.The appearanceof fast tailwarddirado, Boulder.
rectedplasmaflow is often observedbeyond 20-25 RE geo-
2Finnish
Meteorological
Institute,
Helsinki.
centric distance.This phaseis often associatedwith a promi3Space
Sciences
Laboratory,
University
ofCalifornia,
Berkeley. nent thinning of the plasma sheet beyond about 18 RE dis4Max-Planck-Institut
fiirExtraterretrial
Physik,
Garching,
Germany.tance, while nearer the Earth the field becomesmore dipolar.
5Institute
ofGeophysics
andPlanetary
Physics,
University
ofCalifor- The last phaseof the tail activity is termedthe substormre-
nia, Los Angeles.
Copyright1996 by the AmericanGeophysicalUnion.
Papernumber95JA03753.
0148-0227/96/95JA-03753509.00
covery phase.The onset of the recoveryphaseis identified
by the recoveryof negativebaysin the ground-based
auroral
zone magnetogramsand, often, by high-latitudebay development.At aboutthe sametime in the tail the plasmasheet
suddenlyreappearsas a result of its rapid thickening. Such
recoveryeventsare normally associatedwith the appearance
12,975
12,976
BAKER ET AL.' THE NEAR-EARTH NEUTRAL LINE MODEL
THE SUBSTORM
ENERGY
DISSIPATION
SEQUENCE
•OLAR
WIND
ENERGY
INPU•
SOUTHWARD
CONVECTIVE
IMF
"DRIVEN PROCESS
(GLOBAL)
STORAGE
• DI•IPATION
DISSIPATION
EXPLOSIV•
•
ENERGETIC
PARTICLE
ACCELERATION
PARTICLE
PRECIPITATION
JOULE
HEA•G
PLASMA SleET
HEATING
-!
RInG
CURRENT
INJECTION
PLASMOID
FORMATION
"UNLOADING
,
AURO1/AL
:
LUMINOSrrY
RADIATION
PROCESS"
RETURN TO
SOLAR WIND
l
MAGNETOTAlL
IONOSPHERE
Figure 1. The flow of energyfrom the solarwind into, andthrough,the magnetosphere-ionosphere
system.
of fast earthwardplasmatransport.Figure 1 describesthe energy flow within the magnetosphere-ionosphere
system during the substormprocess.
The substormneutral line model representsan attempt to
place this broad range of observations into a consistent
framework. It was originally conceived independently in
theoretical investigations of large-scale magnetotail instabilities and through interpretation of magnetotail observations. Both observational and theoretical approachesbase
substormactivity on the phenomenon of magnetic merging
which allows magnetic field lines to be severedand subsequently reconnected.Thus magnetic reconnection involves
large scalemass,magnetic flux, and energy transport which
would otherwise be impossible. Magnetic reconnection in
this context refers to processesat a near-Earthreconnection
site, which was originally assumedto be a magnetic neutral
line. This gave rise to the name near-Earth neutral line
expansiontailward and earthwardof the reconnectionsite,
respectively,and the presenceof plasmoidsin the distant
tail. All of these observations
can be related to substorm on-
set by a variety of timing arguments.Furthermore,the reduction of the open magneticflux as evidencedby low-altitude
spacecraftand ground-basedoptical auroral observationsis
often taken as strong supportfor the NENL model.
Despite its many successessuch as its ability to explain
and combine phenomena observed at widely separatedlocations in space,the NENL model has been subject to substantial criticism. Examples include the lack of detailedand repeatedobservationalevidencefor neutral lines and questions
about field-aligned current generation. These criticisms have
been addressedby improved theory and simulations using
three-dimensional
tail
models.
Such three-dimensional
mod-
els seemto producea neutral line as a responseto enhanced
flux loading into the tail, and also show that shearsin the
earthwardflows generatedby magnetic reconnection cause
(NENL) model.
Early theoretical work and continuing numerical simula- the formation of region 1-type field-aligned currents.
Further refinements
of the model include the substormtions have been highly successfulin the explanation of observationalphenomenasuch as tailward flow with southward related bursty bulk flows which are explainedon the basis of
Bz and velocity dispersionwhen the separatrixlayer linked localized magnetic reconnection.Moreover, the energy flow
to a reconnection site crosses the satellite location. Also,
pattern required both globally and locally in the inner tail to
the model explainsquite naturally plasmasheetthinning and power substormevolution has recently been addressed.The
BAKER ET AL.: THE NEAR-EARTH
earthwardtransportof magneticflux requiredto explain the
dipolarization of the magnetic field in the near-tail, and
large-scaleauroral featuressuch as the westwardtraveling
surge,identifiedas the ionosphericfootprint of the western
section of the substormcurrent wedge, have now been included in the NENL
model.
Recent criticism of the NENL model is based primarily on
observations in the inner tail, for example, near geosynchronousorbit, and on the ground.Traditionally, the NF3•
NEUTRAL
LINE MODEL
12,977
nual variationof geomagnetic
activity.They notedthat each
yearthe Earth'sdipolereachesmaximaltilt (35ø) relativeto
the normalto the ecliptic plane at the equinoxes.
At these
times the interplanetarymagneticfield is more likely to
havea largeprojectionon the Earth'sdipoleaxis allowing
magneticreconnection
to generatestrongermagneticactivity.
IMF controlof the ring current(Dst index) was quantita-
tively demonstrated
by Burtonet al. [1975]. Their relation
modelhas coveredprimarily the midtail and far-tail regions was based on the work of Dessler and Parker [1959], who
of the magnetosphereand establishedlarge-scale connec- showedthat the magneticperturbation(AB)producedby a
ring currentat the Earth'scenteris directlyprotions betweenthese regions and the polar cap and auroral symmetric
to the totalenergy(Ur)of particlesdriftingin the
ionosphere.Recent observationalevidence,however,indi- portional
cates that a variety of previously unappreciatedphenomena radiationbelts: AB =2/3 Ur/UB. In this relation, UB is the
energyof the Earth'sfield. Sincethe Dst inoccur in the inner tail region. In this region, substormonset totalmagnetic
to AB, the time rateof change
brings about the rapid dipolarizationof the magneticfield, dexis a goodapproximation
to the rateat whichthe energyof ring
includingstrongmagneticfluctuationswith timescalescom- of Dst is proportional
parable to local ion gyrofrequencies.At the same time, currentparticleschanges.Ring currentenergyis increased
by particleloss dueto
ground-based
optical observationsindicatethat the first au- by particleinjectionand decreased
roral brighteningoccursnear the equatorward
boundaryof the chargeexchangewith neutralparticles.
Later, Perreault and Akasofu [1978] used a somewhat
auroral oval. Furthermore, spacecraftobservations demonstrate the existence of strong spiky electric fields at the similar approachto studythe developmentof magnetic
trappingboundaryin the inner magnetosphere.
Theseobser- storms.However,sinceDst is proportionalto energy they
functionQ(t) mustalso
vations,togetherwith the commonlyobservedsubstormin- arguedthattheringcurrentinjection
jection of energeticparticles in the inner magnetosphere, haveenergyunits.Sinceit wasevidentthat the IMF plays
role, theychosethe magnitude
of the solarwind
have led to other substormmodels, which suggest that the an essential
processthat initiates the substormdynamicalevolution is Poyntingvectoras one factorin the injectionfunction,so
forthestrongdepenapparentlylocatedin the inner tail, inside of ~10 RE from thatQ(t) o•E x H o•VB2. To account
activityon the orientation
of the IMF,
the Earth. This view is further supportedby magnetic map- denceof geomagnetic
of differentangular
gatingfunctions
g(O)
ping of variousauroraland ionospheric
featuresinto the pre- theytrieda number
onset magnetotail, which again suggestsnear-tail activity as a secondfactorin the injectionfunction.Becausetheybelievedmagneticactivitywasdirectlydrivenandthat no enoccurringsimultaneously
with the auroralbrightening.
theyselected
a gating
In this paper we review the observationaland theoretical ergyis storedin the magnetosphere,
function that had finite value for all IMF clock angles. This
basis for the NENL model of substorms. We recount new observationsand modelingresultswhich tend to supportthis allows a finite correlationbetweenactivity indices and the
model.We also examine the issuesand problems that pres- IMF even when the IMF has a northward component.
Two-and-one-half-minuteaverageswere utilized by Clauer
ent difficulties in the NENL model. We conclude with a secet al. [1981, 1983] to investigatethe relation of the AU, AL,
tion that looks to future refinementsof the NENL approach.
and Asym indices to the solar wind electric field at high time
resolution.
2. Global Substorm Processes:The Original
The authors
showed that the AL filter
is 3 hours
long and has the form of a negative bay peaking at about 1
NENL Model
hour [see Baker et al., 1981]. Bargatze et al. [1985]
investigated the possibility that the prediction filter for the
AL index changesin a systematic way with the level of ac2.1
Solar
Wind
and IMF
Control
tivity. They found that at all levels of activity there is a
The first demonstration
of IMF control of substorms was
peak in the impulse responseat about 20 min. In addition,
by Fairfield and Cahill [1966], who establishedthat auroral at moderatelevels of activity there is a secondpeak at about
zone bay disturbancesoccurredduring intervals of southward 1 hour. They interpreted these two peaks in the impulse reIMF. Several years, later Aubry and McPherron [1971] dem- sponseas evidencethat two distinct processescontributeto
the AL index. Since it is the rectified solar wind electric field
onstrated that substorms followed a definite sequence of
growth, expansion,and recovery.A southwardturning of the that is correlatedwith AL, they assumedthat magnetic reIMF (i.e., antiparallel to the Earth's field) is followed by an connectionunderliesboth. Thus they attributedthe first peak
interval during which the tail field systematically increases to daysidereconnection and the secondto nightside reconuntil the onset of the expansion phase, and then decreases nection.
during the expansion phase. Their results showed that the
High time resolution data were also usedby McPherronet
bestAE correlation(a measureof the strengthof bay activity)
al. [1986a] and Fay et al. [1986] to obtain linear prediction
was with the rate at which southwardmagneticflux arrived at filters for the relations between the solar wind electric field
the subsolarbow shock,that is, with VBs (Bs = -Bz if Bz <0,
and dynamic pressureand the Dst index. The pressurefilter
=0 if Bz >0). Furthermore,the best prediction of AEwas ac- was a delta function at approximately zero delay when the
complished by regressing the current and two preceding solar wind input was first propagatedto the magnetopause.
The Dst filter was identical to that obtained by lyemori et al.
hourly valuesof VBs againstthe currenthourly AE.
Subsequently,Russell and McPherron [1973a] used the [1979] although with higher time resolution. The authors
hypothesisof magnetic reconnectionto explain the semian- also obtained a filter for the injection function defined as
12,978
BAKERET AL.: THE NEAR-EARTHNEUTRALLINE MODEL
Another consequenceof magnetic field lines passing
through the magnetopauseis the application of the solar
peakappears
at thesametimedelayas the first peakin the wind motional electric field to the magnetic field lines of the
AL filter andpresumably
indicates
that ring currentinjection tail lobes and to the polar caps. It is this electric field that
occursin response
to the sameprocess
thatdrivesthe initial drives the cross polar cap portion of the region 1 ionospheric current closure. It also causesthe field lines of the
response
of theAL index,that is, daysidereconnection.
tail lobes to slowly drift toward the plasma sheet. Close to
the Earth this drift begins soon after dayside reconnection
2.2. Growth phase Sequence
begins. However, at the distant X line (100-200 RE down
The clearest manifestation of the substorm occurs when
tail) it may take 30-40 min before it begins. Recent obserthe field is southward for about 1 hour. Under such condivations of reconnection in the tail during substorms by
tions, three phasesof the substormcan be identified:the ionospheric radar [Blanchard et al., 1995] show that recongrowthphase,the expansion
phase,and the recoveryphase. nection of open field lines does not begin until shortly after
The growthphaseis an interval duringwhich energyis ex- the expansion phase onset. Thus it appears that recontractedfrom the solarwind and storedin the magnetotail and nection does not commenceat the distant X line during the
ring current,while it is simultaneously
dissipatedin the substormgrowth phase and so magnetic flux accumulatesin
Q(t)= [dDst/dt+ Dst/'rWhen•: wastakento be 8 hoursthe
filter wasa Gaussianpulsecenteredat 20-min delay. This
ionosphere
[Bakeret al., 1984a].Duringthegrowthphasethe the tail lobes.
During the growth phase, an increasing amount of closed
magnetosphere
undergoesa sequence
of changesin configuration that eventually lead to a global instability and the magnetic flux is removed from the dayside which then appearsin the lobes as open flux. These changesare apparent
explosivereleaseof energyduringthe expansion
phase.
Finite ionospheric
resistancedissipatesenergyassociated in the ionosphereas an increasein the size of the polar cap
with the region 1 and 2 currentsas they closethroughthe [Akasofu, 1968; Baker et al., 1994a]. The auroral oval moves
ionosphere.
Also, the self-inductance
of the field-alignedcur- equatorward[Feldsteinand Starkov,1967] and the aurorasberents,in conjunctionwith the resistance,retardsthe rate at come more intense.Multiple auroralarcs are observedin the
which the currentsdevelop. As a consequence,
the iono- auroral oval all drifting equatorward across the oval
sphericcurrentscannotdevelopas rapidlyas needed
to force [Akasofu, 1964]. The strength of the DP-2 currentincreases
nightsidemagneticflux to returnto the daysideat the rate with time (the AU and AL indices increase), with more and
requiredby daysidereconnection[Coroniti and Kennel, more current closing outwardfrom the Harang discontinuity
1972a]. To compensate,the reconnectionregion moves along field lines. This current continues westward in the
earthwarderodingthe daysidemagnetopause
[Aubryet al.,
equatorial plane closing along field lines in the afternoon
1970; Holzer and Slavin, 1978]. Magnetopausepressurebal- sector.The magnetic effects of this current are observedas a
anceis maintainedby a reductionof magnetosphericmag- partial ring current and are measuredby an increase in the
netic field causedby the magneticperturbationsof the re- asymmetry (Asym) index.
As the growth phase progresses,the near-Earth current
gion 1 and2 currentsystem[Coronitiand Kennel,1972b].
sheet
becomesthinner and more intense causingmuch of the
On the daysideof the Earth, erosion causesthe cross section of the magnetosphere
to decrease,while further from the magnetic flux normally crossing this region to close at
Earth the addition of magneticflux to the lobes causesit to greaterradial distances[e.g., Bakerand Pulkkinen, 1991].
increase [Maezawa, 1975; Baker et al., 1984b]. These The field becomesvery weak and tail-like in the equatorial
changes produce increased flaring of the magnetopause plane [Kokubunand McPherron, 1981]. Energeticparticles
[Fairfield, 1985]. The increasedangle of attack of the solar normally drifting though this region with 90 deg pitch anin particle
wind on the flared magnetopauseenables the solar wind to gles move closerto the Earth causinga decrease
exert greaterpressurethan before. This enhancedpressureis fluxes. Particles of small pitch angles are less affectedso
balancedby increasedmagneticfield pressurewithin the tail that pitch angle distributionsbecome highly elongatedor
[Bakeret al., 1978].
lobes. Lobe magnetic pressureis transmittedto the plasma "cigar-shaped"
Baker and McPherron [1990] examined the extreme thinsheetcausingit to thin. At the same time, plasma and magnetic flux transportedthroughthe plasmasheetto the day- ning of the cross-tailcurrentsheet that occursremarkably
closeto the Earthduringthe late growth phase.They argued
side is associatedwith thinning as well.
on closedfield lines in the more
In addition to producingthe above effects, magnetic re- that magneticreconnection
connectionopens the magnetosphere.Field lines from the distanttail couldhelp drive this extremethinning.McPherpolar caps pass through the magnetopause
into the solar ron et al. [ 1987], Sergeevet al. [ 1990a], and Sannyet al.
wind. Tailward of the neutral points these field lines pass [1994] presentedobservationsdemonstratingsuchextreme
throughthe portion of the magnetopause
wherethe currents currentsheetthinning and usedempirical modelingmethods
flow in the dusk-to-dawn direction. Within the boundary
to assessthis behavior. Pulkkinen [1991] used modifications
there is a sunwarddirectedforce on the plasma, and by reaction, the solar wind applies a tangential force to the
boundary.This drag pulls the Earth'stail and its contentsan-
of the Tsyganenkomodelsto studymagneticfield and current configurations
near substormonset.This work was later
extendedto providea time-dependent
modelingof the nightside currentsand field configurationsthroughoutthe entire
growthphasesequence[Pulkkinenet al., 1991a, 1992]. In
Figure2, for example,is shownthe integratedcross-tailcurrent, K, and the magneticfield configurationin a particular
magnetic meridian for the CDAW-9 analysis period
[Pulkkinenetal., 1991a]. The upperpanels show the values
tisunward.To maintain equilibrium, the Earth must pull
harderon the tail. This is accomplishedby the tail current
movingcloserto the Earth so that the gradientsin the fringing field of the tail lobes can interactmore strongly with
the Earth'sdipolemoment[Unti and Atkinson, 1968' Siscoe,
1966].
BAKER ET AL.: THE NEAR-EARTH NEUTRAL LINE MODEL
12,979
5
K[•-] a)50
300
Z[RE]
200/•tal
....
o
/ ".......•Ring
100
[.......
