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. . Their relation modelhas coveredprimarily the midtail and far-tail regions was based on the work of Dessler and Parker , 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  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.  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 , 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  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.  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  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  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  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  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 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 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 , 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.  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.  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 $0 ............... 50 4O -;50 -15 0 15 ;50 45 60 75 90 105 120 1;55 150 Time[minutesfrom onset] Figure 4. Timing of the substormrecoveryat severallocations. was presentedby Baker et al. . They used numerous high-altitudesatellites as well as global auroralimages and ground-based data to studythe substormgrowthand early expansion phase. The magneticmodeling methodsof Pulkkinen et al. [1991a,b] were also employed.Figure 5 summarizes •s$ 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.  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.  andFedderet al.  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. ). 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  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  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  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.  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. . 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.  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  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.  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.  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. . 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. . the near-Earthplasmasheet[Sergeevet al., 1991]. The above Drnitrieva and Sergeev  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 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. , McPherron et al. [1986b], and Dmitrieva and Sergeev  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. , Ohtani et al. [1992c], Lopez et al. phree et al., 1991]. Multispacecraftdata and ground-based , and Angelopouloset al. . 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.  showedthe importanceof nightside earthward flux transportand its association with geomagneticactivity. may still be weakly activated. Koskinenet al.  and Nakamura et al. [1994b] showed Lopezet al.  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.  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.  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.  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  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.  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.  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. . 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.  paper. It is likely that the ionospheric manifestation of the short-lived, localized, tailward retreating magnetotail activations of Angelopoulos et al.  are the progressively poleward moving short-lived auroral activationsof Bosinger and Yahnin . Sergeevet al. [1995a] also provide further evidencefor this process. In addition,Angelopouloset al.  showedthat IMP 8 NEUTRAL LINE MODEL 12,993  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.  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  and Chen and Wolf  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.  re- moid/mantle interface.Mukaiet al. 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.  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.  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.  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.  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.  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.  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.  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.  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.  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.  (-700 km/s) andNishida et al. [1995a] (300 -1000 krn/s). Franket al.  presentedmagneticfield and plasma observationsof a plasmoid observedby Geotail at X=-125 RE (seeFigure13). Ion distributions from that plasmoidwere presented by Frank and Paterson . 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. andFigure4 of Frank and Paterson). 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,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. , 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 , 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. , Frank and Paterson , 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.  addressedthe issue of the tailward retreat of plasmoids and acceleration region in the courseof substorms.Angelopouloset al.  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.  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.  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  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-  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 . 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.,  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.  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.  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 , 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.  