Flux transport, dipolarization, and current sheetevolution during a double‐onset substorm

R. Nakamura,1 W. Baumjohann,1 E. Panov,1 A. A. Petrukovich,2 V. Angelopoulos,3

M. Volwerk,1 W. Magnes,1 Y. Nishimura,3 A. Runov,3 C. T. Russell,3 J. M. Weygand,3

O. Amm,4 H.‐U. Auster,5 J. Bonnell,6 H. Frey,6 D. Larson,6 and J. McFadden6

Received 26 June 2010; revised 21 February 2011; accepted 25 February 2011; published 27 May 2011.

[1] We study a substorm with two onsets (at 0220 and 0243 UT) that occurred during agradual northward interplanetary magnetic field (IMF) turning on 16 February 2008. Atthese times, Time History of Events and Macroscale Interactions during Substorms(THEMIS) and GOES spacecraft were distributed between 6.6 and 18 RE downtail. Priorto the weak auroral electrojet onset at 0220 UT, a thin current sheet was extended near10–11 RE. After the onset, Earthward fast flows with dipolarization fronts followed bysignatures of magnetic flux pileup were detected in this region. The 0243 UT onsetdisturbances were more intense and centered at higher latitudes. The reconnection regiontailward of 18 RE became activated and reached the lobe flux. We suggest that activations ofreconnection Earthward of 18 RE associated with the 0220 UT event led to pileup of flux andredistribution ofBZ to form a thin current sheet with smallBZ in themidtail region. This madeconditions favorable for reconnection tailward of 18 RE involving lobe flux for the 0243 UTonset. The reconfiguration process in the current sheet between the two onsets possiblyenabled a relatively strong, high‐latitude substorm despite the rather weak IMF driver. Thenear‐Earth dipolarization observed after the 0243 UT onset was accompanied by morelocalized Earthward flows and flow reversals. Differences in dipolarization signatures couldbe caused by ambient plasma condition and field configuration between these two events.Our observations of the double‐onset substorm suggest that the plasma sheet can bepreconditioned by both the IMF driver and internal magnetotail processes.

Citation: Nakamura, R., et al. (2011), Flux transport, dipolarization, and current sheet evolution during a double‐onsetsubstorm, J. Geophys. Res., 116, A00I36, doi:10.1029/2010JA015865.

1. Introduction

[2] A magnetospheric substorm is the sudden release ofmagnetotail energy that was transported from the dayside intothe auroral ionosphere and nightside magnetosphere. Duringsubstorms, high‐speed plasma flows, called bursty bulk flow(BBF), and magnetic field dipolarization, enhancement(s) inBZ, are observed in the near‐Earth tail. These processes changethe distribution of the tail current locally and/or globally.[3] One of the most widely accepted mechanisms for

creating fast flows is magnetic reconnection. Although the

occurrence rate of BBFs, which indicate enhanced magneticflux transport, is known to be well correlated with substormactivity [Baumjohann et al., 1990], the role of the flow insubstorm evolution, in particular, the timing, location, andpresence of the reconnection site [e.g., Baumjohann et al.,2007; Lui et al., 2008; Angelopoulos, 2008], is still debated.Some observations support reconnection as the cause of sub-storm initiation; others consider it to be the result of substorminitiation. It should be noted that although BBFs also existduring nonsubstorm intervals, they are always associatedwith some auroral precipitation [Nakamura et al., 2001] and adistinct ionospheric equivalent current pattern [Juusola et al.,2009].[4] Two types of dipolarization have been identified in

the near‐Earth magnetotail. One, also called a dipolarizationfront, is reported to be associated mainly with Earthwardflows exceeding 100 km/s. Such a dipolarization, attributedto magnetic flux transported Earthward [e.g., Angelopouloset al., 1994; Sergeev et al., 1996; Nakamura et al., 2002;Sigsbee et al., 2005; Runov et al., 2009], is considered to bea rather thin front structure that precedes the fast flow. Theother type of dipolarization, associated with recovery from athin current sheet state to a dipolar configuration, propagates

1Space Research Institute, Austrian Academy of Sciences, Graz,Austria.

2Space Research Institute, Russian Academy of Sciences, Moscow,Russia.

3Institute of Geophysics and Planetary Physics, University of California,Los Angeles, California, USA.

4Finnish Meteorological Institute, Helsinki, Finland.5Institut für Geophysik und Extraterrestrische Physik, Technische

Universität Braunschweig, Braunschweig, Germany.6Space Science Laboratory, University of California, Berkeley, California,

USA.