']'2:.-......-..-..2............•'5
Lon•146'
TS,,Kp•5-mAø
-5
i
K[•-]
5
5
b)
300
i
Z[RE]
200
0
100
-5
-5
0
10
20
30 0
Radialdistance[ RE ]
i
i
i
5
10
15
20
Radialdistance[ RE ]
Figure 2. (left) Currentintensitiesintegratedover the currentsheetthickness(in mA/m) in the ISEE 1 meridian(a) in the
basicT89 model,and (b) in the modifiedmodelwith maximalparametervalues.Total current(solid line), ring current
(dottedline), andtail current(dashedline) are shownfor bothcases.The addedcurrentsheetis shownby dashed-dotted
line.(right)Field
lineprojections
to theISEE1 meridian
(146ø)computer
using
(c) basicT89model
and(d) themodified
model.The regionof chaoticelectronmotionfor 1 keV electronsis shownhatched[from Pulkkinenet al., 1991a].
of K (Figure 2) and the field lines (Figure 2) for the unmodified Tsyganenko[1989] model.The lower panels (Figure 2b
and 2d) show the correspondingresults for the modified,
time-dependentmodel which is constrainedby multisatellite
observations in the midnight sector. Tremendous field
stretchingand cross-tailcurrent enhancementleads to a very
the vertical magneticfield throughthe currentsheet. When
some portion of the currentsheet reachesan appropriate
threshold,reconnectionbegins spontaneously
at the center
of the sheet. At the instant of formation the X line is of
limited azimuthal extent and connected to an O line at each
end. As reconnection
proceeds,closedmagneticfield lines in
the plasmasheetare reconnected
formingloopsaroundthe O
line and shorterclosedfield lines movingearthwardfrom the
2.3. Magnetotail
Storage and Release
X line [Fairfield, 1984]. Within the plasmasheet,plasma
Essentially all of the energy to drive substormscomes flows verticallyinto the X line and then horizontallyout of
from solar wind input; this input is greatly enhancedwhen the X line. On the tailwardsideit fills the loopsof field surthe IMF tums southward. However, as discussed above, there roundingthe O line causingit to move tailwardaway from
may also be significant solar wind coupling even under the X line stretchingthe remainingclosedfield lines of the
the plasmoid.Figure3a is
mildly northward IMF conditions [Perreault and Akasofu, plasmasheetthat still surround
in the
1978;Bakeret al., 1986].A notablepoint [e.g., Bakeret al., the classicdiagramshowingthis substormsequence
thin current sheet structure.
1984a] is that the directly driven aspectsof substormsare
manifestedin the ionosphereas a substantialdissipation associatedwith the global convection process. As this global
convective dissipation occurs, the magnetotail is also loaded
with energy thus making "storage", or the substormgrowth
phase, an integral part of the driven aspectsof substorms.
The changesin the distributionof magnetic flux occurring
during the substorm growth phase cannot continue indefinitely. Eventually, no flux would cross the nightside equatorial plane and the plasmasheetwould thin to the scale of an
ion gyroradius.Alternatively, magneticflux wouldbe completely eroded from the dayside.Before either of these happens a global instability develops that allows closed field
lines to return to the dayside. In the NENL model this instability is the onset of reconnection [e.g., Schindler, 1974].
At first this reconnectionis on closedfield lines, but quite
soon it moves into the open field lines of the lobe, thus adjusting the balance of open and closedfield lines.
The onset of reconnectionis attributedto growth phase
thinning of the plasma and current sheet, and reduction in
magnetotail [Hones, 1984].
The substormcurrentwedgeis a predictedconsequence
of
a localizedX line [McPherron et al., 1973]. When the X line
forms, the net current across the tail in the meridian of the X
line is decreased[Birn and Hesse,1991]. The original current
may be divertedin severaldifferent ways: earthwardof the X
line enhancingthe inner edge of the tail current[Bakerand
McPherron, 1990], tailwardof the X line alongthe centerof
the plasmoid,and to the ionospherealong field lines connectingto the ends of the X line. This is the beginning of
the substorm.currentwedgeas shownin Figure 3b. Plasma
flowing earthwardfrom the X line will occupy a narrow
channelwith strongvelocity shearsat the edge of the channel. The sense of these shears is such that tail current will
be divertedinto the growing substormcurrentwedge.When
the flow reachesthe strongfield in the inner magnetosphere,
it is slowedand the embedded
field is compressed.
The flow
then splits and begins to move aroundthe Earth. The vorticity associatedwith the splittingof the flow toward dawn and
dusk diverts additional
tail current into the substorm current
12,980
BAKER ET AL.: THE NEAR-EARTH NEUTRAL LINE MODEL
PLASMA
SHEET
CONFIGURATION
CHANGES
(•
DURING
A SUBSTORM
1•$45W
/-Field aligned
s\•
Tail
fie•'d"-
\ •. collapse
---.,,
Inne.redge "•
Figure3. (a) Theclassic
diagram
of themagnetotail
substorm
sequence
including
plasmoid
formation
[fromHones,1984].
(b) Theclassic
diagram
of thesubstorm
current
wedgerelationship
to near-Earth
current
disruption
[fromMcPherronet al.,
1973].
wedge.Numericalsimulationsshow that the maximumcur- magneticrecords.As the flow develops,the moving plasma
(westward)electric
field
rent diversion occurs behind the interface between the dipo- polarizescreatinga dawn-to-dusk
larizedregionandthetail field of theplasmasheet[Birn and acrossthe flow channel. This electric field is propagatedto
the ionosphereas an Alfv6n wave. The field-alignedcurrent
Hesse, 1991].
An importantconsequence
of the initiation of earthward associated with the wave initiates the substorm current
flow is the generationof Pi 2 pulsations[Baumjohann
and wedge.When the electric field reachesthe ionosphere,it
However,inGlassmeier, 1986, and referencestherein]observedin ground startsionosphericplasmaflowing equatorward.
BAKER ET AL.: THE NEAR-EARTH NEUTRAL LINE MODEL
ertia retards the flow, and the electric field is mostly reflected. The initial wave reverberatesbetween the plasma
sheet and ionospherea number of times eventually establishing the dc current necessary to drive the equatorwardflow
against ionospheric resistance.
Particle precipitation is expectedin the turbulent region
of stagnation and flow diversion behind the interface between dipolar and tail-like fields. This precipitation creates
the unstructuredaurora of the auroral bulge [Nakamuraet al.,
1992]. The interface projects as the poleward edge of the
auroral bulge [see Rostoker, 1991]. Subsequentearthward
flow addsadditional magnetic flux to the region of dipolar
field and the interface moves tailward. Its projectioninto the
ionosphere moves polewardexpanding the region of bright
luminosity. Within the auroral bulge ionospheric conductivity is much enhancedrelative to the surroundingionosphere.
A westwardelectric field producedby the high-velocity
earthwardflow projects into this region and drives Pedersen
currentswestwardand Hall currents poleward. Becauseof the
strong conductivity gradients at the latitudinal edges of the
auroral bulge, the Hall current cannot close in the ionosphereand the bulge polarizes becoming a Cowling channel
[Coroniti and Kennel, 1972a; Baumjohannet al., 1981]. At
the westernend of the channelthere is a strongupward fieldaligned current returningdiverted tail currentto the equatorial
plane. Above a threshold current density the magnetosphere
is unable to supply sufficient electrons to carry the required
current, and an instability develops creating a field-aligned
potential drop [Knight, 1973]. This potential accelerates
electronsdownward causingthe auroral surge, and accelerates
oxygen atoms upward injecting them into the magnetosphere
[Daglis et al., 1991]. A similar effect can occur in the outward sheetof field-aligned current at the poleward edge of the
auroral bulge. At locations where the outwardcurrent density
is sufficiently high, potential drops can develop and surge-
12,981
plasmoid continues to grow as its central axis moves radi-
ally away from the Earth. In the ionospherethe plasmoJd
and the X line that defines its inner edge are not visible.
However, field lines that map just earthwardof the X line
and just tailward of the last reconnectedfield lines (well beyondthe O line) mustcontainplasmaof quitedifferentproperties. Within 5-15 min, reconnectionreachesthe boundary
of the plasmasheet,and the last closedfield lines begin to
be severed [Coroniti, 1985; Baker and McPherron, 1990].
Plasmain the ionospherebegins to flow acrossthe polar
cap boundary.At this time the near-EarthX line connects to
the distantX line and the plasmoidbeginsto be enfoldedby
recently reconnectedlobe field lines. The plasmoid is then
free to move tailward. As it departs,it createsa thin plasma
sheet extending some distance tailward of the original X
line. At the center of this sheet southwardmagnetic flux is
transportedtailward. At the edges of the sheet tailward flow
from the X line becomesfield-aligned.
If the IMF remains southward, the above situation could
remain stableprovidedthat closedflux flows out of the dipolar region to the daysideas fast as it is added.This may explain the observations of convection bays in which strong
electrojet activity continues without evidence of substorm
expansion[Pytte et al., 1978; Sergeev et al., this issue]. It is
more likely, however, that the flow channel earthward of the
X line will be clogged by reconnectedflux on the night side.
This situation is the inverse of dayside erosion, that is, the
nightsidemagnetospheregrows more dipolar by accretion of
closedfield lines. If the IMF turns northward, daysidereconnection stops and the internal convection transporting flux
to the daysideslows down. Again, it might be expectedthat
the X line
will
move
tailward
to make
room
for additional
closed flux. In either case, as the X line moves away from
the Earth the plasma flow reverses locally as the X line
passesand earthwardof the X line the plasma sheet expands.
like forms will result.
This is the beginning of the recovery phase.
At distancesbeyond the interfacebetweendipolar and tailThere is no reason to expect that reconnection will allike field lines (but earthward of the X line), the plasma ways continue sufficiently long at a given X and O line pair
flows associatedwith the near-EarthX line project into the to reach the plasma sheet boundary. If it stops, a small
ionosphere as equatorwardflows. Above and below the X plasmoid will be producedin the plasma sheet. Closed field
line plasma flows toward the neutral sheet also projecting as lines that wrap aroundthis structuremay provide sufficient
equatorwardmotion. Most auroral arcs probably map within tension to overcome the radial pressure gradients in the
this active region. The details of the arc formation are likely plasma sheet and will pull it earthward. In this case, southdictatedby low-altitudeprocessessuchthat individualarcsdo ward flux will appearto be transportedearthwardeverywhere
not necessarilyhave a correspondingstructurein the magne- tailward of the O line. Such events may explain pseutotail. In the ionosphere the distinguishing characteristicof dobreakups.In the framework of the NENL model, these are
this phaseof the substormis the absenceof flow acrossthe defined as bursts of reconnection that do not sever the last
separatrixbetween open and closedfield lines; that is, there closed field lines and hence do not producea detachedplasis not reconnection of open field lines. Ionospheric plasma moid. If subsequentlya new X and O line pair forms earthin the polar cap will be flowing antisunward,but the bound- ward of the first, and the X line succeedsin reconnecting
ary of open and closed field lines moves equatorwardat the through the plasma sheet a disconnectedplasmoid with comsame rate as the plasma convects on either side of the plex internal structurewill be producedand travel down the
boundary. Above and below the X line the plasma sheet tail. These entities would be expected to evolve with adjathins as closed field lines are progressively cut, and their cent O lines attracting each other and eventually coalescing
plasmais acceleratedboth earthwardand tailward [Nishida et into a single O line [Richardsonet al., 1987]. Conceivably
al., 1981].
several X and O line pairs could exist simultaneously, alAs reconnectionproceedswithin the plasma sheet, fewer though how their flows would interact and affect each other
and fewer closed field lines separate the X line from the if they were all active is not clear. Once disconnectiontakes
separatrixdefining the plasma sheet boundary. In the iono- place any events occurringwithin the plasmoid would be inspherethe auroralbulgeexpandspolewardas well as east- visible in the ionosphere.earthward of the disconnectionthe
west. If the plasma density near the plasma sheet outer edge earthward flow and northward flux make it unlikely that addiis low, precipitation can be quite weak, and thus it is not tional X and O lines would form.
As shown in Figure 1, a key ingredientof any viable subnecessarythat the visible bulge reachesthe boundary. The
12,982
BAKER ET AL.: THE NEAR-EARTH NEUTRAL LINE MODEL
storm model is the explosive dissipation that occursin the
expansion phase. Initially at least, the dissipation is more
localized on the nightside of the Earth than is the storage,
or loading, phase. The dominant ionosphericenergy dissipa-
auroral activity moves from the (pre-midnight)midnight sector to the morning sector, where diffuse pulsating patches,
pulsating auroras, or omega bands [e.g., Davidson, 1990;
Opgenoorth et al., 1983] are often observed.In the AE index,
tion is in the form of Jouleheatingand particle precipita- a two-step behavior can be identified: A decreasein the action. In the magnetotail, effects includeplasma sheet heat- tivity is causedby the substormcurrent wedge and auroral
ing, ring current injection, energetic particle acceleration, bulge decay. The subsequentslower decreaseis due to the
and plasmoidformationandrelease.In additionto plasmoids morning sector oval activation, which maintainsthe activity
that carry magnetotailenergy back to the solar wind [Hones at an elevated level.
et al., 1984a],a significantportionof the storedenergymay
The precipitation patternsin the morning sector are conalso be so far tailward that it is not geoeffective(and there- siderably different from those found in the evening sector
fore this far-tail energyis returnedto the solar wind as well). during growth or expansion phases. In the morning sector,
Hesse[1995] has arguedthat lobe energybeyondthe near- the Hall conductivity is high, whereasthe Pedersenconductail reconnectionsite does not contributesubstantiallyto tivity remains at a relatively low level. This results in connear-Earth substorm effects.
ductivity ratio of-4 broadly throughoutthe morning sector,
which is about twice the typical values (other than in arcs)
foundin the e•,eningsectorexpansionphase auroraliono2.4.
Recovery
Phase
Within the NENL framework,the recovery phase begins
when the plasmoidstartsto retreattailward [e.g., Honeset
al., 1984a]. In the midtail region (15-30 RE), the rapid
plasma sheet expansion [Sauvaudet al., 1984] and earthward
plasmaflow [Hones, 1992] have been interpretedto be signatures of the recovery phase. Statistically, such plasma
sheet expansionsare found over a large range of z values,
indicatingthat the thicknesschange is quite large. On average, the plasma sheet recovery was found to occur about 45
min after the substorm onset as timed from the AE index.
These plasma sheet expansions were also found to correlate
with energetic particle flux enhancementsat geostationary
orbit.
Such enhancements
were found in all local
time
sec-
tors other than local noon, and they were interpretedto be
causedby relatively small magnetic field reconfiguration
events associatedwith the midtail plasma sheet expansions
[Baker et al., 1994b]. The tailward motion is further supported by the observationsof plasmoidsor traveling compression regions in the distant tail [Slavin et al., 1993] as
sphere[Opgenoorthet al., 1994]. This suggeststhat the precipitation during the recovery phase into the morning sector
is more energetic than that into the evening sector during
earlier substormphases(see Plate 1).
The ionospheric recovery phase signatures need not always be temporally separatedfrom the expansion phase features: Often, expansion phase auroral forms are still visible
in the evening sector simultaneously with recovery phase
characteristics already appearing in the morning sector
[Pulkkinenet al., 1991c]. On the other hand, recoveryphase
signaturesmay be observable at higher latitudes and in the
morning sector, while at lower latitudes and in the evening
sector signaturesof a new growth phase are already visible
[Pellinen et al., 1992].
During many recovery phases,a new discreteprecipitation
region is formed poleward of the preexistingauroral oval. As
shown in Plate 2, this "doubleoval" is separatedfrom the
main oval by a few-degree-widegap almostvoid of precipita-
tion [Elphinstoneet al., 1995b]. Magnetic field modelingresults indicate that the equatorward(preexisting) oval, together with the hard precipitationand omega bands, maps to
the inner magnetotail close to geostationary orbit
[Pulkkinenet al., 199!c]. The precipitation gap is associated
will be discussed in section 3.6.
with the midtail region (-20-40 RE) but also the poleward
In the near-Earthtail, the recovery phase appearsas the oval maps mostly within lunar distance [Pulkkinen et al.,
decayof the substormcurrentwedgeand the subsequent
slow 1995b]. From the magnetic field pattern suggestedby the
recoveryof the magneticfield towardthe quiettime configu NENL model, it is not clear what causesthe region of weak
ration after the neutral line has started its tailward motion.
luminosity between the two bright bands. It could be enviThe rebuildingof the near-tailcurrentsystemhas been found sioned that the more poleward band maps to subsequently
to last for more than 2 hours.Perhapssurprisingly,both the larger distancesas the neutral line retreats, but this remains
start time of the field recovery and its duration seem to be an open question.
controlledby internal magnetosphericprocessesrather than
Figure 4 shows a comparison of the different timescales
variationsin the IMF direction [Pulkkinenet al., 1994]. How- associatedwith recovery phase observationsin various magever, the final state strongly depends,of course,on the ex- netosphericregions. The midtail recovery occursrelatively
ternal driving conditions.
rapidly, over a period of few tens of minutes [Baker et al.,
Using ground-baseddata, several other definitions for the
1994b], whereas the ionospheric and near-tail observations
substormrecovery phasecan be found: It is often defined as show a slow and gradual decay toward the quiet time state
the time when the substorm electrojet starts to decay rPulkkinenet al., 1994b]. Both the similar recovery times(maximum in the AE index) or as the time when the auroras cales and magnetic mapping studiessuggestthat the mornreach their most poleward extent [see Pellinen et al., 1992]. ing sectoractivation is connectedwith the near-EarthmagBoth usually appear before the tailward retreat of the neutral netic field recovery (see Figure 4). It could be envisioned
line [Baker et al., 1994b], whereas these two definitions that the increasing current system drives high-energy partiroughly coincide.
cles into the loss cone giving rise to the auroralpatterns in
The auroral behavior during the recovery phase shows in- the morning sector. However, it is not clear either what the
teresting features whose significance to the NENL scenario precipitation processis or what in the tail causesthe sighas not yet been fully clarified. During this period, the nificant dawn-duskasymmetryobservedin the auroral oval.