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  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.  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  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  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  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  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.  constructedaveragetracesof the 70. Minutes Processes During disturbedtimes, the plasma sheet changesdramatically. Baumjohannet al.  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  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. , 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. , Fairfield et al. , and speedion bulk flows detectedby the IRM plasmainstrument Moldwinand Hughes  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.  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.  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.  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. , aroundsubstormonsets that occurredduring the expansion Frank and Paterson  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. 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.  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. ) 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 References magnetosphere andnear-Earthtail [Angelopoulos et al., 1992] Akasofu, S.-I., Thedevelopment of theauroral substorm, Planet.Space exist, but the modelsfor the electricfield patternare much Sci., 12, 273-282, 1964. lessdetailedthanthecorresponding modelsfor the magnetic Akasofu,S.-I., PolarandMagnetospheric Substorms, D. Reidel,Norfield. well, Mass., 1968. For the modelstudies,it is importantto recognizethe Akasofu,S. oI., Assessing the magnetic reconnection paradigm, Eos need of the various-scale models. Mesoscale models are re- Trans.AGU, 75, 248-, 1994. V., W. Baumjohann, C. F. Kennel,F. V. Coroniti,M. G. quiredfor studying thedetailedprocesses takingplacein the Angelopoulos, Kivelson, R. Pellat,R.J.Walker,H. Liihr,andG. Paschmann, Bursty currentsheetat the instability onset, whereasthe global bulkflowsin theinnercentralplasma sheet,J. Geophys. Res.,97, MHD simulations cangive us cluesaboutthe large-scale re4027-4039, 1992. sponseof the magnetosphere to the solarwinddriving.FurAngelopoulos, V., C. F. Kennel,F. V. Coroniti,W. C. Feldman,J. T. thermore, fully kinetic(small-scale) numericalandanalytical Gosling,M. G. Kivelson,R. J. Walker, and C. T. Russell,Observastudiesare requiredfor the instabilitymodeanalysis. tionsof a quasi-static plasma sheetboundary, Geophys. Res.Lett.,20, The larger databasesand improvedcomputerfacilities 2813-2816, 1993. havereached a level wherethe symmetryrequirements tradi- Angelopoulos, V., C. F. Kennel,F. V. Coroniti, R. Pellat,M. G. Kivelson, tionallyassumed in the modelsmayno longerbe necessary. R. J.Walker,C. T. Russell,W. Baumjohann, W. C. Feldman, andJ.T. The dawn-dusk asymmetryboth in the magneticfield and Gosling,Statistical characteristics of burstybulkflowevents,J. Geophys.Res., 99, 21,257-21,280, 1994a. plasmaparameters, the asymmetricring currentclosing V., V. A. Sergeev,D. N. Baker,D. G. Mitchell,G. D. throughthe ionosphere,andnonzerotilt anglesare all ef- Angelopoulos, fects whose contribution to the substorm evolution have not Reeves,C. T. Russell, andH. J. Singer,Multi-point studyof bursty been investigatedin detail. In particular,the large-scale field-aligned currentpatterns, the region1 andregion2 cur- bulkflow eventsduringa sequenceof smallsubstorms, in Proceed- rents,are not represented in mostpresent-daymodels. ingsof theSecondInternational Conference on Substorms, pp.643650, AGU, Washington, D.C., 1994b. Angelopoulos, V., et al.,Growthandevolution of a plasmoid associated with a small isolatedsubstorm:IMP 8 and GEOTAIL measurements 5. Summary In this paper we have reviewed the near-Earthneutral line modelof substorms. We have attempted to providean historical framework for the model, and we have tried to show in themagnetotail, Geophys. Res.Lett.,2, 3011-3014,1995b. Angelopoulos, V., F. V. Coroniti,C. F. Kennel,M. G. Kivelson,R. J. Walker, C. T. Russell,R. L. McPherron,W. Baumjohann, E. Sanchez, C.-I. Meng,W. Baumjohann, G. D. Reeves,R. D. Belian, N. Sato,E. Friis-Christensen, P. R. Sutcliffe,K. Yumoto,and T. Har- the observationaland theoreticalunderpinnings of this ris,Multi-point analysis ofburstybulkflowevents,I, April 11, 1985, framework.In our opinion, the NENL approachis still the J. Geophys.Res.,in press,1996. only one whichconstitutes a completemodelof the global Aubry,M.P., andR. L. McPherron, Magnetotail changes in relationto cycle of growth, expansion,andrecoveryof substorms.In thesolarwindmagnetic fieldandmagnetospheric substorms, J. Geophys.Res., 76, 4381-4401, 1971. particular,it treatsthe large-scale interactionof the magnetospherewith the solar wind, and it also addressesthe cou- Aubry,M.P., C. T. Russell,and M. G. Kivelson,Inward motionof the magnetopause before a substorm, J. Geophys.Res., 75, 7018-7031, pling of the magnetosphere to the polar and auroraliono1970. sphere.Any modelthatwouldattemptto supplantthe NENL theoryof high-latitude geomodelmustaddress all of theseglobal aspectsof substorm Axford,W. I., andC. O. Hines,A unifying physical phenomena and geomagnetic storms, Can.J. Phys.,39, dynamicsat least as well as the NENL model. We have tried in this review to discuss the weaknesses of theNENL modelas well as its strengths.New observations forceoneto improveandrefinecontinuously the model'sde- 1433-1464, 1961. Baker,D. N., and R. L. McPherron, Extremeenergeticparticledecreasesneargeostationary orbit: A manifestationof currentdiversion withintheinnerplasmasheet,J. Geophys. 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