Copyright 2011 by the American Geophysical Union.0148‐0227/11/2010JA015865

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more tailward [Baumjohann et al., 1999] or in the azimuthaldirection [e.g., Nagai, 1982]. The latter dipolarization maybe associated with pileup of Earthward transported magneticflux in the near‐Earth region where the flows encounter astrong field and a high‐pressure region and are decelerated (orbraked) [Shiokawa et al., 1997; Baumjohann, 2002]. Pileupof the magnetic flux then leads to a tailward motion of thedeceleration region [e.g.,Hesse and Birn, 1991]. Instabilities,such as ballooning [Roux et al., 1991] or cross‐field currentinstabilities [Lui et al., 1991], have also been proposed toproduce a dipolarization signature and subsequent tailwardand azimuthal propagation [Baumjohann et al., 2007]. RecentCluster observations [Nakamura et al., 2009] that showedsystematic relationships between these two types of dipolar-ization; (an Earthward, BBF‐associated dipolarization frontfollowed by tailward propagating dipolarization) support thepileup effect for those events.[5] The location and time, with respect to substorm onset,

of near‐Earth magnetic reconnection and other key processesin the magnetotail have been reported in several statisticalstudies using the extensive data set fromGeotail [Asano et al.,2004; Miyashita et al., 2004; Nagai et al., 2005; Miyashitaet al., 2009]. According to Asano et al. [2004], the near‐Earth magnetic reconnection site is located close to thetailward edge of a thin current sheet that develops betweenX ∼ −5 and −20 RE [Miyashita et al., 2009]. Miyashita et al.[2009] also determined that on average, magnetic recon-nection in the premidnight tail at X ∼ −16 to −20 RE anddipolarization, which occurs between X ∼ −7 and −10 RE,take place almost simultaneously, about 2 min before onset.However, for intense substorms, the reconnection site islocated closer to the Earth, which is consistent with thedependence of ionospheric onset latitude on substorm inten-sity [Miyashita et al., 2004]. The location of the reconnectionsite within the magnetotail has also been found to be depen-dent on the solar wind electric field value, −Vx × Bs, where Vx

is the x component of the solar wind velocity and Bs is thesouthward component of the interplanetary magnetic fieldprior to the onset [Nagai et al., 2005]. Hence, the radial dis-tances where critical current sheet processes occur can differfor each substorm depending on internal current sheet con-figuration and external solar wind condition.[6] Substorms often have multiple onset signatures in both

the ionosphere and the magnetotail. One distinct type ofmultiple onset sequence is a major onset with clear polewardexpansion preceded by weak onsets with no such expansionduring the growth phase. These weak onsets are referred to aspseudobreakups, or more accurately, growth phase pseudo-breakups [Kullen et al., 2010]. Although pseudobreakupshave typical magnetospheric signatures of substorms, theirstrength and magnetotail features are quite different fromthose of a major onset [Koskinen et al., 1993; Ohtani et al.,1993; Nakamura et al., 1994]. For example, the currentwedge and the particle injections are confined to a localizedregion and expand very little tailward, Earthward [Koskinenet al., 1993; Ohtani et al., 1993], and in the azimuthal direc-tion [Ohtani et al., 1993; Nakamura et al., 1994]. Auroralsignatures during growth phase pseudobreakups are observedto be localized in azimuth and typically occur at a higher

latitude than the major onset region [Nakamura et al., 1994].Statistical results of substorm studies in which a strongeronset was associated with a thin current sheet and a recon-nection region closer to the Earth [Miyashita et al., 2004]suggest such relationships.[7] Another suggested difference between major onsets

and pseudobreakups during multionset substorms is thatmajor onsets are associated with a lobe reconnection, whereaspseudobreakups involve only closed magnetic field lineswithin the plasma sheet [Russell, 2000; Mishin et al., 2001;Sergeev et al., 2008; Pu et al., 2010; Tang et al., 2010]. Ifclosed field line reconnection evolves toward a lobe recon-nection, then major onsets can occur poleward of the firstonset in the ionosphere, which explains observations of apoleward expansion of onset disturbances in the ionosphere[Russell, 2000; Sergeev et al., 2008].[8] While poleward expansion of the onset can be produced

by evolution of the same reconnection site toward the lobe[Russell, 2000; Sergeev et al., 2008], it can be also associatedwith tailward excursion of the reconnection region [Tanget al., 2010]. Tailward retreat of the activation sites hasbeen observed in the midtail region by IMP8 and Geotail[Angelopoulos et al., 1996]. Although there are a number ofstudies of double‐onset substorms, the evolution of the cur-rent sheet and flux transport processes in the magnetotailare not yet well understood. Monitoring of current sheetconfiguration, the location of the reconnection site andsimultaneous observations of the ionosphere are essential tounderstand the evolution of multiple‐onset substorm.[9] The Time History of Events and Macroscale Inter-