BAKER ET AL.' THE NEAR-EARTH
Evening Sector
1986-04-01
1842
NEUTRAL
LINE MODEL
Evening Sector
UT
1986-04-01
1904
Morning Sector
UT
1986-04-03
•Height-integrated
conductivity
60 Height-integrated
conductivity
12,983
0147
UT
Height-integrated conductivity
• Hall
• I •Pealersen
•
40
''
o
.
68
70
72
68
Latitude of field line at 1t30 kin
70
72
Latitude of field line at 100 km
68
70
72
Latitude of field line at 100 km
5 Ratio
between
HallandPedersen 5 Ratio
between
HallandPealersen
Ratio between Hall and Pedersen
4
3
2
1
0
68
70
72
Latitudeof field lineat 100km
68
70
Latitude of field line at 100 km
72
68
70
Latitudeof field lineat I00 km
Plate1. Vikingimagesillustrating
the(top)auroralsituations
duringthreeEISCATlatitudescansthroughan eveningsector
arc(ESA),a westward
travelingsurge(WTS),andintensemorningsectorprecipitation
(IMSP).Theseare compared
with
the (middle) latitudedistributionsof height-integrated
Hall and Pedersenconductivities,and (bottom)their ratios as derived
from EISCAT measurements
[from Opgenoorthet al., 1994].
3. Recent Observations
With the NENL Model
3.1.
Linking
Tail
groundradar systems.These tools have been combined with
and Concerns
and Low-Altitude
Substorm
Effects
Within the last decadeor so there have been many im-
the latestdata distributionand data analysis tools to permit
major stridesin the study of substorms.Individual researchers usingthe latest in graphic visualization tools as well as
group participantsin CoordinatedData Analysis Workshops
[e.g., McGuire et al., 1993] havebeenable to performextensive multispacecraftstudies[Lopezet al., 1993; Pulkkinenet
al.,
1991a].
provements
in space-based
and ground-based
observationsof
Severalauthorsover the past several years have come to
magnetosphericprocesses.This has includedglobal auroral the conclusionthat substormsinitiate in the inner magnetoimages, imaging riometers, arrays of all-sky cameras,and sphere(6-8 RE). They seemto acceptthe notion of a highly
•... ?•.•¾-- •..• -::
!•'•;•;•E •': *•'
!•??•• •' ',. •'...•
•E
--'5-.',
'
•,•.i:•'* 0
-.•..... ,..•
'
ß
.
80
80
.
•'
•
"2
0
3
• 1822'-'1•2:UT
I
AE
ooo
.
2
.
• ..,
_ __e .........
'.3
0759-08• UT
April
--I ...................
I
I '' 'I
800
600
400
200
I
o
I
ß.
.
I
20:00
15:oo
I
01:00
:;•..
•
. •:,.
>•::,, ..:...•
22':-2334
UT
,,
1417-1458 UT
06:00
,
....
11:00 UT
..
1213'•1•6
UT
Plate 2. Illustrationof the doubleauroraloval seenin Viking imagesduringa seriesof substorms
on April 9-10, 1986 [after
Elphinstoneet al., 1995a].
stressed,tail-like configuration near the Earth [Kaufinann,
cessesare poweredfrom an outsidesource(a neutralline lo-
1987; Baker and McPherron,
cated tailward of the near-Earth current sheet disruption region).
1990; Baker and Pulkkinen,
1991] developing during the growth phase as a prelude to
substorm
initiation.
Several
studies
have
used AMFFEJCCE
data to argue that the thin, intensecurrent sheet at the inner
edge of the plasma sheet rapidly and violently "disrupts"at
substorm onset [Takahashi et al., 1987; Lui et al., 1988,
1990; Lopezand Lui, 1990; Lopezet al., 1990; Ohtani et al.,
1992a]. As also summarizedby Lui [1991], the currentdisruption model of substormspostulates that a plasma instability [Lui et al., 1990] of the cross-tail current grows and
spreadsradially outwardfrom the near-tail region [Lui et al.,
1992; Lopez et al., 1992], and that the growth of this instability powers the near-Earth field reconfiguration. The NENL
model takes an opposite view, proposing that these pro-
Jacqueyet al. [1991] andOhtaniet al. [1992b]usedISEE 1
and 2 data at somewhatlarger geocentricdistancesto support
the idea of currentdisruptionand outwardradial propagation
of such a disruption. The partial breakdownand tailward
propagationof the disruptionregion can accountfor measurementsboth at geostationaryorbit (GEOS) and in the neartail (ISEE) accordingto Jacqueyet al. [ 1993]. Indeed, the
magnetic signaturesseen by ISEE in many casesseem very
supportiveof such simpledisruptionand propagationscenarios; other more complex substorm events are not as con-
vincirig,however[seeJacqueyand Sauvaud,1993].
A rather comprehensivestudy of a large substormevent
BAKER
ET AL.' THE NEAR-EARTH
............................
-200
....................
•=-400
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
ISEE-2
•
10-z
12,985
setof spacecraft
near local midnightin the radialrange5-9
RE made it possibleto studythe strongcross-tailcurrentdevelopmentduring the substormgrowthphase(seealso Figure 2 describingthe sameevent) and the currentdisruption
andcurrentwedgedevelopmentduringthe expansionphase.
The time-evolvingmagneticfield modelpermittedmapping
of observedauroral featuresout into the magnetospheric
-600
10-•
LINE MODEL
the principal findings of the Bakeret al. [1993] study. The
substormwas simultaneouslyimaged in the southernauroral
oval by DE 1 and the northern auroraloval by Viking. The
29 MARCH 1979
o
NEUTRAL
Plasma
she,_.,et
recovery
ß
SEE-2
10
equatorial plane. There was both a dominant-eastwardand a
0
weakerwestwardprogressionof activity following the ex-
pansionphase.A clearlatitudinalseparation
(_>10ø) of the
-10
1.30
2.00
2.30
3.00
3.30
4.00
4.30
UT
i-....i
....
i ....
i ....
i ....
i ....
i ....
i ....
i ....
i ....
i ....
i ß ß ß ...i
80
7O
$0
...............
50
4O
-;50
-15
0
15
;50
45
60
75
90
105
120
1;55 150
Time[minutesfrom onset]
Figure 4. Timing of the substormrecoveryat severallocations.
was presentedby Baker et al. [1993]. They used numerous
high-altitudesatellites as well as global auroralimages and
ground-based
data to studythe substormgrowthand early expansion phase. The magneticmodeling methodsof Pulkkinen et al. [1991a,b] were also employed.Figure 5 summarizes
•s$
LATEGROWTH
PHASE
SgI•IRS•EONRTM
.....
FL
.... EAR
,.
•sEo
uGrlceONo•
FAC)
initial region of auroralbrightening and the region of intense westward electrojet current was identified. The com-
bined ground,near-tail, and imaging data providedthe opportunity to investigate tail currentdevelopment,field line
mapping, and substorm onset mechanisms. Evidence was
found for strong current enhancementwithin the near-tail
plasma sheet during the late growth phase [Baker and
McPherron, 1990] and strong currentdisruptionand fieldalignedcurrentformationfrom deeperin the tail at substorm
onset.Baker et al. [1993] invoked a model of magneticneutral line formation in the late growth phase which causes
plasma sheet current diversion before the substorm onset.
The expansionphase onset was concludedto occur when re-
connectionof lobelike magneticfield lines began, roughly
concurrentwith the cross-tailcurrentdisruptionin the inner
plasma sheet region.
The near-Earthneutral line model has been remarkably
successfulin organizing observation in the near-Earthand
distanttail. In fact, the outstandingsuccessof the modelwas
the predictionof the structureof plasmoidsand flux ropes as
they were subsequentlyobservedin the distant tail [Honeset
al., 1984a;Baker et al., 1987]. The modelhas also provided
an observational basis motivating an extensive body of
work in numericalsimulation of spaceplasmas. Although
these simulations do not in detail model the microscopic
processes responsible for magnetic reconnection or
nonadiabaticparticlemotion,they reproduceremarkablywell
the observationssummarizedby the NENL model.
OFINTENSE
]IIIIIIIIIII'•
CROSS-TAIL
I II I I I II I I I
CURRENT III
I I I II I I I
I
I
I
I
At low altitudes the NENL model is less successful in or-
I "INITIAL
I
NEUTRAL
EXPANSION
PHASE
ganizing the host of phenomenaobservedin satellite auroral
images (R. D. Elphinstoneet al., What is a global auroral
substorm,submittedto Reviewsof Geophysics,1995), or the
details of developmentof the auroral electrojets [Rostoker,
1979]. However, on a macroscopicscale the NENL model
doespredictmany aspectsof substormdevelopment.For example, the recent simulation results of Raeder et al. [1995]
andFedderet al. [1995] that on a large scale are consistent
with the predictions of the NENL model, include the substom current wedge, the auroralbulge, the polewardexpansion of the aurora,the westwardtravelingsurge,and a double
FLOW
SHEAR
oval structure.
A major problem in substormstudiesis the mapping of
auroralbreakupto the equatorialplane. Many researchersat• INITIAL
tempt to project the X line to the breakup arc. This appears
NEUT•L
to be quite difficult. The distinguishing signature of the
NENL, namely,tailwardflow threadedby southwardmagnetic
field, is seldom seen inside of-20 RE [Baumjohannet al.,
Figure 5. A schematicof the near-Earth plasma sheet and cross-tail 1990; Angelopouloset al., 1993], suggestingthat the X line
currentsduringthe (top) late growthphaseand (bottom)early expansion typically forms beyond this distance.However, auroral lumiphaseof a large substorm(adaptedfrom Baker et al. [1993]).
nosityjust before auroralbreakupis difficult to map beyond
••LINE
(X-LINE)
12,986
BAKER ET AL.: THE NEAR-EARTH NEUTRAL LINE MODEL
10-15 RE, even with very stretched field configurations
[Pulkkinenet al., 1991a]. Instead,the breakup arc appearsto
map to the thin current sheetjust beyond synchronousorbit,
a location which is almost certainly near the inner edge of
the tail current, and well earthwardof the probable location
of the X line. However, reconnection at the NENL would not
causesignificant field-aligned flows; rather, the most notable signaturesare the tailwardand earthwardflows within the
neutral sheet. Thus it is only when this flow travels earthward driving FAC that a groundonsetis seen. In this sense,
the onsetarc mappingearthwardof the NENL is a quite natural result of the reconnectionprocess.
The above discussionrelates also to another NENL problem in the view of some researchers,namely that the field
line on which the breakupoccursis almost certainly closed.
Viking auroralimagesshowa wide auroraloval with breakup
beginning in the equatorward region of luminosity
[Elphinstoneet al., 1995b;Baker et al., 1993]. Also, meridian
scanningphotometerson the groundoften show substantial
luminosity more than 5ø poleward of the breakup arc
[Samsonet al., 1992]. Typically, it takes 5-10 min for the
auroraldisturbanceto propagatepolewardfrom the region of
onset to the high-latitude region whose polewardborder is
taken to be the polar cap boundary.The problem arisesbecausein some previous versions of the NENL model auroral
expansiononset was associatedwith the releaseof the plasmoid by severanceof the last closed field line. Clearly, this
associationof plasmoidreleasewith auroral onset cannot be
correctif reconnectionof open field lines doesnot begin until the auroral expansion reachesthe polar cap boundary
[Baker and McPherron, 1990].
The realization that the arc that brightens first during a
substormshouldmap near the inner edge of the plasma sheet
[Samsonet al., 1993; Elphinstoneet al., 1991; Galperin and
Feldstein, 1991; Mauk and Meng, 1991; Rouxet al., 1991]
suggeststhat most often the substormexpansionis a phenomenon that is associatedwith closed plasma sheet field
zation that occur in the inner portion of the plasma sheet
typically require substantialenergy inflow into the disruption regionfrom outside.Hesseand Birn [1993] thereforeargue that a large-scalemagnetotail instability is requiredto
provide the energy which flows into the near-tail, closed
field line region.
Multiple auroralarcsare a problemfor the NENL model as
is true for most models. Such arcs are often observeddrifting
equatorwardduringthe substormgrowth phase, and the conventional view is that it is the most equatorwardarc that is
activatedat onset.If this arc projectswell earthwardof the X
line, then the remainingarcs are presumablybetweenthe region of activation and the X line. This is the region of
strongearthwardflow out of the X line. It is difficult to understandhow such a flow could exist through the region of
the magnetosphericprojectionof the arcs and not causeany
disruption in the ionosphere.
The NENL pictureis mademuchmorecomplicatedby con-
sideration
of finiteByeffects.Typically,the plasma
sheet
contains
a finiteBy component
transverse
to thetailaxis.In
its presencereconnectionproducesa flux rope rather than a
simple plasmoid [Hughesand Sibeck, 1987; Slavin et al.,
1995].If thesignof Byis positivethe flux ropewill be
connectedto the northern hemisphereat the dawn end and
the southernhemisphere at the dusk end. Projected into a
meridianplane the flux rope will have a form similar to that
of a plasmoidexceptthat there will be field lines at the ends
connectedto the ionosphere.If reconnectionproceedsat the
same rate everywherealong the flux rope the structurewill
grow in a uniform manner with ionospheric connectivity
only at its ends. More likely, however, the reconnectionrate
is fastestnear the center of the flux rope and closedflux is
addedmore rapidly there than at the ends. Also, the length
of the flux rope probably increases with time carrying reconnection to other meridians in the tail. The evolving
structurewill be very complex with different layers of flux
being addedat different locations. Each layer will be connected to different polar regions at its two ends, and there
will be multiple field lines connecting each layer to the
ionospheres. In this more realistic geometry, the "neutral
line" becomesa finite size "neutral region" within which the
reconnection-associated
energy dissipationoccurs.
lines that are located near the transitional region between
dipole-like and tail-like field lines. The breakuparc is also
associatedwith the Harang discontinuity [Koskinenand
Pulkkinen, 1995] and the outwardfield-alignedcurrentin this
region. It is probably no coincidencethat the outwardcurrent of the substormcurrentwedgehas the same sense and
occursin the sameregion as this current.However, the level 3.2. High-Speed Plasma Sheet Flows
of detailsin the descriptionof near-tailand low-altitudepheThe realization that during the courseof magnetospheric
nomena within the NENL model is not sufficient to explain substormsthe outermostparts of the plasma sheet are often
this fact.
populatedby counterstreaming
beams [De Costerand Frank,
Recently, Kan [1993] has discusseda magnetosphere- 1979; Forbes et al., 1981] led to the definition of the plasma
ionosphere coupling (MIC)model of substorms.In this sheetboundarylayer as an important region of magnetotail
view, the substormexpansiononsetoccurs as a consequence dynamics.Recent studiesof the relative importanceof the
of the ionospheric response to enhanced magnetospheric PSBL and CPS by Baumjohannet al. [1989, 1990] have reconvection. It is asserted in the MIC model and other nearvealedthat high-speedflows occur with low but statistically
Earth modelsthat no new X line is neededat expansion on- significant occurrenceprobability in all plasma sheet reset. Instead, the explosivedissipationof the substormonset gions, and that when the flow velocity is high the occurresults from an "unloadinginstability" or disruptionwithin rencerate of fast flows at the plasma sheet center is compathe currentwedge region near local midnight.This school of rable to that in the boundarylayer.
thought assertsthat the energy storedon closedfield lines
Fast neutral sheetflows are qualitativelydifferent from the
of the plasmasheetcan power all, or most, of the substorm boundarylayer flows: They are predominantlydue to the drift
expansion phase.
of a single-ion population [Nakamuraet al., 1991], rather
Hesseand Birn [1993] reacheda quite robust and different than the imbalance of counterstreamingbeams, as is typiconclusionconcerningthe sourceof substormpower. They cally the case at the plasma sheet boundary[De Costerand
found that the currentdisruptionand magnetic field dipolari- Frank, 1979; Eastmanet al., 1984]. Baumjohannet al. [1990]
BAKER ET AL.' THE NEAR-EARTH NEUTRAL LINE MODEL
showedthat in the outer central plasma sheet and the plasma
sheet boundary layer, the high-speed flows are nearly fieldaligned. In the neighborhoodof the neutral sheet about 70%
of all high-speedflows have a dominant component perpendicular to the ambient magnetic field. Thus in the inner central plasma sheet most of the high-speed plasma transport
occurs acrossfield lines rather than along them.
Fast flows
near the neutral
sheet were shown
to have
an
average10-min durationand a substructure
of the order of 1
min [Angelopouloset al., 1992]. The 10-min timescalestructures were termed bursty bulk flows (BBFs) and the 1-min
structureswere called flow bursts.Flow bursts correlate positively with magnetic field dipolarizations and ion temperature increases.The importanceof the fast flows for magnetotail transportof energy and magnetic flux was demonstrated
by Angelopouloset al. [1994a]. BBFs are responsiblefor 60100% of the measuredearthwardmass energy and magnetic
flux transport past the satellite in the high-beta plasma
sheet, even if they are observedonly 10-15% of the time in
the midnight plasma sheet. As significant earthwardenergy
and flux transport is expectedduring substorms,BBFs are
good candidatesto attempt to correlatewith geomagneticactivity. Indeed, a positive correlation between fast neutral
sheet flows and AE was observed by Baumjohann et al.