actions during Substorms (THEMIS) mission [Angelopoulos,2008], with its five spacecraft and a dedicated array of groundobservatories located inCanada and the northernUnited States,was designed to study substorm evolution in the midtail andinner magnetosphere on a large scale. In this paper we studymidtail and innermagnetospheric disturbances during a double‐onset substorm beginning at 0220 UT on 16 February 2008.The first weak electrojet onset was located at lower latitudecompared to the second substorm onset at 0243 UT, whichoccurred at higher latitude with a stronger activation. For bothonsets, dipolarization signatures were detected by THEMISinner probes and a comparable or stronger dipolarization wasobserved in the first weak onset. Multipoint magnetosphericobservations together with ground‐based observations enableus to understand large‐scale current sheet evolution and theflux transport process in the course of the two onsets. Suchinvestigations from multipoint measurements are essential tounderstand not only substorm dynamics but also local pro-cesses such as the dipolarization front. In this study we useTHEMIS spacecraft data from the FluxGate Magnetometer(FGM) experiment [Auster et al., 2008], the Electric FieldInstrument (EFI) instrument [Bonnell et al., 2008], the elec-trostatic analyzer (ESA) [McFadden et al., 2008], and theSolid‐State Detector (SST) [Sibeck and Angelopoulos, 2008].In addition, we use data from the THEMIS Ground‐BasedObservatories (GBO) all‐sky cameras (ASI) and magnet-ometers [Mende et al., 2008], as well as geomagnetic fielddata from the International Real‐time Magnetic ObservatoryNetwork (INTERMAG), a global network of cooperatingdigital magnetic observatories. The spacecraft location in the

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magnetosphere and ionospheric foot points are shown inFigure 1.

2. Overview

[10] On 16 February 2008, two westward electrojet inten-sifications accompanied by Pi2 magnetic fluctuation onsetsstarted at 0220 and 0243 UT, as shown in Figure 2. Thegradual westward electrojet enhancement at 0220 UTwas relatively weak (<200 nT) compared with the strong(>500 nT) enhancement of the electrojet at 0243 UT detectedin the high‐latitude magnetogram and clear positive bayenhancement at midlatitude stations. The 0220 UT intensi-fication took place during gradual recovery of the southwardIMF, which was between BZ ∼ −2 and −4 nT for the preceding45 min and then increased to between BZ ∼ −1 and 0 nT. The0243 UT activation took place during a gradual 4 nT north-ward IMF turning. THEMIS spacecraft were distributed inthe premidnight side of the magnetotail, as shown inFigure 1a, and their foot points were located near the localtime sector where enhancement of the westward electrojet,Pi2 pulsations, and the positive bays were detected(Figure 1b). The foot points are calculated using the adaptedtime‐dependent magnetospheric model [Kubyshkina et al.,2009], in which the standard Tsyganenko model (T96) ismodified to find the best fit to the observed field from all therelevant spacecraft in 5 min steps.[11] The strongest westward electrojet intensification dur-

ing this period was detected at Iqaluit (IQA), which is locatedmore than 4° northward of the foot point of P3‐5(THD,E,A)and northward of the standard AE stations. Such high‐latitudesubstorms have also been called contracted‐oval substorms[Akasofu et al., 1973; Lui et al., 1976]. The strong electrojetand preceding southward IMF interval, however, are dis-similar to characteristics of the original contracted‐ovalsubstorm events, which were associated with quiet geomag-netic activity and northward IMF periods.[12] Auroral activity and nightside magnetospheric ob-

servations during the two onsets are summarized in Figures 3

and 4. The keograms in Figures 3b and 3c show gradualequatorward motion of the arc associated with the growthphase (indicated by slanted arrows) at Kuujjuaq (KUUJ) andSanikiluaq (SNKQ) before a weak intensification at around0216 UT at KUUJ (indicated by the vertical arrow directedupward) and 0217 UT near SNKQ. The intensification nearSNKQ cannot be easily identified in the keogram due todifferent longitudinal location, but can be seen in the all‐skyimage in Figure 5, as discussed later. At 0220 UT a clearauroral onset was recorded at SNKQ (downward directedarrow) and then from 0222 UT the aurora was also observedat KUUJ (downward directed arrow) followed by bothequatorward and poleward auroral expansion. As the auroralarc further intensified, another onset of poleward expansionstarted at 0243 UT at SNKQ (upward directed arrow) andthen at 0244 UT at KUUJ (upward directed arrow). Thisactivity also reached the field of view of Rankin Inlet(RANK), as plotted in Figure 3a. The keogram of KUUJ(Figure 3c) also shows that the aurora expanded fartherequatorward than the previous onset at 0220 UT. The footpoints of the THEMIS spacecraft were distributed aroundthese auroral regions. In particular, the foot points ofP3(THD) and P4(THE) were within the field of view ofKUUJ during the interval (see Figure 5 and its description fordetails).[13] Figures 3d and 3e show the total pressure and the sum

of the magnetic field pressure and ion pressure. The magneticfield pressure is calculated using only the BX and BY com-ponents assuming pressure balance in a planar tail currentsheet, as in previous studies [e.g., Xing et al., 2010]. The ionpressure is calculated using SST and ESA instrument data. Noparticle measurement data were available for P1(THB). Tobetter examine the current sheet signatures associated withthe two onsets, we plotted the magnetic field data forP5(THA), P3(THD), and P4(THE) in Figures 3 and 4 in acoordinate system that takes into account the tilt of the dipoleand the hinging distance. The location of the hinging distanceand the effective dipole tilt for the event were calculated usinga model by Tsyganenko and Fairfield [2004] with IMF BZ =

Figure 1. (a) Location of THEMIS and GOES 10 and 12 spacecraft in the equatorial plane in GSM coor-dinates. (b) Foot point location of THEMIS, GOES 10 and 12 spacecraft, and the closest ground‐basedmag-netometers and/or all‐sky imagers in geographic coordinates.