[1989]. BBF occurrencerates and AE were correlatedby Angelopouloset al. [1994b]. A one-to-one correlation between
BBFs and substormphase is not apparent [Angelopouloset
al., 1992] and representsan area of active researchtoday.
The newest results from the IRM
and ISEE satellites
show
that most of the plasmatransportalong the length of the
tail is achieved by means of high-speed flow bursts. The
left-hand panel of Figure 6 shows the occurrencerates of
high-speedflows (bulk flows in excessof 400 km/s) in two
different layers of the plasmasheetfor different rangesof AE
again using the IRM tail survey data. If one averagesover
all levels of disturbance, the following picture emerges:
From all 4.5-s ion flow samplestaken in the plasma sheet
boundarylayer, about 5% exhibit velocitiesin excessof 400
km/s. This rate drops to 1% in the outer central plasmasheet
(not shownhere) but rises again to an averageof about 3%
of the samplesobtained in the inner central plasma sheet,
8.
w
I
*
'
!
12,987
that is, the neighborhoodof the neutral sheet. Thus there is
a 3:1:5 chance to detect ion flows with velocities greater
than 400 km/s when going from the neutral sheet to the
boundarylayer. However,if one considersthe behaviorof
the fast flow occurrencerate during substormactivity, it varies by lessthan a factor of 2 in the plasmasheetboundary
layer [Baumjohannet al., 1990]. It is only near the neutral
sheet that the fast flow occurrencerate strongly increases
with increasingAE, reaching the same or even slightly
higherlevels than in the plasmasheetboundarylayer during
strongly disturbedconditions. Thus the high-speedplasma
flow in the plasmasheetboundarylayer is less affectedby
substormactivity than that near the neutral sheet, similar to
the behavior of the ion temperature.
From all high-speedion flows with the velocityin excess
of 400 km/s observedby the IRM satelliteduringthe two 4month periodsit spent in the magnetotailat distancesbetween9 and 19 RE, the overwhelmingmajority was directed
earthward.Less than 100 samples had a distinct tailward
component.The same holds for the ISEE observation in the
samedistancerange[Angelopoulos
et al., 1993]. However,i n
the ISEE data set, about 20% of the samples taken between
19 and 22 RE had a distinct tailward component.These observationsindicate that a near-Earth neutral line is typically
locatedoutsideof-20 RE, however,on rare occasionsit may
comeas closeto the Earth as 13 RE [McPherronand Manka,
1985; Paschmann et al., 1985; Lin et al., 1991].
The right-hand panel of Figure 5 shows that high-speed
flows are rather bursty.The majority of all flows stay uninterruptedlyat high-speed levels for no more than 10 s.
About 35% of all flow burstswere observed during only one
4.5 s measurementinterval. Typically, severalof these highspeedflow burstsare groupedinto an event lasting for some
minutes. Of course,with data from just one satellite one is
unable to decide whether the burstinessresultsfrom temporal
or spatial variations,that is, whetherthe ions in the whole
plasmasheetmove at such high velocities for short periods
of time or whether the short durations are causedby rapid
traversals of thin layers of plasma streaming at high velocities and intermittent regions of slower plasma motion.
Figure 7 showsa superposed
epochanalysisof the behav40.
*
30.
20.
10.
O.
O.
,
t
300.
,
I
600.
AE Index, nT
,
O.
900.
O.
10.
20.
30.
40.
50.
60.
Iligh-Speed Flow Duration, See
Figure 6. (left) Occurrence
rate (in percent)of high-speedion bulk flow samplesin the inner centralplasmasheet(solid
line) andin theplasmasheetboundarylayer(dashedline) as a functionof theAE index.(right)Occurrencerate of highspeedionbulkflow samples
in theplasmasheetasa functionof continuous
durationof the flow event [from Baumjohann
et
al., 1990].
12,988
BAKERETAL.'THENEAR-EARTH
NEUTRAL
LINEMODEL
250.
o.3
200.
.
5o.
0.25
o.o
,oo.
•0.
-30.
-20.
•inutes
-10.
o.
Around
10.
20.
Plow
Burst
30.
• 0.10
-30.
-20.
•inutes
-10.
o.
Around
10.
20.
Plow
Burst
•0.
•igure•. R•sults
o•asup•os•d•poch•alysisapplied
onplasma
andmagnetic
•ld data•oundmore
than100bursty
bulkflow(BBF)•v•ntsobs•d byIRMinth•inner
c•ntr•plasma
sh•t du6ng
1985.
Th•plasma
andmagnetic
fi•lddata
w•m tim•-c•nt•md
•oundth• flowburstc•nt•r(t = 0) andaveraged
in 3-minbins•or-30bins•or-30• t •30 min.
ior of plasma and magnetic field aroundthe center of more
than 100 BBF events observedby the IRM satellite in 1985.
The typical BBF event lasts about 10-15 min and is accompanied by a strong dipolarization. Moreover, the plasma
temperaturealso increasesstrongly during a BBF event. Altogether, a BBF event looks much the same as a substorm
intensification,and it is quite reasonableto assumethat it is
causedby the same mechanism,namely a burst of reconnection somewheretailward of 20 RE, which heats the plasma
rameter values obtained are in concert with direct satellite
and accelerates it earthward.
[Buchnerand Zelenyi, 1987] that "chaotization"of thermal
electronmotion may trigger near-Earthtearing mode onset
[e.g.,Pulkkinenet al., 1992].Dagliset al. [1991, 1992] have
3.3.
Thin
Current
Sheet Instability
The tail region responsiblefor the global reconfiguration
during substormshas been extensively debatedfor the past
decade.The resultsfrom AMPTE/CCE showedthat processes
in the near-Earth tail are closely correlatedin time with the
ground signatures of substorm onset [e.g., Lopez et al.,
1990]. Empirical magnetic field models also suggestedthat
the auroral brightenings map to the near-Earth tail
[Pulkkinen et al., 1992, 1995b]. However, magnetic field and
plasma observations have suggestedthat in most cases the
neutral line is formed in the region beyond-20 RE, somewhat tailward of the active inner tail region [Hones et al.,
1984a]. Figure 8 summarizesschematically our present view
of tail
observations[Sergeev,1992;Sannyet al., 1994].
Recent studies of substorms have included estimates of
current sheet thicknesses at the inner edge of the plasma
sheet and suggestionsof an "explosive" growth phase
[Ohtaniet al., 1992a].Also, recentstudiesby Mitchell et al.
[1990] have attemptedto identify the currentcarriersin the
near-Earthplasmasheetduringall partsof the growthphase.
There has also been observational support for the idea
GROWTH
INNER EDGE
EARTHWARD
SW•-
DAYS,DE
I
RECONNECTON|
IMF
TAIL CURRENT
SHEET
NENLON
CLOSED
FIELD
observations.
The changes in the near-Earth magnetic field configuration have been extensivelystudied using empirical magnetic
field models. These studies utilized the Tsyganenko [1989]
magneticfield models, which were modified to give a best fit
to satellite magnetic field measurements[Pulkkinenet al.,
1992]. The most important result from these modeling studies was that during each of the growth phases modeled, a
thin and intense current sheet was formed in the relatively
near-Earth region. The inner edge of the thin current sheet
was located between 6.5 and 9 RE (see Figure 9, for example). The minimum current sheet thickness was in many
cases only a fraction of an RE, being comparableto the
thermal ion Larmor radius in the correspondinglobe region
(see also the review by Sergeevet al., [this issue]). Further-
_••?•___
• • •M'PAUSE
• ------PS
THINNING
,•,/SHEET
PLASMA
•
RECONNECTION
FLOW
DURING
RECONNECTION
LATE
GROWTH
CURRENT DIVERSION
WffHIN CURRENT SHEET
ONSET
AND
EXPANSION
-;.
NENL
OPEN'
FIELDON
LINES
NEG.
H-BAY
Pi2 .••
PULSATIONS
AURORAL
•
.'
BRIGHTENI
DIPOLARIZ
•
FAST TAILWARD
FLOWS
TURBULENCE
INJECTIONS
FLOW SHEAR
REGIONS
more, small regionsof very small Bz valueswere foundto
Figure8. Schematic
summary
of obse•ations
in thetail duringthe
form in the near-Earth tail [Pulkkinenet al., 1992]. The pa-
growth,onset,andexpansion
phases
of substorms.
BAKER ET AL.' THE NEAR-EARTH
NEUTRAL
LINE MODEL
12,989
Substormonseton May 3, 1986, 0111 UT
Equatorialplaneprojection:
Tail satellite data:
substormcurrent wedge
o111
Magnetic field model:
Inner edgeof the
thincurrent
sheet
40
(B min -- 2.1 nT,
h min-- 0.2 Re)
-10
10
2o Y
Dusk
Earth/l
ToSun
Figure 9. Thin currentsheetstructureobservedon May 3, 1986 [from Pulkkinenet al. 1995b].
examined the role of ionosphericions in triggering ion tear- relative to substorm onset. Whenever the central plasma
sheet thins, chancesincreasethat the spacecraftwill be in
ing mode onset during substorms.
Several numerical modeling studieshave recently empha- the plasmasheetboundarylayer or lobe; thus the occurrence
sized the importance of thin current sheets in the substorm rate is an indirect measureof the central plasma sheet thickprocess.Both MHD simulations[Vogeet al., 1994] and two- ness.
dimensional kinetic simulations [Pritchett and Coroniti,
The right-handpanel in Figure 10 showsthe resultsof the
epochanalysis,again separatelyfor the two dis1994] have shown that changesin the boundaryconditions superposed
(or changesin the convectionelectric field) can result in the tance ranges, that is, those onsets where IRM was located
formation of a very thin current sheet in the transition re- between 10 and 16 RE and those substormsduring which the
gion, where the tail-like field merges with the dipolar near- satellitewas situatedfurther out, at 16-19 RE. The difference
Earth field. These results are in concert with the empirical in the absolutenumbersis readily explainedby a larger abmodel results of the location, magnetic field, and current solute thickness of the central plasma sheet closer in.
sheet thickness values. Furthermore, Birn et al. [ 1994] Throughoutthe whole region between10 and 19 RE the
showed that thin
current sheets
can be formed
even
if the
thinningstarts30'45 min beforesubstorm
onsetandper-
siststhroughoutthe growth phase.After substormonset, the
centralplasmasheetexpands.Note that the timescaleof the
Recent observations from midtail spacecrafthave further averaging is not sufficient to judge whether the minimum
wasobtained
priorto, or after,the substorm
onilluminatedthe thin current sheetissue.Figure 10 shows the thickness
averagebehavior of the AE index, separatelyfor those on- set. Furthermore,Figure 12 combines two physically very
plasma is relatively isotropic, contrary to what is often assumed.
setswhere IRM was locatedbetween 10 and 16 RE and those different processes:The thinning during the growth phase is
substormsduring which the satellite was situatedfurther out, slow and linear, and thus the probability curve shown can be
associated with the actual current sheet thickness.
However
at radial distancesof 16 to 19 RE [Baumjohannet al., 1992].
(on
this
timescale),
the
plasma
sheet
recovery
is
a
step-like
The AE traces look quite similar for both setsof events, thus
there seemto be no systematicdifferencesor biases between process. Thus the part of the curve after substorm onset is
not a measure of actual current sheet thickness but rather a
the two radial distanceranges.
To show relative changes in the thickness of the central measu.reof the probability that the plasma sheet recovery
plasma sheet, it is easiestto determinethe number of meas- has occurred.These probabilities are in agreementwith anurementswhere IRM was located in the central plasma sheet other statisticalstudy by Baker et al. [1994b].
These results are consistent with the following scenario:
relative to the total numberof (central plasma sheet, plasma
sheetboundarylayer, and lobe) samplesand then look for During the growth phase, magnetic flux is addedto the tail
temporal changesin the central plasma sheet occurrencerate lobes, increasingthe lobe pressureand stretchingthe tail
12,990
BAKER
,500.
'
I
ET AL.: THE NEAR-EARTH
'
I
NEUTRAL
LINE MODEL
1.0
*
:
ß
.
450.
400.
i
350.
•
•00.
--•
0.4
-
ß
•
•
250.
P.,00.
-45.
Minutes
0.2
o.
45.
around
90.
Substorm
Onset
t
-4.5.
Minutes
I
I
I
o.
around
45.
Substorm
90.
Onset
Figure10.(left)Superposed
AEtraces
withrespect
to 19substorm
onsets
during
whichIRM waslocated
at 10<R<16
RE
(solidline)and20 onsets
whenthesatellite
wassituated
further
out(16<R<19
RE, dashed
line),constructed
byaveraging
the
19and20AE tracesin 11.25-min
bins.(right)Relativefractionof central
plasma
sheetsamples
withrespectto the 10 or 20
substorm
onsets,
constructed
by dividing
thenumberof IRM measurements
takenin thecentralplasmasheetby thetotal
numberof tail measurements
in a particular11.25-minbin. The dashedverticallinesmarksubstorm
onsetandthe startof the
recoveryphase[from Baumjohannet al., 1992].
magneticfield. To keep the vertical pressureprofile constant
and to adapt to the changing magnetic field configuration,
the plasma sheet must shrink in its vertical extent. Independently, the transition region near the inner edge of the
plasma sheet developsa thin current sheet structureand lowB region, which is embeddedin a thicker plasma sheet, and
is formed as a responseto the changingexternal conditions.
Theoretically, a key questionfor the substormonset process determination has been the plasma and magnetic field
conditions that would allow reconnection to proceedin the
relatively near-Earthregion, especially through the growth
of the ion tearing mode. Various analyseshave given results
both in favor [Kuznetsovaand Zelenyi, 1991] and against
[Pellat et al., 1991] the growth of the ion tearing instability.
To resolve this issue, more advanced theories and models
treating all three dimensions and both particle species are
needed, and better observationsfrom the reconnectionregion
are also required.
We propose that reconnection (presumably due to the
growth of a tearing instability) initiates during the late
growth phase in a region close to or tailward of about 20
RE, in the region of enhancedcurrent density and thinned
plasma sheet. In its initial state, reconnectionwould proceed
slowly within the closed field line region. However, this
It would only be at the stage when the inner tail is driven
unstable that the onset signatures would be visible from
ground:The plasma jetting from the neutral line toward the
Earth causesa shear between the plasma flow regions, feeding field-aligned currents into and out of the ionosphere
[Hesse and Birn, 1992]. These currents form the substorm
current wedge and form a part of the auroral bulge in the
ionosphere.
3.4. Substorm Triggering
and Steady Convection
Soon after the southward
turning of the IMF the growth
phaseof magnetospheric
substormsbegins:magneticflux is
storedin the magnetotail and the near-Earthplasma sheet
thins while its current intensity increases.Soon after that a
magnetosphericsubstormdevelops [Russelland McPherron,
1973b], but the cause of that substorm is sometimes not an
internal, spontaneous instability but a solar-wind-driven
one. Kokubunet al. [1977] notedthat whenthe interplanetary conditionsare right (southwardIMF) then the probability of an interplanetarydiscontinuitycausingthe onset of a
substormis very high (90%), whereaswhen there is no sig-
natureof an enhanced
groundactivityresembling
the growth
phase, the probability of an interplanetary discontinuity
would lead to a feedback effect, which would further enhance triggering a substormis very low (8%). Thus the precondithe thinning of the plasma sheetat its inner edge, and would tioning of the tail current sheet due to a previoussouthward
finally drive the inner tail to an unstablestate [Bakerand turning of the IMF seems to be a necessarycondition for
McPherron, 1990]. The instability that leads to the current substormtriggering. Caan et al. [1977] analyzed 18 events
sheetdisruptionat the inner edge of the plasma sheet is not during which the IMF switched from northward, as it had
yet clear; various kinetic and MHD instabilities have been been for 2 hours or more, to southward.A positive correlasuggested[e.g., Roux et al., 1991' Lui, this issue].This view tion between the amount of flux transportedby the solar
of the near-tail instability being driven by a more distant wind until substormonsetand the amplitudeof the negative
neutral line is further supported by observations during bay at onset was revealed. In addition, a class of events that
steady convection periods' In such events, a thin current weretriggered
by positivechanges
in the (negative)IMF Bz
sheet has been observed to form
and remain
stable
in the
was revealed. In fact, 60% of the events studied were consis-
near-Earth tail for several hours, but in these cases the mid-
tentwith a partialnorthwardrecoveryof the IMF Bz, indicat-
tail field is expanded,
Bz is large,andthereareno signsof a
ing that a substantial number of substormsare apparently
triggeredby IMF changes.
neutral line [Sergeevet al., this issue].
BAKER ET AL.: THE NEAR-EARTH
NEUTRAL
LINE MODEL
12,991
That a significant proportion of substorms are triggered al., 1991]. Accordingto Sergeevet al. [this issue]convection
by the IMF partial positive excursionscan also be inferred bays differ from classical substormsin that the source of the
by the superposedepoch analysis of 20 events by Caan et fast, earthwardmoving plasmais further tailward than during
al. [1975]. The solar wind magnetic field behavior that cor- classical substorms.Although such fast flows contribute
relates with the onset in that study is a partial positive ex- significantly to the flux and energy transport, they do not
cursionof the Bz component(seetheir Figure1). More re- affect the magnetotailtopology very much. The auroralsigcently, McPherron et al. [1986b] revisited the question of nature of a convection bay during which DE 1 images were
substormtriggering from IMF changes and solar wind dis- available [Yahnin et al., 1994] is the one of a double auroral
continuities and concluded that 44% of the substorms studied
oval. Discretearcs are the onesresponsiblefor the luminoswere associated with an excursion of the IMF northward, but
ity at high latitudes [Yahnin et al., 1994] presumablymap29% of the eventsoccurredduring periodsof steady negative ping to the tail activations tailward of the thick plasma
Ba. That thereexistsa classof substorms
that are associated sheet, whereas diffuse precipitation is mostly observed in
with steadysouthwardBa without any evidenceof a solar the equatorialportion of the oval, presumably mapping to
wind trigger was pointed out also by Caan et al. [1977].
the near-Earthplasmasheet[Sergeevet al., 1991]. The above
Drnitrieva and Sergeev [1983] also studiedthe issue of analysesreveal magnetotail activity during convection bays
substormtriggering. They also found that 11 out of the 30 similar to that which occursduring substormstaking place at
events that went into their analysiswere not associatedwith large distancesfrom Earth and mapping to electrojet activaa variation of the IMF Ba to within the uncertaintyof the tions at high latitudes. The fact that the activity is far from
timing. In addition, they showedthat a further decreaseof Earth is responsiblefor the lack of classicalsubstormsignatheBz component
to moresouthward
valuesdoesnot causea tures (Pi 2 values and midlatitudebays).
substorm, but rather causes an increase in the DP2 current
More recently, Lyons [this issue] arguedthat there are no
system.A similar magnitudepositive excursion,however, is clear casesof substormsthat occur during intervals of steady
enough to trigger a substorm.The preferential responseof southwardIMF. Accordingto this work, if an extendeddefi-
the magnetotail
to northwardBz partialexcursions
aftera pe- nition of trigger is introducedin which not only Bz partial
riod of southwardBa indicatesan importantpropertyof the northwardrecoveriesbut also IByl decreasesare included,then
systemthat needs to be addressed
theoretically. In addition,
Drnitrieva and Sergeev[1983] showedthat for the 11 casesof
steadysouthwardIMF with no evidenceof triggering,the duration of loading (i.e., the duration of the growth phase) is
inversely proportional to the magnitudeof the solar wind
electric field. The duration of the growth phasetimes the solar wind electric field was constant.They interpretedtheir results as evidence that the current sheet thickness ought to
reacha critical value beforeit becomesspontaneouslyunstable.
Prolongedperiodsof southwardIMF lead to a state of the
magnetosphere
that has beentermeda convectionbay [Pytte
et al., 1978] or a steadymagnetospheric
convectioninterval
[Sergeev, 1977]. Such events may start with conventional
substorms,but if the IMF is stable and prolonged (4-6
hours),then the magnetosphere
can reach a state whereby
the classicalsubstormsignatures(Pi 2 pulsations,midlatitudebays)are absent.A review of the state of the magnetosphere under such conditions can be found in the work of
Sergeevet al. [thisissue].Here we emphasizesomepertinent
recent resultsconcerningconvectionbays and their association with substorms:(1) During convection bays the nearEarth tail (7-12 RE) resemblesthe substormgrowth phase;
the currentsheetis thin andBz is small [Pulkkinen
et al.,
1992]. (2) The midtail plasmasheet(12-20 RE) is thick, and
the equatorialBa is quite large relative to the averagevalue
[Sergeevet al., this issue]. The midtail plasma sheet resembles the recovery phase plasma sheet. (3) Transient activations similar to the ones observed at substorm times are also
there are no substormsthat are not triggered. Although the
work of Caan et al. [1977], McPherron et al. [1986b], and
Dmitrieva and Sergeev [1983] addressedthis issue and all
these studies showed that indeed there are examples of substormsthat do not have obvious triggersin the solar wind, a
large statisticalstudy that includesthe addition of IByl reduction as a possible trigger has not been performed yet. We
look forward to sucha studyin the future.
The NENL model for substormspostulatesthat an instability resulting from the increase of the cross-tail current
causesinterconnection of the magnetic field lines in the
plasma sheet and eventually at the lobes. The onset of that
interconnection is not necessarily due to an internal instability, but it may be due to external factors. One puzzling
aspectof the partial northwardIMF triggering substormsis
that such an IMF perturbation ought to causea decreasein
the solar wind electric field that operatesacrossthe tail, and
a resulting decreasein the amount of flux transfer in the
nightsidemagnetosphere.This resultsin reducingthe rate of
plasma sheet thinning, not increasing it. Given that thin
plasma sheetsought to be more unstableto plasma instabilities, we concludethat it is counterintuitive that the partial
northward IMF excursion should acts as a substorm trigger.
Is it possible that it is the inductive response to the IMF
changesthat results in substormonset? The physics of substorm triggering due to such IMF changes is very important
not only becauseit is a very frequentphenomenon,but also
becauseit may lead to better understandingof the instability
that causesplasma sheet reconnection, and an enhancement
of our ability to predict substormactivity.
observedin the thick plasma sheet [Sergeevet al., 1990b].
Theseresemblethe burstyflow eventsstudiedby Angelopouloset al. [1994b] and by V. A. Sergeevet al. (Detectionof
3.5. Multiple Onsets, Localized Flow, and
localized plasma bubblesin the midtail plasma sheet, subPseudobreakups
mittedto Journal of GeophysicalResearch, 1995, hereinafter
A common feature of substormsobservedon the ground is
referredto as submittedmanuscript).Occasionally,suchflow
burstsmay protrudeclose enoughto Earth to producea clear multipleonsets.If theseoccurbeforethe main breakup,they
injection signaturein the geosynchronous
region [Sergeevet are calledpseudobreakups
[Koskinenet al., 1993;Nakamuraet
12,992
BAKER ET AL.: THE NEAR-EARTH NEUTRAL LINE MODEL
Angelopouloset al. [1994b] analyzedseverallow AEBBFs
and showedthat they were not registeredby the AE index besubsequentlyhigher latitudes, and somewhat further west causeof the longitudinal localization of the electrojet curthan precedingonsets [Wiensand Rostoker, 1975]. How to rents. One of the low AE events was studiedfurther by Serassociatesuch discreteeventswith a single X line is unclear geevet al. [1995a]. They showedthat the plasma sheet beand a definite issue for the NENL model.
havior during the event was consistent with a magnetotail
Pseudobreakupsare similar to the substorm expansion reconnectiongeometry which starteda few minutesprior to
phase in that they include activation of an auroral arc, en- the onset of Pi 2 pulsationsand geosynchronousinjection
hancementof the westwardelectrojet,and onset of Pi 2 UI_F and producedBBFs on its earthwardside. These observations
with
pulsations [see McPherron, 1991, and referencestherein]. suggestthat the low AE burstybulk flows are associated
Yet these signatures, which occur during many substorm electrojetactivity that may not be seenin the global indices
growth phases,are relatively weak and seem to be rather lo- becauseof its limited spatial extent in the ionosphere.
Fast near-neutralsheet flows during active times have also
calized. Pseudobreakups
have received considerablerenewed
attentionrecently due, for example, to high time resolution been studiedby Paschmannet al. [ 1985], Lin et al. [ 1991],
auroralimages [e.g., Shepherdand Murphree, 1991; Mur- Sergeevet al. [1992], Ohtani et al. [1992c], Lopez et al.
phree et al., 1991]. Multispacecraftdata and ground-based [1994], and Angelopouloset al. [1996]. These studiescommeasurements
have been combinedvery effectively to study plementthe earlier work of many researchers[Franket al.,
the pseudobreakup
process[Koskinenet al., 1993]. This work 1976, Hones and Schindler, 1979; Huang et al., 1987;
suggeststhat the optical and magnetic signaturesmay, in- Paschmannet al., 1985; Coroniti et al., 1978, 1980]. Sergeev
deed,be very localized,but a wide part of the auroraloval et al. [1992] showedthe importanceof nightside earthward
flux transportand its association
with geomagneticactivity.
may still be weakly activated.
Koskinenet al. [1993] and Nakamura et al. [1994b] showed Lopezet al. [1994] studieda bursty bulk flow event and
that pseudobreakups
have almostall of the featuresof a sub- showed that it occurred at substorm onset at the meridian of
storm onset, but the disturbanceseems to "quench"rather the AMPTE/IRM satellite. However, Lopez et al. arguedthat
than proceedto full expansion development.In the event some auroral activity was present in another time sector
studiedby Koskinen et al. [1993] pseudobreakup
subsided prior to the BBF eventdespitethe lack of fast flows at IRM
quickly and the actual substormexpansion phase onset did prior to the BBF event onset. Thus it is possiblethat fast
not occur for another 20 min. Nakamura et al. [1994b] used flows are not alwaysrelatedwith auroralarc formationat the
all-sky cameraand near-tail spacecraftdata to delineatethe meridianof the fast flows. Angelopouloset al. [1996] also
tight localization of pseudobreakup
featuresboth in the studied a high-latitude substorm intensification which ocal., 1994b]. After onset they are referred to as intensifications.
There is evidence
that each intensification
occurs at
curredat the meridian of AMFFE/IRM in good temporal association with a BBF event onset. The authors arguedthat
the good temporal associationbetween ground and space was
a result of the fortuitouspresenceof AMPTE/IRM at the meKrimigis and Sarris, 1979]. Sergeevetal. [1986] also pro- ridian of the substorm intensification. The auroral activity
vided evidenceof localized (a few RE) extent of magnetotail spreadover more than 5 hours of local time within 10 min,
activations. Honesand Schindler [1979] attributed the poor yet the energy and magnetic flux transport estimatesin the
correlation between fast magnetotail flows and substormsto magnetotail suggest that the BBF event was localized to
the longitudinal localization of the magnetotail phenomena. within 1-2 RE. It is possible that the auroral arc formation
Lin et al. [1991 ] used two satellite measurementsand inferred and electrojet enhancementdo not map to the location of the
that the scale size of flux ropes earthward of the presumed fast flows themselves but rather to the location of the decellocation of the plasmoid was of the order of a few RE, sug- eration of those flows near Earth.
Additional information as to the evolution of the magnegesting that the magnetotail activations within the retotail
acceleration during substorms comes from a multiconnection region have a small scale size. More recently,
Angelopouloset al. [1996] used magnetic flux and energy spacecraftstudy by Angelopoulos et al. (Tailward progrestransport arguments to infer that the scale size of a BBF sion of maagnetotail acceleration centers: Relationship to
event which correlated well with a substorm intensification
substormcurrentedge, submittedto Journal of Geophysical
was of the order of 1-2 RE. Ohtani et al. [1992c] studieda Research, 1995). The authors observed that the "tailward relimited duration fast flow event and also inferred a maximum
treat of a neutral line" during the active period studiedwas in
scale size of 6 RE. Sergeev et al. (submitted manuscript, reality a gradual progressionof multiple, time dependentand
1995) arguedbased on ISEE 1 and 2 measurementsthat the localized acceleration regions. They provided evidence that
scale size of the plasma bubbles is of the order of 1-3 RE. occasionally there is more than one particle acceleration
The above studies suggestthat both during expansion phase sourcein the plasma sheetand thereforethere is not a single
onset and during the recovery phase of substorms,the mid- X line associatedwith a substormbut many localized acceltail (IXI- 15 to 30 RE) magnetotail accelerationphenomena eration sites. Sergeevet al. [1992] also argued that impulsive
are localized. Within the simple picture of the NENL model, dissipation events observed in energetic particle, electric
it is difficult to explain either the highly localized nature of field and plasma data in the magnetotailhave exhibited, durthe activations (if the NENL is assumed to have a macro- ing fortuitous conjunctionswith the ground, a good one-toscopic scale size) or the breakups and quenchingsof small- one correlation with ionosphericactivity intensifications. A
scale activations (if reconnectionat a large NENL is assumed good correlation betweenenergetic particle intensifications
to proceed explosively). Thus more details need to be added and ground activity has also been observedin an event studto the NENL model before these phenomenacan be accounted ied by Jacqueyet al. [1994]. Ionosphericintensifications are
for.
composedof a progressivepolewardre-appearanceof localionosphereand in the near-tail.
Early evidenceof localization of magnetotail phenomena
was presentedby investigators of energetic particle signatures at the IMP 6, 7, and 8 satellites [Sarris et al., 1976;
BAKER ET AL.: THE NEAR-EARTH
ized, short-lived (a few minutes) activations [Bosingerand
Yahnin, 1987] that are reminiscent of the progressive reappearance of the acceleration centers discussedin the Angelopouloset al. [1996] paper. It is likely that the ionospheric manifestation of the short-lived, localized, tailward
retreating magnetotail activations of Angelopoulos et al.
[1996] are the progressively poleward moving short-lived
auroral activationsof Bosinger and Yahnin [1987]. Sergeevet
al. [1995a] also provide further evidencefor this process.
In addition,Angelopouloset al. [1995] showedthat IMP 8
NEUTRAL LINE MODEL
12,993
[1987] that the connection of plasmoids to the ionosphere
and the solar wind may changealong the plasmoid'shistory.
The compositionof the distant tail is influencedduring substorms not only via plasmoids transporting near-Earth
plasma but also via more complex magnetic topologies with
only moderatenorth-southbipolar Bz signaturesthreading
tailward convecting plasma. Christonet al. [1994] reported
thatduringa veryactiveperiod(Kp=7-)molecular
ions(O2+,
NO+, N2+) hada measurable
abundance
ratio to O+ of 1-2%.
These ions emanatedfrom the ionosphereprior to the major
locatedat X--35 RE observeda large Bz increaseinterpreted substormonsetbut during an interval when the auroralelecas current wedge formation, at least several minutes after the trojet was active. The authorsarguedthat the molecularions
tailward progressionof the accelerationcenterspast the Geo- loadedthe plasmasheetat a distanceof 80-100 RE and were
tail satellite, located at X--70 RE. The result provides further subsequentlyheatedand convectedto the satellitelocation at
evidence that the location of magnetotail acceleration and
the tailward boundaryof the current wedge are far from each
other at late expansion/earlyrecovery phaseof substorms(at
least 30 RE apart in this case. These resultsare in agreement
with the conceptual evolution of the near-Earthneutral line
model that suggeststhat the energy that powers the substorms may be due to magnetotail reconnection but that
auroraeand electrojetcurrentsmay map very close to Earth,
X=-145 RE.
The ionosphereis an active modifier of plasmoid composition not only becauseof the connectionof plasmoidsto
the ionosphereor becauseof the ionosphericloading of the
plasmasheetplasma prior to plasmoid formation. Cold ions
from the mantle get acceleratedand heated at the plas-
much closer than the location
ExB-driftingtowardthe plasmasheetwith the samevelocity
and had the same field-aligned speed,just as expectedfrom
the ion spectrometer
model of mantle plasma.Althoughobservationsof cold ionosphericoutflow at X=-23 RE had been
of the X line.
Recently, Pontius and Wolf [1990] and Chen and Wolf
[1993] arguedthat if the plasmasheet is loadedwith plasma
in the distant tail in an inhomogeneousfashion, flux tubes
of low pressurewill tend to move earthward, and protrude
with high speedinto the surroundinghigh thermal pressure
plasma sheet. The observational signatureof these bubbles
would be fast flow and short duration, as well as low thermal
pressurerelative to the surroundingmedium. Sergeevet al.
[1995a] arguedthat a subsetof the bursty bulk flow events,
the ones that are seen in a thick plasma sheet, predominantly at the recovery phase of substorms,exhibits such
thermal pressuredecreaseand thus adheresto the "bubble"
paradigm.Thus high-speedflows are not always evidencefor
nearby reconnection but may be due to reconnection much
further downtail
from
the satellite.
These features add a sub-
stantial amount of complexity to the traditional view of
laminar plasma flow in the tail. More theoretical work on
the interactionof many small-scale activations and their integratedeffects on the flow pattern are requiredbefore their
meaningto the NENL picturecan be fully estimated.
3.6.
Plasmoids
Distant
and
Flux
Ropes in the
Tail
The Geotail satellite has verified the presence and relevance of plasmoidsfor substormdynamics. It has also provided new insight into the structure,dynamics, composition,
and evolution of plasmoids. Meanwhile recent new analyses
from the ISEE 3 satellite and the Galileo Earth flyby have
complementedthe Geotail results.
Plasmoid composition may give significant clues as to
the plasmoid history and connectivity. Lui et al. [1994] re-
moid/mantle
interface.Mukaiet al. [1994]reported
on H+ and
O+ coldion beamsobserved
at X=-42 RE. Thesebeamswere
made with the ISEE 2 satellite [Orsini et al., 1990],
the
Mukai et al. [1994] observationspointed out the dyngtmicand
localized nature of the ionosphericloading. The electric field
associatedwith the equatorwardconvectionof the cold beams
was 1.5 mV/m
and therefore
could not extend
across the en-
tire tail: The process had to be localized in Y. In addition,
Hirahara et al. [1994] pointed out that the cold ions were acceleratedand heated after the passageof a plasmoid during
the courseof the substorm.However, the O+ beam was already traveling at a speedof 100-200 km/s when observedat
the mantle; this speedis greater than the typical outflow velocities from the dayside cusp/cleftregion and places limitations on the possible circulation mechanismsof ionospheric
plasma.