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0, IMF BY = 4 nT, solar wind dynamic pressure of 1.5 nPa,and solar wind speed of 630 km/s. The hinging distance wasestimated to be 11.0 RE, which indicates that the three innermagnetospheric THEMIS spacecraft, P5(THA), P3(THD),and P4(THE), were Earthward of this location. The effectivedipole tilts for the three spacecraft were estimated to be−15.2°, −14.7°, and −15.7°, respectively. To calculate thecoordinate system for the three innermagnetospheric THEMISspacecraft, we used the average tilt, −15°, and plotted themagnetic field for the spacecraft in a tilted coordinate system(see Figures 3g, 3i, and 4a). The midtail data from P1(THB)and P2(THC) are plotted in GSM coordinates (see Figures 3fand 3h). For GOES 10 and 12 (shown in Figures 4b and 4c),we used the conventional dipole coordinate (HDV) system.Figure 3j shows the cumulative magnetic flux transport fromP2(THC) and P4(THE), which is obtained by integrating EY.This parameter has been found to be a useful quantity toshow the enhanced flux transport rate and to detect the onsetof reconnection [Angelopoulos et al., 2009; Liu et al., 2010].We integrated the Y component of the −V × B electric fieldstarting from 0200 UT.[14] Prior to the 0220 UT onset, typical growth phase sig-

natures, such as an increase in the total pressure (Figures 3dand 3e), were observed. This suggests that magnetic fluxaccumulated in the tail because of enhanced dayside recon-nection during southward IMF BZ. A decrease in BZ

(Figures 3h and 3i) can be identified most clearly at P1(THB)and P4(THE), but not at P5(THA), suggesting progressivestretching of the tail in the premidnight region. Furthermore,the absolute value of BX increased at all spacecraft (Figures 3fand 3g), suggesting large‐scale thinning of the current sheet.Such a sequence of stretching and thinning has been reportedas characteristic of the growth phase [Petrukovich et al.,2007]. All the three inner magnetospheric THEMIS space-craft were located at a similar distance (about 10–11 RE) anddistributed in both hemispheres. The enhanced current den-sity in this region can be seen in the difference between BX inthe northern hemisphere (average of P3(THD) and P4(THE))andBX in the southern hemisphere (from P5(THA)), as shownin Figure 4a. Figure 4a also gives our estimate of the plasmasheet thickness, L. We estimated L using the Harris currentsheet assumption with theseBX values and the lobe field valueobtained from the average total pressure and by assuming thatthe plasma pressure is negligibly small in the lobe. Beforedipolarization, the plasma sheet thickness decreased gradu-ally to about 0.9 RE, thinner than an average plasma sheet,6 RE [Baumjohann and Paschmann, 1990] (obtained fromAMPTE/IRM observations), but not thin enough for thetypical near‐Earth current sheet instability, which requirescurrent sheet thickness less than an ion scale. The ion inertiallength and the gyroradius were in the 300∼600 km range forthis event.

Figure 2. Overview of IMF and ground magnetic activity between 1 and 4 UT on 16 February 2008.Shown are (a) the IMF BZ from ACE, (b) the H component from six high‐latitude magnetograms, (c) thePi2 pulsations observed at five stations, and (d) the H component from two lower‐latitude stations. TheACE observations are shifted by 37 min to represent the IMF values at the magnetopause. The vertical linesshow the two ground magnetogram onsets at 0220 and 0243 UT. Locations of the magnetogram stationsused in this plot are shown in Figure 2b.

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Figure 3. Auroral and magnetospheric observations between 2 and 3 UT. Keogram from (a) RANK,(b) SNKQ, (c) KUUJ, (d–e) total pressure, (f–g) BX from THEMIS spacecraft in the midtail P1‐2(THB‐C)and in the inner magnetosphere P3‐5(THD‐E,A), respectively, and (h–i) BZ in the same format as BX.For total pressure, only data from P2(THC) are shown during this event. (j) Cumulative flux transport fromP2(THC) andP5(THE) calculated by integrating the y component of the electric field determinedwith−(V×B).The flux transport value for P2(THC) is multiplied by two to show the temporal change more clearly. Thevertical lines indicate the onset times at 0220 and 0243 UT.