Nishida et al. [1994a] reported on observations of fast
tailward flow at X=-70 RE, which were consistent with the
passageof a plasmoid. They showed that the slowly flowing
plasma sheet ahead of the plasmoid was acceleratedto high
speedsupon the plasmoid arrival. Contrary to the high-speed
beams observedat the plasma sheet boundary, the authors
clarified
that their observations
were consistent
with convec-
tive motionsof the plasmasheet.Machida et al. [1994] also
investigated the interaction of the plasmoid with the surroundingmantle plasma and found that the cold mantle ions
are pushed away from the neutral sheet plane before the
plasmoid arrival, whereas they convect toward the neutral
plane after the passageof the plasmoid. This was also true
in the Mukai et al. [1994] case.
A statistical survey of plasmoid signaturesdefined as bi-
portedobservations
of the abundance
ratiosof He++/H+ and polarsignatures
in bothBz andBy withpeakto peakampliO+/H+ in plasmoids.The relativeabundance
of ionospheric tude of > 3 nT [Moldwin and Hughes, 1992] revealed a sigto solar wind sourcesis variable not only from one plasmoid
to another but also in different regions within the same
plasmoid. The authors concludedthat both ionospheric and
solar wind plasma sourcesremain operational and dynamically influence the compositionalcontent of plasmoids.This
result is in agreement with the idea of Hughes and Sibeck
nificant number of earthwardmoving plasmoids. The earthward moving plasmoids are preferentially located close to
Earth and have average speed (based on electron measurements) equal to 218 km/s, which is half the averagespeed of
tailward moving plasmoids(498 km/s). Also, Nishida et al.
[1986] had noted that slow flow (V<300 km/s) plasmoids
12,994
BAKER ET AL.' THE NEAR-EARTH NEUTRAL LINE MODEL
800
I
I
I
I
I
_
"
¾EL
SUPERPOSED
EPOCH
I
I
I
I
I
I
PLASMOID
--
t
"
I
pose interesting problems.
The plasmoid association with substormshas been approachedin a statisticalmannerand on a case-by-casebasis.
Moldwin and Hughes[1992, 1993] identified 366 plasmoids
I
in the ISEE
4OO
--
o
_B
4
o
I--
4--
3 database and found that more than 84%
events occurred between
Bz
of the
onset.
I
did not have a correlation
I
index or in geosynchronousspacecraftenergetic particle injections. The authors discountedthe possibility that plasmoids are not related to substormsby noting that the quiet
time plasmoids occurredwithin prolonged periods of geomagnetic quiescencewhen (1) it is more likely for the AE
stationsto be outside the nominal position of the contracted
auroral oval; and (2) the inner edge of the plasma sheet and
the possible plasmoid related injections are much further
away from 6.6 RE. The location of the near-Earthand the
I
I
--
•'2•_ ..•
o
•'-2
-•
after substorm
I
I
ß1
LogFlux
,1• ? _
6.6 Re
4 and 60 min
Large, isolated substormshad a good one-to-one correlation
with plasmoids.Prolongedperiodsof activity that may have
multiple intensificationsdid not appear in one-to-one correspondencewith plasmoids.A fraction of all plasmoids(16%)
8
co
distanttail part of the Geotail mission. The examinationof
the plasmaion data for plasmoidsand other tail phenomena
hasbeen an importantelementthat was missing from previous studies.These data have brought new insight but also
I
I
I
--
GROWTH
I
distant tail reconnection
with
substorms
sites was inferred
in either
from
the AE
the times
of
the earthwardand tailward encountersof the plasmoids relative to onset and the plasmoid speed as measuredby the
5'-1 i
electrondetectoron ISEE 3. The averagetailward flow velocSUBSTORMI >$0 key
0
II
ity was 439 km/s. The resulting earthwardand tailward neuAL
I
tral lines at the time of onset were variable and were placed
-anywherefrom 10 to 140 RE.
-200I
Slavin et al. [1993] also noted an excellent agreementbe-400I
tween isolatedor paired TCRs and small substorms.The TCR
delay times relative to onset based on many indicators
-600
(AKR, AL index, Pi 2 pulsations, geosynchronousinjections) were all found to be in good positive correlation with
I I I I I
the distanceof TCR observation.The speedinferred from the
-120
-80
-40
0
40
80
120
TIME (Minute.)
plot of delay times versusdistancevaried from 580 to 1200
Figure 11. A superposed
epochanalysisfor ISEE 3 plasmaandfield km/s depending on the substormindicator used. The lowest
data(topthreepanels)and for electrondata(at L = 6.6) andAL index speed,580 km/s was determinedfrom the AL index as a subdata (bottompanels).Zero epochcorresponds
to plasmoidarrival at storm indicator, and the highest was determinedby using
ISEE 3 at -220 RE in thetail [fromBakeret al., 1987].
AKR as an indicator. Slavin et al. [1992] also pointed out
that
may be dueto simultaneousoperationof the near-Earthand
distant tail neutral lines prior to the onset of explosive,
lobe field line reconnection. More recently, Kawano et al.
[1995] presenteda bipolar magneticfield signatureobserved
undergeomagneticallyquiet conditions(Kp=0) and also interpretedthat in termsof two plasmasources:A near-Earth
and a distant-tail source.Their quasi-stagnantplasmoid was
also moving with a speedless than 300 krn/s. The modeling
results of Khurana et al. [1995a] help one approachthese
observationsfrom a different angle: Is it possible that the
quasi-stagnant
plasmoidsare low total currentstructures
that
are inconsequentialfor substormdynamics?
As shown in Figure 11, the ISEE 3 observationshave
shown a remarkable correlation
between substorms and dis-
tant tail phenomenasuch as tail expansions at growth
phase,plasmoidrelease,and passageof traveling compressionregions[Honeset al., 1984a; Bakeret al., 1984c, 1987;
Slavin et al., 1985; Moldwin and Hughes, 1993]. Many of
these results have been corroborated from the results of the
the correlation
between
substorm
onset
and TCR
or
plasmoidobservationwas not simply a statistical result but
is evident during periodsof recurrentsubstormsduring which
the magnetotail is monitored continuously. No plasmoids/TCRs were observed during a low geomagneticactivity period surroundingthe recurrent substorm period. The
time delays between plasmoid observation and onset were
consistentwith plasmoid formation due to a near-Earthneutral line at the time of onset with an inferred plasmoid speed
of 400 km/s.
Recent
Geotail
observations
have confirmed
the statistical
results from the ISEE 3 mission [e.g., Baker et al., 1987].
Nagai et al. [1994] used geosynchronousinjection data for
substorm identification and Pi 2 data for onset timing and
classifiedthe magnetic signaturesobservedat Geotail. Of the
89 events in their study, 53 had a clear bipolar signaturein
Bz. A timing analysison those eventsproduced
a goodcorrelation between delay times and observationdistance with a
slope of-775 krn/s. The inferred location of the near-Earth
neutralline was at X=-30 RE and the centerof the newly created plasmoid at X=-60 RE. Estimates of the near-Earthre-
BAKER ET AL.' THE NEAR-EARTH
NEUTRAL
LINE MODEL
12,995
GEOTAIIdMGF/LEP
g 1o
14
0
-10
• 10
0
-10
•
6
•
o
<1
o
:>' o
3
[.,,
1
o
x
GEOTAII-./LEP/I•,A-i
ION
OCT. 8. 1993
(a) 12:03:5912:04:11
UT
(b) 12:05:2,212:05:34
UT
(c) 12:05:3412.'O5:46
tYi'
i)awnwatd
Dawnward
Dawnward
o
Duskward
Duskw•ud
Du6kward
L(:X•(I•)
Figure12. Geotailobservations
of thermalionsinsidea plasmoid
(adapted
fromFigures2 and4 of Machidaet al. [ 1995]).
Toppanelsshowthemagnetic
fieldandplasma
moments
duringthepassage
of a plasmoid.
Themeasured
ionflowvelocityis
-700 km/s.Bottompanelshows
threeiondistribution
functions
duringselected
times:(a) At theseparatrix
layera cold,dense
ionbeamtravelingat --300km/sappears
simultaneously
witha hotion beamtravelingat 2100km/s.(b) The ion distribution
hasa character
similarto theprevious
one.(c) Ion distribution
insidetheplasmoid:The hot ion beamand the portionof the
coldionsthatcrossed
theplasmoid
boundary
areassimilating
to becomeonecomponent
movingtailwardat an averagevelocityof-700 km/s.
BAKER ET AL.' THE NEAR-EARTH NEUTRAL LI•
12,996
connectionline location basedon the assumptionthat the
MODEL
All the above observations
are consistent with a near-
plasmoidspeedis comparableto the local Alfv6n speed Earthlocationof the plasmoidearthwardedge.However,is it
(V-300-1000 km/s) and using AKR observationsfor sub- not possible to determinevery aCCurately(to bett6r than
storm
Onset
timingalsogavea distance
of X=-35REforthe several minutes) when the tailward plasma acceleration
near-Earth position.
startedin the plasma sheet. In addition, the above calculationseitherdependon inferenceof the local plasmoidspeed
basedon electronmeasurements,
or energeticparticle measurements,or predict a plasmoidspeedthat is consistentwith
suchmeasurements.
Observationsfrom the distanttail portionsof the Galileo and Geotail satelliteshave provideddirect measurements
of the plasmoidion flow velocity and
haveresultedin interesting
but•controversial
results.
Nishidaetal. [1995a]identifiedbasedon magneticfield
measurements
severaltypes.of neutralsheetcrossings.One
of the typeswas the type "B" crossing,that is, thosewith a
bipolar Bz signature.One suchsignaturewas examinedon
the basis of electric field and magneticfield data. The
authorsshowedthat the crossingwas consistentwith a tailwardmoving plasmoid with a speedof-430 km/s and was
centeredat X=-60 RE at the time of the correlatedAKR enhancementas shownin Figure 12. A large plasmoidspeed,
consistentwith the plasmoidspeedsinferred from the ISEE 3
instrumentationwas also foundin the plasmoidsstudiedby
Machida et al. [1994] (-700 km/s) andNishida et al. [1995a]
(300 -1000 krn/s).
Franket al. [1994] presentedmagneticfield and plasma
observationsof a plasmoid observedby Geotail at X=-125
RE (seeFigure13). Ion distributions
from that plasmoidwere
presented
by Frank and Paterson [1994]. The authorsargued
that the ion distributionwithin the plasmoidis composedof
a hot(Ti > 107K), tailward
streaming
(Vx=--500
km/s)and
low-density
(Ni -0.06 cm-3) ion population
anda colder,
slower(Vx=-200krn/s)anddenser
(Ni -0.1 cm-3) compo-
OX.
nT
nent. The number density of the cold ions was similar to the
mantle ion beams that surroundedthe plasmoid. The elec-
tronscanbe approximated
witha hot isotropic
Maxwelhan
By
distribution of a tailward
flow similar
to the ion flow and a
density equalto the sum of the densitiesof the two ion
I
I
populations.Thus quasi-neutralitywas observed,but the net
speedof the ions was only -250 km/s, that is, lower than
the speed of the electrons. The imbalance between electron
and ion speedswas indicative of a significant earthwardcur-
BZ
-,o
: ,-:
rentof-5-10 nA/m2. Theauthors
argued
thattheseresults
201- ' '
O!
,
1230
were consistent
ß 1 • 'IT ! •,
I3C)O
I
ß
1330 UT
GE:OTAIL OPT -HP
PIlOTON V•:LOClTY DISTRIBUTION
SOLAR ECLIPTIC CC)4]K)INATES
with
both
from the ISEE 3 satellite
the electron
flow
measurements
and the inferred flow
velocities
from the energetic particle instrumentson the ISEE 3 satellite. However,the ISEE 3 measurements
which were typically
500 -1000 krn/s are not representativeof the bulk flow of
the plasmoid becauseof the presenceof the cold ion com-
ponent.
The low (_<200 krn/s)plasmoid
speedis not a prop-
Vz
erty of only the plasmoid whose ion distribution functions
c-zzs3/cuS
iC;25
1306-1314 UT
31 MARCH Igg3
Figure 13. Geotailobservations
of thermalionsinsidea plasmoid
(adaptedfromFigures1 and2 of Franket al. [1995]andFigure4 of
Frank and Paterson[1994]). Top panelsshowplasmamoments
and
magnetic
fieldmeasurements
duringthepassage
of a i•lasmoid.
The verticallinesdelineatetheearthwardandtailwardsidesof the plasmoid.
Themeasured
ion flow velocityduringthisplasmoid
is -250 km/s.The
ionsdistribution
function
(bottom
panel)showsa coldanda hotcompo-
nent.
Thehotcomponent
isfiowin•g
tailward
witha flowvelocity
of 200
km/sandhasa density
of 0.1cm'ø. Thecoldcomponent
is necessary
to
satisfy
qUasi-neutrality.
The totaliondistribution
is flowingslowlybecauseof thepresence
of thedensecoldcomponent.
BAKER
ET AL.' THE NEAR-EARTH
NEUTRAL
LINE MODEL
12,997
were presented
by Frank and Paterson[1994],but of all four an increase in the near-Earth cross-tail current, in agreement
plasmoidsseenbetween0500 UT on March 30, 1993, and with direct observations. The details of the numerical models
affectthe stability of the tail, but eventuallyin all simula1306 UT on March 31, 1993.
Khuranaet al. [1995b] also presentedobservations
of flux tions the tail is driven to an unstable state, and global reropesseenby the Galileo satellite. There, too, a slow flow configurationof the magneticfield patternfollows. For ex(Vx --150 krn/s) flux rope was associatedwith the onsetof a ample,in the simulationby Walkeret al. [1993], even during
substorm. It was modeled with a structure that contained
a periodthat wouldbe characterized
as the late growthphase,
about a quarterof the total plasmasheetmagneticflux per a quasi-stableneutral line forms in the near-Earthtail (Figure
unit Y distance.The authorsarguedthat the near-Earthneutral 14). During this period, slow reconnectionbegins on closed
field lines, but this does not causea large-scale field reconline for that substormwas formedat a distanceof X=-50 RE.
The issue of the plasmoid ion flow velocity is quite important. It lies at the heart of the basic problem that faces
the near-Earth neutral line model: A low plasmoid speedresults in a plasmoidejection far from Earth and a location for
figuration. When reconnectionproceedsto open field lines,
a plasmoid is severed and begins to move away from the
Earth. Formation of a neutral line during the late growth
phase has been earlier suggestedby Baker and McPherron
the near-Earth
[1990], based on observational results obtained in the nearEarth magnetotail.
neutral
line
that
is far from
where
the
sub-
storm dissipationregion is expectedto map. It is important
to assesswhether the plasmoidsanalyzedby Frank et al.
[1994], Frank and Paterson [1994], and Khurana et al.
[1995b] are typical of the Geotail databaseof plasmoidsor
whether they representunusualevents, possibly due to the
geomagneticconditions prior to their observation.
Recently, with the use of two satellites (IMP 8 and Geotail) in the tail, Angelopouloset al. [1995] addressedthe issue of the tailward retreat of plasmoids and acceleration region in the courseof substorms.Angelopouloset al. [1995]
analyzed the release of a plasmoid during a small isolated
substorm. They showed that the plasmoid formed within
closedfield lines and gradually evolved to encompassmuch
of the flux of the plasma sheet. These observations also
agree with the observationsof Machida et al. [1994] and earlier observationsby Kettmann et al. [ 1990] that reconnection
evolved from closed field lines and gradually proceedsto the
last closedfield line. In the Angelopouloset al. [1995] event
the plasmoidcore moved tailward of X--70 RE and the earthward ion beamswere seenat IMP 8 at X--35 RE prior to the
poleward expansionof the activity at an auroral station. The
authorspresentedevidence that the active electrojet mapped
to the near-Earth environment (aroundX=-10 RE). They argued on the basis of the time delay that the accelerationregion (interpretedas the X line) and the equatorial projection
of the active electrojet were very far apart from each other
(one was tailward of X=-35 RE and the other was as close in
as X=-10 RE). These multi-point observationsfrom the tail
further elucidate the progression of the X line and its relationship to the near-Earth activity during the course of a
substorm.They support the earlier idea [Baker and McPher-
The large-scalefluid modelsconsideringonly a portion of
the solar wind-magnetosphere-ionosphere
system have basically producedsimilar results (see, e.g., the review by Birn et
al. [this issue]. In these models, the dynamical evolution is
initiated either by imposing a finite resistivity within the
plasma sheet or by enhancing convection through changes
in the boundaryconditions[Birn and Hesse, 1991; Janhunen
et al., 1995]. The finite resistivity triggers magnetic reconnection, and an unstable configuration develops. It is notable that in most simulations the energy dissipation is realized through magneticreconnection.
z 20Its
,
T = 38 min
Reconnection
on
z 20Rs
T = 47min
Reconnection
on
•1
open
field
lines
ron, 1990] and the numerical simulation results of Birn and
Hesse [1991] that the location of the X line and the location
of the current diversioninto the ionosphereare at two different places and can be far apart from each other at substorm
recovery.
z 20Rs
T = 57 min
3.7.
MHD
Simulations
Tailward
motion
Global and large-scale fluid simulations of magnetosphericdynamicsand of the solar wind-magnetosphere
interaction consistentlyshow that the responseof the magnetotail to external energy input (southwardIMF)is the formation and tailward ejection of an isolated plasma structure,a
plasmoid[e.g., Walker et al., 1993; Raeder, 1994]. This has
been true for a wide variety of simulation codes and external Figure 14. Development of magnetic field structuresin a threeparametersused in the simulation runs.
dimensionalsimulationshowingreconnectionand plasmoidformation
Enhancedenergy input is seenin the MHD simulationsas [from Walker et al., 1993].