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[15] At around 0218 UT, P4(THE) observed enhancementsin BZ and BX, most likely associated with localized, shortauroral intensifications at 0216 UT (Figure 3b). In the mid-tail at P2(THC), the pressure began to decrease at around0219 UT (Figure 3d), indicating the start of unloading ormagnetic flux decrease in this region. The pressure in thenear‐Earth tail region, on the other hand, increased further(Figure 3e) at the same time. These midtail and near‐Earthmagnetotail pressure changes were reported as a typical sig-nature in the tail during substorm onset [e.g.,Miyashita et al.,2009; Xing et al., 2010]. At P2(THC), BZ turned negative(Figure 3h) as the flux transport started to increase (Figure 3j).Such signatures are expected with onset of reconnection[Angelopoulos et al., 2009; Liu et al., 2010], and the recon-nection site was located Earthward of the P2(THC) spacecraftbased on the negative excursion in BZ. The sharp dipolar-ization at P3(THD) and P4(THE) in the near‐Earth tail startedat around 0222 UT (Figure 3i), and flux transport at P4(THE)increased significantly (Figure 3j). This dipolarization is asso-ciated with enhanced Earthward transport of the magneticflux. During this onset interval, no clear signature of dipo-larization was observed at P5(THA)(Figure 3i) and at geo-synchronous orbit (Figure 4b) except some small fluctuationat GOES 10 starting at 0219 UT. We interpret these obser-vations tomean that for the 0220UT onset, dipolarizationwaslimited to a local region in the near‐Earth plasma sheet.[16] During the ∼15 min interval between the two onsets,

the total pressure in the midtail (THC) decreased on averagecompared to its value before the 0220 onset, suggesting thatmagnetic flux was removed from the midtail region whilethe IMF BZ was between 0 and −1 nT, indicating a veryweak solar wind driver. Furthermore, the absolute BX valuesincreased gradually in the midtail at P1(THB) but not atP2(THC) (Figure 3f), indicating that the current sheet wasstill thinning at the location of P1(THB). At P2(THC), BZ

changed sign from negative to positive, suggesting that thepossible reconnection region changed from the Earthward to

the tailward side of the spacecraft. Evolution of the possiblereconnection region will be discussed in more detail insection 3. In the near‐Earth tail region, on the other hand,the total pressure gradually increased mainly at P5(THA) andslightly increased also at P3‐4(THD‐E). The local currentdensity decreased, and the thickness of the plasma sheetincreased (Figure 4a). Progressive reconfiguration toward athicker plasma sheet with dipolar configuration was particu-larly prominent at the innermost spacecraft, P5(THA), whereBZ increased the most. Although the assumption of a 1DHarris current sheet might not accurately describe the currentsheet configuration after dipolarization due to these increasesin BZ and the growing difference in the total pressure amongthe spacecraft, it is still valid to argue that the plasma sheetdoes thicken. Flux transport increased further during thisinterval, but only in the midtail P2(THC) (Figure 3j). Theseobservations suggest that the magnetic flux transported fromthe midtail accumulated in the near‐Earth tail and innermagnetosphere region during this interval.[17] The second onset (at 0243 UT) was associated with

a decrease in pressure (Figure 3d) and an increase in BZ atP2(THC) and at P1(THB). Both P2(THC) and P1(THB)detected a decrease in the absolute value of BX, most likelydue to plasma sheet expansion. The inner THEMIS probesmeasured further plasma sheet thickening and dipolarization.During this second onset, GOES 10 and 12 also observeda brief decrease in BH and an increase in BD (GOES 10 pre-ceded GOES 12 by about 5 min). Such signatures have beeninterpreted as the westward propagation of the duskside ofthe current wedge located tailward of the spacecraft [Nagai,1982].[18] Ionospheric equivalent currents and aurora for selected

times during the two substorms are shown in Figure 5.The ionospheric equivalent currents are obtained using theSpherical Elementary Current Systems (SECS) technique[Amm, 1997; Amm and Viljanen, 1999]. To determine theseequivalent ionospheric currents, ground magnetometer data

Figure 4. (a) TheDBX (local current density) and L (current sheet thickness) determined using THEMISdata, (b) H, and (c) D components from the GOES 10 and 12 magnetic field instruments. The vertical linesindicate the onset times at 0220 and 0243 UT.

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from the THEMIS, CANMOS, CARISMA, GIMA, andMACCS arrays in Northern America and from the Greenlandarray are used. Mosaic images obtained from THEMIS GBOall‐sky cameras at RANK, SNKQ, and KUUJ are shownat the right side of each panel. Magnetic foot points fromTHEMIS and the equivalent current vectors are also shownin the auroral plots. The foot points marked with asterisks(and colored dots in the equivalent current plots) are obtainedfrom the adapted time‐dependent model [Kubyshkina et al.,2009]; those marked with diamonds are calculated from theTsyganenko T89 magnetic field model for comparison. Themain difference between these foot point positions for thisevent is the latitude. T89 predicted that the foot point of thespacecraft would be located 1–2 degrees higher in latitudefor all probes but P1(THB). As previously discussed, theKubyshkina et al. adapted modeling is a time‐dependentmodel that takes into account changes in spacecraft fields andinput parameters. Hence, adapted time‐dependent modelingis believed to provide better estimates of foot point location.[19] A very localized auroral activation between KUUJ and