12,998
BAKER ET AL.: THE NEAR-EARTH NEUTRAL LINE MODEL
Since we still lack global imagingof the entire magneto- sion for the magnetospheric
processesrepresented
by AL.
sphereor the possibilityto monitoreachkey regionwith a
For example,resultsof suchanalysisby Vassiliadiset al.
dense network of instruments, global fluid simulations, to- [1990] showed a correlation dimension of-3.5.
gether with comparisonwith observations, provide the best
State-spacereconstruction
using autonomous
methodsfor
way to studythe large-scaleresponseof the magnetosphere AE and AL tend to show low-dimensional behavior with a
to the variations in the external conditions. The global correlation
dimension
_<4.Thisresultwouldimply that the
MHD simulations have consistently verified the NENL coupledsolar wind-magnetosphere
systemsettlesonto a lowschemefor magnetosphericsubstorms,producingeffects of dimensional
attractor.This meansthatjust threeor four ordithe growth phase, a plasmoid during the expansion phase, nary first-orderdifferentialequationscould describethe esand a recovery phasecharacterizedby the tailward motion of senceof magnetospheric
dynamics.Hencea complexplasma
the plasmoid [e.g., Raeder, 1994]. However, there are still system with an essentially infinite number of degreesof
some aspectsin the present-daymodels which may affect the freedommay manifestjust a few of thesedegreesof freedom.
dynamicsequence
that the systemundergoes
undersolarWind This suggesteda high degreeof possible coherencein the
systemand the fractionaldimensionsuggested
a fractalstruc-
driving.
A key issuein substormresearchduring the past few years
has been the location of the substormOnset region in the
very innermost part of the magnetotail, that is, in the vicinity of the geostationaryorbit. This region is particularly difficult to representwith the global MHD models for two obvious reasons:First, the field and plasma gradients in that
region are large, and thus a small grid size is required.This
requiresboth enhancedcomputerresourcesand the development of variable-size grids. Second, the near-Earthplasma
sheetis largely populatedby particles originating from the
dense ionosphere.Because most of the MHD models do not
have an ionosphericsource,the ring current region is underpopulated, and the magnetic field and plasma sheet descriptions do not give an accuraterepresentationof the real magnetosphere.Accurate currentand plasma distributions can be
particularly important in determining the time scales of the
growthphaseand in determiningthe location wherethe instability growth occurs. Furthermore, the details of the current sheet disruption observed near the geostationary orbit
also require an accuraterepresentationof the local current
systems[Pulkkinenet al., 1995a].
ture and an underlying"strange"attractor.
More recent examinations of magnetosphericdynamics
have made clear that one must consider the solar wind and
the magnetosphere
as a completesystem.This is becausethe
magnetosphere,and the auroralactivity representedby AE,
dependsfor virtuallyall of the dissipatedenergyon the external driving by the solar wind. Hence one must consider
the magnetosphereas a nonautonomous rather than autonomous system. Input-output methods are therefore necessary as was pointed out by Prichard and Price [1992]. Certainly, it is possible to constructdata bases which associate
solar wind inputs with magnetospheric
outputs[e.g., Bargatzeet al., 1985; Price et al., 1994]. Vassiliadiset al., [1995]
have found evidencefrom nonautonomous
analysesthat nonlinearity of optimal activity predictorsmay be equivalentto
nonlinearity of the solar wind-magnetospheric
coupling.
This issueremainsto be fully resolvedand is the subjectof
very active research[see Klimas et al., this issue].
Building on the apparentevolution of linear prediction
filter elements with increasing geomagneticactivity [see
Bargatzeet al., 1985],Bakeret al. [1990] developeda simple
The formation
of thin current sheets and their effects on
mechanicalanaloguemodel of substormdynamics.As shown
the substormevolution has also been under lively discussion in Figure 15, the modelconsidersa masson a spring. The
recently [e.g., Pritchett and Coroniti, 1994; Sergeevet al., massincreasesat a fixed loadingrate thL until a critical disthis issue]. Some of the MHD models show quite consider- tentionD=D c is reached.At this critical point, a portion of
able thinning of the near-Earthcurrent sheetprior to the on- the massis released
from the springat a rate (rhu) that is
set of reconnection, but this has not been the case for all governedby the velocity of the mass D at the critical dissimulations [e.g., Pulkkinen et al., 1995a]. Further investigationsare neededto find the controllingparametersfor
the thin current sheet formation, to ensure that the simula-
placement
point.Thesetof equations
for /5 and /3, as
closed
by theconditions
on rh, leadto a nonlinear
system
of equations.
As the thL increases,
the systemgoesfrom
tions representreality the best possible way.
weakperiodicunloadingto highlychaotic,nonlinearbehav-
3.8.
Low-Dimensional
The NENL
Nonlinear
Dynamics
model is one of the few models which
consid-
DrippingFaucetAnalogueModel
ers the complete, global substormframework.However, the
very general aspectsof solar wind-magnetospherecoupling,
have been examined by several authors using the tools of
nonlineardynamics[e.g., Vassiliadiset al., 1990; Robertset
al., 1991]. In this "state-spacereconstruction"technique,the
AL index is treated as a general time series that reflects the
basic dynamicsof the magnetosphere-ionosphere
system. A
state vector is constructed
from the extensive AL time series.
The generationof state vectors createsa new m-dimensional
spaceout of the AL time series. Within this m-dimensional
space,one can examine the intrinsic correlation characteris(a)
tics of the dynamicalsystem.The correlation dimensionv
for higher and higher m-space embeddingdimensions be- Figure 15. (a) A mechanical
analogue
of thedrippingfaucet.(b) Analcomesa good estimateof the dynamical "attractor"dimen- ogyof plasmoidformationto a leakyfaucet[fromBakeret al., 1994b].
BAKER ET AL.: THE NEAR-EARTH
ior. Thus the loading and unloading of the mass leads to
complex interferencesas the loading rate increases.
The mass-on-springmodel of Baker et al. [1990] took
cognizanceof an analogybetweensubstormunloadingprocessesand a drippingfaucet [Hones, 1979], as shown in Figure 15. In the magnetotail, the plasma sheet distendsand
stretchesduring the growth phase.At substormonset (in the
NENL model), a part of the plasmasheetpinchesoff to form
the substormplasmoid. The plasmoidmoves down the tail
and out of the system at high speed while the residual
plasmasheetsnapsback toward the Earth. The upperpart of
Figure 15 shows that the dripping faucetbehavesin a very
analogousway to the unloading magnetotail during sub-
NEUTRAL
flux is reconnected.
As shown by Hesseand Birn [1993], an energy balance
equationneedsto be satisfiedin the substormcurrent disruption region V. In general, one can integrate the energy equation over the volume
nal dynamicsin the tail near the NENL position;and (3) the
time-varying magnetic flux content of the tail lobes. By
prescribing a critical lobe flux content, the Faraday loop
model is similar to the drippy faucetmodel. Flux loads into
the tail at a rate determinedby the external (solar wind) electric field Eo. When the critical flux level is reached,the tail
unloadsby forming a plasmoid, thereby dumping much of
the free energy in the tail. This is very consistent with the
substormcycle depictedin Figure 15.
An advantageof the Faradayloop model over the mechanical analogue model is that it is possible to drive the
Faradayloop using realistic solar wind electric field variations [Klimas et al., 1992, 1994]. By using observedVBs
valuesfrom the Bargatzeet al. [1985] data set, it was possible to comparethe analoguemodel responseto that of the
real magnetosphere.Moreover, in the Faraday loop model,
one can also examine aspectsof loading and unloading in
ways that are not controllable in the real magnetosphere.
The very simple global analoguemodels, including as they
do the essenceof the NENL model, are able to represent substormdynamicsto a remarkabledegree[Klimas et al., this issue].
3.9.
Energy
Budgets
A problem with powering the substorm by a near-Earth
currentdisruption is the source of the energy for the substorm expansion phase. An analysis of the near-Earth
plasma sheet carriedout by Hesseand Birn [1993] demonstratedthat there is not sufficient energy in. the closed field
line region of the inner plasma sheet to power a substorm
expansion.Thus it is essentialto tap the reservoir of energy
storedin the tail lobes [Bakeret al., 1995a]. A similar argument can be applied to flux balance.The buildup of the lobe
field is generally attributed to opening of field lines on the
daysideand their transportto the tail lobes. It is difficult to
V to show that
+
¾ v 2#0 2
+
cross-tail
electric
fieldin thecentralcurrent
sheet,Ey;(2)
therateof change
of Ey dueto external
loadingand/orinter-
12,999
understandhow the return of the lobe field to presubstorm
values can occur during an expansion phase as is observed
statistically [Caan et al., 1978], and yet none of the open
storms.
The mechanicalanaloguemodelof Baker et al. [ 1990] was
able to model several aspectsof substorms;the growth, expansion,and recovery cycle was accountedfor. Also, as the
loadingrate went up, the analoguesystem exhibited nonlinear evolution, period-doublingand bifurcation, and eventually full-fledgedchaotic dynamics.For a rather simple system, this is a quite successfulrepresentation.However, the
mechanicalanalogy is somewhatlimited and so Klimas et al.
[1992] developeda plasma physical analogue model called
the Faradayloop model. This is a dynamic global convection model with three degreesof freedom: (1) the average
LINE MODEL
dsn'
•o3
ExB +
P0
+P_l_+
v + 2p_l_
+
dV
v_l-
2
+ vx+•v,v,•+q}• 0.
(1)
This is the most general formulation assuminganisotropic,
hot flowing plasmas. The equation shows that the time derivative of the volume integral of the magnetic, kinetic, and
internal energies of the plasma is equal to the surfaceintegral of the energy fluxes. These are, respectively, the
Poynting flux, the kinetic energy flux, the enthalpy flux,
and the heat flux.
This very general relation is often greatly simplified. For
example, Hesse and Birn [1993] note that if one can define a
suitable
volume
which
is "closed"
and across whose surface
the energy fluxes are negligible, then the first integral in (1)
should be zero. For a fixed volume
V this relation
can be re-
written for some initial time to and somefinal time tI as
2//0
+0vg
2
2//0 2
-•- V.
(2)
Hesseand Birn [1993] go on to showthat for observedmagnetic field and plasma flow conditions, this balance cannot
be obtainedin the inner plasma sheetregion. They therefore
concludethat energy must flow into the "disruption"region
from a strongenergy sourceoutsidethe near-tail.
If the closed field lines of the plasma sheet are not adequate to power the substorm,then one must look to the open
field lines of the tail lobes connection to the polar cap.
Baker et al. [1995a] have applied the energy budget formalism above to a specific event. Four specific times were identified: The beginning of the growth phase, the expansion
onset, the beginning of the recovery phase, and the end of
the recoveryphase.A good magneticfield model for the first
and last time steps was the unmodified Tsyganenko [1989]
model. The model at substorm onset was the maximally
modified growth phasemodel with a thin and intense current
sheet in the near-Earth tail, and at the beginning of the recovery phase the field was strongly dipolarized and the
plasmasheetwas expanded[Pulkkinenet al., 1992, 1994].
For each of the time steps,the magnetic energy given by
the first term of (1) was evaluated for both the open field
13,000
BAKER ET AL.: THE NEAR-EARTH
line and plasma sheetregionsseparately.In addition,for the
plasma sheetregion the kinetic energy given by the following three terms in (1) was evaluatedusing (constant) observational values for the velocity and pressure(which was assumedisotropic).The region of the tail included in this volume integralwas chosento be from X=-4 to X=-20 RE, -15 <
Y < 15 RE, and Z from the currentsheetcenterto the magnetopausedefinedby the field models. As the magnetic field
model doesnot specify the open-closedfield line boundary,
the plasma sheet region was definedby mapping the poleward boundary of the oval to the tail. Evaluation of the
above integralsrevealed that the lobe region is the only region to increaseits energy contentduring the growth phase.
The plasmasheetmagneticenergydecreasesdue to the thinning of the current sheetand consequentdecreaseof B in the
plasma sheet. The plasma energy decreasesbecause the
plasmais cooled and the density is reduced.
During the expansion phase, the lobe energy is dramatically decreased.During this period, the plasma sheet magnetic and plasma energy increase:B increaseddue to the field
dipolarizationand the plasma was heatedand the flow velocity increased.During the recovery phase,the systemreturned
to the original Tsyganenko model. Lobe magnetic field increased,leading to an increasein the lobe energy, and in the
plasma sheetthe magneticfield was reduced.
Overall, the work of D.N. Baker et al. (A quantitative assessmentof energy storageand releasein the Earth's magne-
NEUTRAL LINE MODEL
totail, submittedto Journal of GeophysicalResearch, 1995,
hereinafterreferred to as Baker et al., submittedmanuscript,
1995a) andHesseand Birn [1993] strongly suggeststhat the
inner plasma sheet region cannot supply very much of the
energy to power substorms.Therefore, an alternative power
supply is clearly required. As shown by Baker et al.
(submittedmanuscript, 1995a), a more than adequatesupply
of energy can be obtainedby tapping the lobes of the magnetotail through near-Earth magnetic reconnection. More
eventsneed to be evaluatedin this comprehensiveway using
the fall suite of ISTP spacecraft.
3.10.
Plasma
substorm onsets, where the IRM
450.
65.
400.
80.
"•
350.
55.
•
300ß
50.
• 250.
45.
2oo.
40.
,
0.30
,
I
-45.
ß
I
Substorm
'
,
I
Onset
Minutes
•.0
'
,
,
.
O.
...
ß
,
around
!
.
45.
90.
Substorm
,
Onset
.
ß
ß
:
:
'•
•
-
35.
-45.
around
satellite was located be-
tween 10 and 19 RE. The AE trace looks typical for a moderate-to-major substorm.On average, the 45 rain following the
onsets can be characterized as expansion phase with AE
reachingpeak values around 500 nT. During the last 45 min,
AE decreasesagain, and this interval can thus be characterized as the first part of the recovery phase.
Baumjohannet al. [1991] constructedaveragetracesof the
70.
Minutes
Processes
During disturbedtimes, the plasma sheet changesdramatically. Baumjohannet al. [1991] did a superposedepoch study
of the behavior of the plasma sheet around substorm onset
in order to study these changes.The upper left panel in Figure 16 showsthe average behavior of the AE index during 39
•00.
150.
Heating
ß
:
0.25
0
'•
0.20
.
?.0
e
ß
0.15
o
6.6
0.10
:
.
:
:
:
0.05
-45.
Minutes
.
:
:
;
O.
around
.
:
:
.
;
45.
Substorm
.
6.0
90.
Onset
-45.
Minutes
:
:
:
:
.
,
O.
around
45.
Substorm
O0.
Onset
Figure 16. Superposed
AE indextrace,constructed
by averaging
39 AE indextracesin 9-minbinswithrespectto 39 substormonsets(upperleft). The otherthreepanelsgive the averageion temperature
(upperright),the specificentropy,
kTi/Ni
2/3(lower
left),
and
theaverage
ion-to-electron
temperature
ration
(lower
right)
inthecentral
plasma
sheet
during
the
same39 substorms.
Thedashed
verticallinesmarksubstorm
onsetandthe startof the recoveryphase[ Baumjohann
et al.,
1991].
BAKER ET AL.: THE NEAR-EARTH
NEUTRAL LINE MODEL
13,001
development
of relevantmagneticfield andplasmaparame- model. One has to await hybrid, or at least two-fluid, simulaters by binning measurements
taken between45 min before tions to see whether the ion tearing mode actually heats ions
and 90 min after each of the 39 substormonsetswith respect and electronsin parallel.
to the particularonsetinto 9-min-wide bins and then averaging over all the samples in a particular bin. Their result is
consistent
with those achieved
in earlier
statistical
studies
which binned the data with respect to the AE index
[Lennartssonand Shelley, 1986; Huang and Frank, 1986;
Baumjohannet al., 1989]: During a substormthe magnetic
field gets more dipolar, the ion temperatureincreasesmarkedly, and the plasma density in the central plasmasheetis
somewhatdepleted.
IRM data [Baurnjohannet al., 1991] as well as ISEE data
[Huang et al., 1992] show that the heating of the central
plasma sheet during the expansion phase to about twice its
presubstormtemperatureis accompanied by a density decreaseof about 25%. This clearly indicatesthat the substorm
heating cannot occur in an adiabaticfashion, where temperature and density changes must be positively correlated according to the polytropic equationfor adiabatic spaceplas-
masP = atN5/3,whereat is the specificentropyof the
plasma.
The lower left panel of Figure 16 [from Baumjohannet al.,
1991] showsthe superposed
epochtrace of the averagespe-
4.
Future
4.1.
Aurora
Directions
and
Near
Tail
Is it possibleto explain recent observationsof aurora and
magnetic field in terms of the NENL model? We believe the
answer is yes, but not without modification of the model.
Such modifications
should not be considered a failure of the
model but the necessaryconsequencesof increasedunderstanding brought about by new observations. It should be
rememberedthat a substormis a collection of physical processesorganizedin time and space.Additional processescan
be addedto this collectionwithout changingthe basic nature
of the model. In our opinionthe fundamentalquestionraised
by recent observations is whether the current disruption
mechanism triggers reconnection after a short delay, or
whether current disruptionis a near-Earth manifestation of a
somewhat
more distant
X line.
The most crucial factor in the determination
of an inner
cificentropyat = kTi /Ni 2/3.Thespecific
entropy
trace tail substormonsetis relative timing in the observationsof
showsa rapid change during the expansion phase. Actually,
the specific entropy increaseseven more strongly than the
ion temperatureduring the expansionphase and, as a result,
presubstormand postsubstorm
plasmaare at totally different
specific entropy levels. Thus the substormheating process
must clearly be a nonadiabatic process, most likely current
sheet heating or heating by reconnection.