SNKQ began at 02:16:12 along the faint aurora at 68° lati-tude, westward and northward of the foot point of P4(THE)(see 0217 UT panel). This auroral activation did not expanduntil about 0220UTwhen a surge‐like structure (indicated byan arrow in the 0220UT panel) began to form between SNKQand KUUJ. This surge developed westward, reaching thezenith meridian of SNKQ (left arrow in 0223 UT panel).East of that surge, around the local time of the foot point ofP4(THE) and P3(THD), two slanted, north‐south alignedauroral signatures developed (right arrow). The region of thenorth‐south‐aligned aurora then intensified and expandedlocally poleward (shown in 0225 UT panel). By 0227 UT thewestward expansion stopped and began to dim (not shown).A localized westward electrojet region centered at around69° geomagnetic latitude, east of the surge and north‐south‐aligned aurora, developed by 0223 UT.[20] The second onset began with intensification of an

auroral arc at around 70° to the west of the SNKQ all‐skyimager at 02:42:42 UT. The intensification became visiblewithin the field of view of KUUJ, expanding northwardas well as westward (arrow in the 0243 UT panel), untilit also became visible at RANK. As the expanding aurora’spoleward edge moved out of KUUJ’s field of view, diffuseaurora mixed with fine structured, north‐south filamentaryaurora developed equatorward (arrow in the 0247 UT panel)where the foot points of P4(THE) and P3(THD) werelocated. The westward electrojet for the second onset wasstronger and distributed over a wider region than for theprevious onset. It was also centered at a very high latituderegion (∼75°) with its eastern edge most likely located eastof the Greenland stations.

3. Observation in the Midtailand the Flow‐Braking Region

[21] As described in section 2, P2(THC), which waslocated in the southern hemisphere in the outer edge of theplasma sheet, observed plasma sheet thinning and expansionassociated with the two auroral onsets. Figures 6a–6d showmagnetic field and ion velocity; the latter is a merged productbetween the two particle instruments, ESA and SST. The

Y component of the −V × B electric field, cumulative fluxtransport, ion energy spectra from ESA and SST, and electronspectra from ESA are shown in Figures 6e–6h. Figures 6i–6kare the electron pitch angle spectra from selected times,indicated by the solid bars at the base of Figure 6h. Note thatthe resolutions in the bottom plots of Figures 6j and 6k aredifferent from that in the bottom plot of Figure 6i due todifferent modes of operation for the instrument, i.e., burstmode and reduced mode [see McFadden et al., 2008].[22] Both ion and electron energy decreased before

0220 UT during current sheet thinning. The spacecraftremained in the outer plasma sheet during the 0220 UT onset.BZ turned negative just before onset (Figure 6b); EY turnedpositive (Figure 6e); and cumulative flux transport (Figure 6f),which is associated with transient enhancement in VZ and −BZ,started to increase from 02:20:30 UT (indicated by dashedline). Such EY and VZ enhancements have been interpreted asevidence of X line activation [Angelopoulos et al., 2009].After the onset BZ stayed mainly negative; BY increased; andthe flow direction became more persistently tailward andmostly parallel to the field.[23] At 02:30:00 UT another increase in the flux transport

occurred along with an enhancement in VZ, −BZ, −VX, and BY

(indicated by dashed line), again suggesting activation ofan X line Earthward of the spacecraft. The slight differ-ence between the electron energy flux for pitch angle 0° andthat for 180° (Figure 6i) supports this interpretation. After0236 UT BZ turned mainly positive.[24] A third increase in the flux transport, which took place

at 02:38:20 UT, was associated with an increase in VZ, BZ

and VX, suggesting that this event is due to the X line tailwardof the spacecraft. At 0243 UT when VZ and BZ increased, thefirst signatures of an Earthward electron beam with an energyof a few keV were detected when the lower‐energy (500 eV)electrons were restricted in the tailward direction (Figure 6j).This suggests that the spacecraft was near the region of theopen lobe field line, where low‐energy electrons comeexclusively from the Earth, and very close to the X line atthe tailward side of the spacecraft, where Earthward directedenergetic beams could be produced. Following this obser-vation, the spacecraft shortly moved into the lobe and thenreentered deeply into the plasma sheet, where it measuredEarthward fast flows associated with the final increase inthe flux transport rate at about 02:44 UT (indicated withthe dashed vertical line). During this flux transport increase,more energetic Earthward electron beams (10 keV) wereobserved together with tailward streaming lower‐energybeams (Figure 6k). These electron beam properties have beenobserved in many previous reconnection studies [e.g.,Fujimoto et al., 2001;Nagai et al., 2003; Angelopoulos et al.,2008].[25] The flux transport profile and the particle andmagnetic

field signatures at P2(THC) suggest that four main recon-nection events took place during the interval. The first twoenhancements took place due to reconnection Earthward ofthe spacecraft; the latter two took place tailward of the space-craft. Thus, a transient tailward retreat of the reconnectionregion took place with these multiple activations.[26] Figure 7 summarizes the field and plasma signatures of

the dipolarization events observed at P3(THD) and P4(THE).P4(THE) was located 1.2 RE Earthward, 0.5 RE duskward,

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Figure 6

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Figure 6. Fields and plasma data observed by P2(THC) between 0215 and 0250 UT. (a) BX, (b) BY and BZ components of themagnetic field, (c) three components of the ion velocity, (d) VX and VZ components of the ion velocity perpendicular to themagnetic field, (e) EY calculated from −(V × B)Y, (f) cumulative flux transport, (g) ion energy spectra from ESA and SST,and (h) electron energy spectra from ESA. The vertical solid lines show the ground onset times at 0220 and 0243 UT, andthe vertical dashed lines show the activation times of the reconnection. (i–k) The electron pitch angle profiles for selectedtimes, which are marked with a thick vertical bar at the bottom of Figure 6h. For Figures 6i–6k, the upper parts show the energyspectra for different pitch angles; the lower parts show the phase‐space density profile for parallel (0°), antiparallel (180°), andperpendicular (90°) directions to the field lines.

Figure 7. Field and plasma observations from P3 (THD in the top row) and P4 (THE in the bottom row)between (left) 0221 and 0227 UT and (right) 0243 and 0249 UT. Shown in each quadrant are three compo-nents of the magnetic field (first panel), three components of the E × B velocity (second panel), the pressure(third panel), and the electron energy spectra (fourth panel) from P3(THD) and P4(THE).

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and 0.5 RE northward of P3(THD). In Figure 7 as well asFigures 8–10, we usedE × B/B2 velocity in GSM coordinatesobtained from the EFI measurements. The two spin planecomponents were used for determining the spin axis com-ponent by assuming E · B = 0. To check for consistency, wealso compared the E × B/B2 velocity with the velocity fromthe ion moment obtained by combining EFT and SST data.Since we are interested in the flux transport process andtherefore in the perpendicular component of the flow, we usedthe flow velocity obtained from the EFI measurement, whichhas the advantage of high temporal resolution, for furtherdetailed analysis below.

[27] Associated with the 0220 UT onset, a sharp enhance-ment in BZ accompanied by a subsequent Earthward fastflow disturbance was observed at both spacecraft starting at02:21:59 UT for P3(THD) and at 02:22:17 UT for P4(THE)(indicated by the vertical lines in Figure 7 (left)). The Earth-ward flow and the dawn‐dusk electric field (not shown) wereabout 3 times larger for P3(THD), indicating that a flow‐braking process was taking place. The dipolarization waspreceded by an increase in the plasma pressure due to a densityenhancement (not shown). The plasma was therefore com-pressed at the head of the dipolarization front. For bothspacecraft, enhancements in the BZ component and fast flows

Figure 8. Changes in the equatorial flow pattern observed by (top) P3(THD) and (bottom) P4(THE)between (left) 0221 and 0226 UT and (right) 0244 and 0249 UT. Shown in each quadrant are the BZ com-ponent (first panel), the VX component (second panel), the flow vectors (third panel), azimuthal angle of theflow (fourth panel) from P3(THD) and P4(THE) in GSM. The flow data shown here are the E × B driftvelocity. Four Hz data are plotted for the BZ, VX, and azimuthal angle panels and every 30th point (8.5 s)is given for the flow vectors. The vertical lines indicate time of determination of the dipolarization frontdirection listed in Table 1. The crosses in the VX panels indicate the times for which the flow velocityvector is also plotted in Figure 9 as a representative value for the fast flow. The crosses in the azimuth angleplot indicate times when the horizontal flow speed exceeded 200 km/s.

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were associated with an increase in BX in the front, also sug-gesting enhancement in the local tail current density (or thin-ning of the current sheet). In the disturbed fast flows after0223 UT, however, BZ enhancements were associated witha decrease in BX, and the plasma sheet magnetic field con-figuration became more dipolar (BZ > BX), more apparent atP4(THE).[28] The dipolarization for the 0243 UT onset shown in the

right column of Figure 7 looks quite different from that forthe 0220 UT onset shown in the left column of Figure 7. Themagnetic field and plasma signatures were also differentbetween the P3(THD) and P4(THE) spacecraft. The firstenhancement in BZ was associated with predominantlydawnward flow at P3(THD) starting at 02:44:16 UT andP4(THE) at 02:44:47 UT (indicated by the first vertical line inFigure 7 (left)). Dipolarization fronts associated with fastEarthward flow were also observed starting at 02:45:42 UTin P3(THD) and 02:46:14 UT at P4(THE) (indicated by thesecond vertical line). The flow direction at P3(THD) turnedtailward and again Earthward within minutes, suggestinghighly structured flows. Such frequent flow reversals werenot detected by P4(THE), but the flow had multiple peaks.While the changes in the pressure associated with the dipo-larization in P4(THE) were similar to the dipolarization frontobserved in the 0220 UT event, changes in the pressure wereless clear at P3(THD).[29] When the dipolarization fronts were encountered,

the electron energy was enhanced for most events. But thiswas not the case for the first dipolarization event observed by