It is not only the ion temperaturethat increasesstrongly
with geomagnetic activity, the electron temperaturevaries
the same way, too. The two temperaturesshow a very high
degreeof correlation, with Ti/T e -- 7 [Baumjohannet al.,
1989]. The linear relationbetweenTi and Te holds for temperature variations over nearly two decades,that is, both in
a hot and cold plasma sheetand is totally independentof the
the occurrenceof different phenomena [McPherron, 1978].
While ground-based observations can often be timed to
within tens of seconds,space-basedobservationsoften have
temporal uncertainties of the order of a few minutes. These
are due to the fluctuation levels present in the data at most
times and problems with one-point measurementsto locate
the relative position of the activated region. In addition to
theselimitations,magneticmapsbasedon averagemagnetic
field modelstend to be exceedinglyinaccurateduring times
of high geomagneticactivity, or just prior to substormonset, when thin current sheets change the magnetic field geometry to be much more tail-like. More meaningful magnetic
mapping requires specially tailored field models, which have
been developed in some cases [Pulkkinen et al., 1991a,
disturbance level.
1992]. In these cases, auroral featuresare mapped signifiIn fact, one can see from the lower right panel of Figure cantly farther downtail than expectedfrom average models.
16 that the temperatureratio in the central plasma sheet Thereforean exact cause-effectrelation betweenprocessesin
staysbetween6.5 < Ti/T e < 7.5 during the courseof a sub- the ionosphere, the inner tail, and the midtail and outer tail
storm. There are temporal variations of this ratio, but they is, at best, difficult to establish and suffers from a high deare not related to the substormphases in any systematic gree of uncertainty.
way, thereby corroboratingthe conclusion of Christon et al.
This uncertainty, however, does not take away from the
[1988] that the plasma as well as energetic ion and electron fact that these processesindeed occur. Since these features
populationsrespond collectively as a single unified particle are an importantand repeatablepart of substormdynamics,a
population during plasma sheet temperaturetransitions.
completemodel of substormevolution needsto include a satHence any substorm theory has to include a mechanism isfactory explanation of near-tail events. Possible causal
that heats the plasma in a nonadiabaticway, increasing its connectionsto other phenomenaand to substormprocesses
entropy and treating ions and electrons as a single fluid. like magnetic reconnection must be established. One such
MHD simulationsof magnetotailreconnection[Hesseand connectioncan be madeon the basis of global arguments,
Birn, •992] tell us thatnonadiabatic
heatingandincreaseof which show that magneticreconnectionis required to tap the
plasmaentropy are a natural consequence
of the ion tearing lobe energy and magnetic flux reservoir as an energy source
mode.Thesedramaticchanges
in the centralplasmasheetare for the majority of the substormenergy dissipation(Baker et
hence consistent with the neat-Earth
neutral line model.
However, the parallel heating of ions has not been explained yet, either in the framework of the near-Earthneutral
line model or by any other theory. Since such a behavior
can, by definition, not be seen in the single fluid MHD
simulations as presently performed,this is not necessarily
an argumentagainst the validity of the near-Earthneutral
al., submitted
manuscript,
1995a),anda flux sourcerequired
to dipolarize large sections of the inner plasma sheet. This
fact doesnot, however,precludethe possibility that the
auroralbrighteningmaps away from the actualreconnection
site, for example, somewherecloser to the Earth instead.
This scenariois suggestedby the large spatial distancebetweenthe reconnection
site, placedat or outwardof-20 RE
BAKER
13,002
ET AL.: THE NEAR-EARTH
downtail from the Earth, and the inner magnetosphere.If a
near-Earthsequenceof eventsshouldoccurfirst, magneticreconnection(and thus a large-scaletail instability) must then
be triggered by the original onset process through some
complicatedinteraction, Thus one important goal of future
research should be to establish this connection
and its direc-
tion.
4.2.
Midtail
Region
UsingIMP 6 and8 plasma
data,Nakamura
etal. [1994a]
found that high-speedflow events with velocities in excess
NEUTRAL
LINE
MODEL
near the neutral sheet at tailward distances between 11 and
19 RE. It is quite apparent that the probability to observe
even short-lived high-speedflows drops sharply inside of
about 15 RE.
It seemsimportant to answer the question of what happens further outward, becauseif the flow is bursty from the
beginning, this would put considerableconstraints on the
source mechanism creating these flows. If, on the other
hand, a coherentsteady flow is convertedinto a more incoherent bursty flow somewherearoundtailward dislancesof
25-30RE, it maytell ussomething
abouthowthekibetic
energyof thefastflowingplasmais converted
ihto thermal
of 300 km/s near tile midtail neutral sheet were essentially
energy.
It
may
also
explain
the
clearly
observablfi
herating
restricted
to tailwarddistances
of XGSM < -24 RE. Setting
the thresholda bit higher, at 400 km/s nearly all high-speed
flows were observedeven further tailward (XGSM <-30 RE,
cf. their Figure 4).
Since the IMP plasmadata had a temporal resolutionof
only 0.5 to 2 rrfin, short burstsof high-speedflow, which
may haveoccurredfurtherinward,wouldhavegoneunnoticed
in the. data set usedby Nakamura et al. [1994a]. Indeed,
Baumjohannet al. [1990], using IRM plasma data with its
muchhigher resolutionof about5 s, foundthat high-speed
ion flows with velocities often near the local Alfv6n speed
can be observed near the neutral sheet at radial distances of
less than 20 RE, but that these flows are very short-lived,
often lasting less than 10 s (see also section 3.2). Hence,
taking into ,,,.account
both studies,there seemto be two
choices.Either relatively steadyflows get burstyinside of
of electronsand ions duringthe substormexpansion phase
(seesection3.10). Finally,the observed
brakingof the bursty high-speed flows around 15 RE has still not been explained theoretically, at least within the framework of the
near-Earth
neutral
line model.
The Geotail spacecrafthas now entereda phaseof its mission whereit is in a 10 x 30 RE geocentricorbit. It has very
comprehensiveplasma measurementcapabilities and should
be well suited for examining plasmaflow and associatedfield
characteristicsin the critical region beyond ~20 RE. These
data will be crucial in addressingthe issueof where and when
the X line forms in relation to substorm expansion phase
onset.
4.3.
Distant
Tail
Time-dependent,
localized accelerationand plasmoids.If
indeed
reconnection
operatesin a time dependentand localXGSM<-35 RE,'yet have suchhigh speedsthere that even
0.5 or 2-min averagesstill yield velocitiesin excessof 400 ized mannerhow do the multiple and often simultaneously
operatingacceleration
centersproducelarge-scaleplasmoids?
km/s.
That there is indeed braking of bursty high-speedion flow Is it possible that during moderatesubstormsthe tail beto much lower velocities, at least further inward, has already haves in a more organizedmannerthan duringlarge subbeen observed.Figure 17 showsthe occurrence
rate of high- storms?Nishida et al. [1988], Fairfield et al. [1989], and
speedion bulk flows detectedby the IRM plasmainstrument Moldwinand Hughes [1993] all commentedon the fact that
duringlarge substormsthere are not alwaysplasmoidsor
25-30 RE or "theflows are alreadyburstyaround-30<
multiple plasmoids observed for each substorm intensification. They also emphasizedthat isolated substormsexhibit a
good one-to-onecorrelationwith one or more plasmoids.
Similarly, Slavinet al. [1993] pointedout that large sub-
stormsproduce
multipleTCRs,whereas
moderate
substorms
.
produceoneor two TCRs with a time delay relative to onset
consistentwith a near-Earthgenerationof a plasmoid.
Is it possible that large substormsare associatedwith a
directenergydissipation
in the ionosphere
withouta loading
phase?The resultsof Nishidaet al. [1994b]suggest
thatit is
indeedpossible for the distant magnetotailto be in a state
w
where no plasmoidsare seenbut insteadneutral sheetfluctua-
0.
-19.
-17.
-15.
XG$M, Earth
-13.
-11.
Radii
Figure 17. Occurrence of high-speedion bulk flows in excessof 400
km/snearthe midnightsectorneutralsheetversustailwarddistancefrom
Earth [after Baurnjohannet al., 1990].
tions of short time scale (and thereforeshort spatial scale
occur).This third stateof the magnetotailat activetimeshas
no analogin the near-Earthplasmasheetwhereeven during
prolongedactive times the flow is bimodal [Sergeevand
Lennartsson, 1988]. Critical in our understanding
of the
magnetotailbehavior duringsuchtimes is an understanding
of the interplanetaryconditionsthat drive the magnetotail.
This is possible for the distant tail portion of the Geotail
missionbecauseIMP 8 datawereavailablefor a large portion of that period, unlike duringthe ISEE 3 excursionsof
the distant tail. The correlations between IMP 8 solar wind
input and the Geotail measurementsof the distanttail behavior may cast light on our understanding
of how energy gets
dissipatedfrom the solar wind to the ionosphere.
BAKER ET AL.: THE NEAR-EARTH
NEUTRAL
LINE MODEL
13,003
count for feedback processesrelated to loading-unloading
which cannot be expressedin terms of moving-averagefilters. The nonlinearity of solar wind couplingis suggestedby
the variability of the numberof filter coefficients necessary
for differentlevels of activity and typesof AL response.The
nonlinearity is even more evident in the superior performance of nonlinear state-input models (obtained from a
timates gave results consistent with a near-Earth location of small number of similar data) to linear state-input models
the NENL. Although the plasmoid speedquotedby Machida (obtained from an average over a large number of data of
et al. [1994] and Nishidaet al. [1994b] is in agreementwith various activity levels). This type of work must be related to
plasmoid speedestimatesfrom the energetic particle instru- detailed magnetotail processes.
ment on ISEE 3 [Richardson et al., 1987] and from the elecBaurnjohannet al. [1995] did a superposedepoch to study
tron flow measurements
[Moldwin and Hughes, 1992, 1993], possible differences in the behavior of the near-Earth tail
the measurementsof plasmoidspeedsby Frank et al. [1994], aroundsubstormonsets that occurredduring the expansion
Frank and Paterson [1995] and Khurana et al. [1995b] are phaseof magneticstormsand those that were not accompamuch lower (at least a factor of 2). As a result the near-Earth nied by a magneticstorm (Dst>-25nT). Comparing these reneutral line location estimate would be much further away sults with the signaturesto be expectedfrom the near-Earth
from Earth than originally postulated.This poses a serious neutralline model [e.g., McPherronet al., 1973; McPherron,
Problem of plasmoid speed.One of the most puzzling
problemsthat has surfacedduring the distant tail portion of
the Geotail mission has been the estimatesof the plasmoid
speed.This lies at the heart of the verification of the nearEarth neutral line model for substormsbecausethe timing
analysesdone in the past using the ISEE 3 satellite data all
dependedon an estimateof the plasmoid velocity. These es-
problemto the NENL modelfor substorms
thathasto be ad- 1991; Hones, 1984], only the substorms
occurringduring
dressedby meansof statisticalstudieson a large set of the stormmain phaseseemedto show the expectedfeatures.
plasmoidsjust as was done with the ISEE 3 data sets by For this type of substorm,a clear dipolarizationof the tail
Moldwin and Hughes[1993].
magneticfield and decreasein the strength of the lobe field
Slow shocks. The recent Geotail plasma measurementswas seen,both of which were expectedto be associatedwith
have providedimportant new information concerningslow the formation of a near-Earth neutral line tailward of the satshocks in the distant tail. In particular,Saito et al. [1995] ellite during substormonset and subsequent
reconnectionof
used quite rigid selection criteria and found several events closedplasmasheetfield lines and open magnetic flux tubes
that satisfied the Rankine-Hygoniot relations across the intermediatelystoredin the tail lobes. During the typical
shock and in addition had an electron heat flux upstream. nonstormsubstorm,a more dipolar field also develops,but
Furtheranalysisof the plasma,magneticfield, energeticpar- the dipolarizationis weakerand maximizesonly near the end
ticle and wave behavior aroundthese entities showsgreat of the expansion phase, after the AE index has reachedits
promiseto provide insighton the microphysicsof magnetic maximum value. Moreover, during this type of substorm,the
reconnection in the distant tail. The realization that slow
lobe field strength stays nearly constant. This work raises
shocksare not seenclose to Earth suggeststhat either near- fundamentalquestionsabout the differencesbetween procEarth reconnectiondoes not proceedin a mannersimilar to essesduring relatively quiet and relatively disturbedcondidistant tail reconnection or that the conditions in the neartions. Certainly more researchis neededin this area.
Earth environmentinhibit formation of slow shocks [Cattell
Results presented by Baker et al. (Re-examination of
et al., 1992]. Understandingthe reason for this near-Earth driven and unloading aspectsof magnetosphericsubstorms,
tail behaviorwill be an important contributionto the near- submittedto Journalof GeophysicalResearch, 1995, hereinEarth neutral line model.
after referred to as Baker et al., submitted manuscript,
1995b)utilized the Faradayloop nonlinear dynamicalmodel
of
substorms.Computationalsimulationswere shown using
4.4. Coupling Studies
realistic solar wind input time series. Baker et al. demonGeneralizing
linear predictionfilters with nonlineardy- stratedthat realistic replicationsof the AL index result from
namical methods, nonlinear filters have been based on a the model simulationsif unloadingis includedin the model.
methodof representing
recordsof past geomagnetic
andso- On the other hand, if unloadingis not permitted, that is, if
lar wind activity as points in a phasespace[Vassiliadiset only a driven response is included, then the resulting AL
al., 1995]. The state-input space representation,where (electrojet current)simulation is unrealistic in both its amnearbypoints correspond
to similargeomagneticand solar plitude and its temporal evolution. Quite importantly, the
wind conditions, is useful in studies of the solar wind-
bimodal shape of the linear response filters (as found by
magnetosphere
couplingsuchas classification,superposedBargatze et al. [1985]) could not be reproducedunless unepochanalysis,and prediction.In the state-inputspace,the loading was included.
magnetospheric
statevectorevolvesin time subjectto solar
The modeling resultspresentedby Baker et al. (submitted
winddrivingconditionsandto internalmagnetospheric
dy- manuscript,1995b) also shednew light on several aspects
namics.By fitting linear filters locally to the magneto- of geomagneticactivity: (1) the nature of the isolated subsphericdynamicsin the state-inputspace,one obtainsnon- stormresponsewas characterizedfor the full dynamical path
linearmoving-average
filtersandstate-input
models.
through growth, expansion, and recovery; (2) the role of
Nonlinearmoving-average
filters assumethat the auroral chaotic and nonchaotic behavior was clarified for realistic
geomagnetic
activity dependssolely on solarwind parame- solar wind input conditions; and (3) the intrinsically biters. They generalizethe linear moving-average
filters and modal responseof the magnetospherewas demonstratedfor
give improvedcorrelations
andreducedsingle-step
prediction moderate activity conditionsboth for the real systemand for
errorsfor syntheticand real AL data[Vassiliadiset al., 1995]. the analoguesystem.This tends to suggestagain (as seen in
State-input-space
modelscorrelatethe presentgeomagnetic earlier work)that the simple analoguemodels including tail
outputwith bothearlieroutputsandinputs.Thesemodelsac- reconnectionare able to capturemuch of the essenceof ac-
13,004
BAKER ET AL.: THE NEAR-EARTH NEUTRAL LINE MODEL
.........
x
x
BAKERET AL.:THENEAR-EARTH
NEUTRALLINEMODEL
13,005
tual magnetospheric
behavior. As shownby Baker et al. tails: This is the essence of science and seems to us in no
(submittedmanuscript,1995b) and Klimaset al. [this issue], way a fatal shortcoming of the basic model. Many of the
theFaradayloopmodelclearlyrequiresan internalunloading most recent observationalresults seem to fit rather naturally
processin order to reproducethe data. It therefore is con-
into the NENL framework. Also, numerical simulations and
cludedthat both driven and loading-unloading
processes
are analoguemodels seem to supportthe general elementsof the
key to describingfully the generalsubstormdynamicalse- NENL approach. Throughout this review we have tried to
quence.
show fruitful directions for continuing research. We are
hopeful that powerful new theoretical modeling methods and
4.5. Modeling and Theory
a new generation of multipoint observationsin the next few
years
will help us to improve the NENL model even more. In
The ISTP program with its multiplicity of observations
particular,
it is hoped that the inner magnetotail can be
from variouskey regionsemphasizes
the role of large-scale
modeled
successfully,
and it is also hoped that observations
modelsin order to interpret the complex, combineddata
on all appropriate scales can reveal clearly the processes
sets.The globalmagneticfield configuration
is highly varithat drive auroral arcs and lead to the substorm onset.
able,thuscomparisons
of datasetsfrom differentregionsrequirestime-dependent
information about the magneticfield
Acknowledgments.
Work at CU/LASPwassupported
by grantnumevolutionas shownin Plate3 [Pulkkinen
et al., 1995b]. Thus ber NAG5-2664 from NASA. The authors thank the two referees for
empirical formulasrepresentingthe various states of the outstanding
assistance
in improvingthispaper.
magnetosphere
are urgentlyneeded.
The EditorthanksV. A. SergeevandG. Rostokerfor their assistance
In addition to the magneticfield configuration,the in evaluatingthispaper.
plasmamotionsare controlledby the large-scaleelectric
field pattern. Simple modelsfor the near-Earthelectric field
[e.g., Volland, 1973] and statistical results for the inner
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