P3(THD). This first dipolarization event was preceded by adecrease in BX, which suggests that the spacecraft may haveencountered the hotter central plasma sheet population beforethe dipolarization front. The observed abrupt electron energyenhancements in the other dipolarization fronts, on the otherhand, are consistent with the idea that these dipolarizationfronts are related to the injection process [Sergeev et al.,2009]. There are also cases, however, in which the electronenergy changes at a boundary not particularly relevant to anEarthward fast flow, such as the first front observed by bothspacecraft for the 0243 UT event and later during the secondinterval.[30] The evolution of the flow pattern near P4(THE) and

P3(THD) (i.e., a scale size of about 1 RE) and the shearstructure around the dipolarization front can be inferred byexamining the flow vectors and the changes in the azimuthalflow angle, ’V = arctan(VY /VX), as shown in Figure 8. Whilethe azimuthal angle is plotted using the 4 Hz field data, theflow vectors are shown only for every 30th data point (8.5 s)to more clearly display the data. In the azimuthal angle panel,times when the equatorial flow speed exceeds 200 km/s aremarked with a cross. For the 0220 UT onset (Figure 8, left),flow evolution sequences were similar at the two spacecraft,except that the flow speed was larger and the azimuthalcomponent stronger for P3(THD) than for P4(THE). Webelieve that the left column of Figure 8 shows that initially thefast flow was directed Earthward and duskward but thenfollowed by another fast flow stream that was directed mainlyEarthward and dawnward. The flow pattern after 0243 UT

Figure 9. Dipolarization front obtained from variance analysis and the direction of the fast flow are shownfor (a, d) the 0222 UT event, (b, e) the 0244 UT event, and (c, f) the 0246 UT event for P3(THD) andP4(THE). The dotted lines show the dipolarization front; the thin arrows perpendicular to the dotted linesshow the estimated velocity of the front; and the thick arrows show the direction of the fast flow in theGSM X‐Y plane (Figures 9a–9c) and in X‐Z plane (Figures 9d–9f).

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onset, on the other hand, was significantly different for thetwo spacecraft. The first dipolarization was associated withdawnward flow at both spacecraft. The flows at P3(THD)then changed to duskward, while at P4(THE), they stayeddawnward. Although the maximum flow was directed Earth-ward and slightly duskward for both spacecraft, no coherentpattern between the two can be identified.[31] In addition to the evolution of the flow pattern, the

angle, ’V, decreases ahead of all the dipolarization fronts.(See the third and sixth panels of Figure 8.) This suggestsclockwise rotation of the flow disturbance viewed from northaround the Earthward moving front. Such flow shear isexpected to create an anticlockwise magnetic perturbationthat corresponds to an upward field‐aligned current [Sergeevet al., 1996].[32] The orientations of the dipolarization fronts marked

with vertical lines in Figure 8 are examined based onminimum (maximum) variance analysis of the magneticand electric fields. The required velocity of the boundary forthe latter analysis was obtained through Minimum FaradayResidue (MFR) analysis [see Sonnerup and Scheible, 1998;Sonnerup et al., 2008; and references therein] (for detailsof these analysis methods). Table 1 summarizes the dipolar-ization front characteristics. In Figure 9 orientation of the

dipolarization front and its normal velocity vectors are plottedwith dotted lines and thin vectors, respectively. The thickarrow shows the maximum speed flow vector around thedipolarization front. The normal direction shown here isobtained from the maximum variance of the electric fieldbecause for the majority of the cases, we obtained a largerratio of the first two eigenvalues, lE1 /lE2 (maximum tointermediate), calculated from variance analysis of the elec-tric field, than the ratio, lB2 /lB3 (intermediate to minimum),which we calculated from the variance analysis of the mag-netic field. In the following we use the boundary coordinatesobtained from the electric field, in which n is the normaldirection to the boundary, l is the minimum variance of theelectric field direction, and m is the intermediate variancedirection.[33] As shown in Table 1, we obtained a reasonable

DeHofmann‐Teller velocity, which means that its componentalong the normal direction, VHT · n, has a value comparableto the normal velocity obtained from MFR method, Un, aswell as to the average plasma normal velocity, hVni, for onlythe first dipolarization event for both spacecraft. For the otherevents, no clear DeHofmann‐Teller velocity existed, or thenormal velocity obtained from the variance analysis differedsignificantly from the DeHofmann‐Teller velocity. Using

Figure 10. (left) The magnetic field vector (first panel), the velocity vector determined from the electricfield drift (second panel), the electric field (third panel), the velocity vector direction (fourth panel), and themagnetic field direction (fifth panel) associated with the 0222UT dipolarization front observed by P3(THD)plotted in maximum variance coordinates of the electric field. The l, m, n indicate the minimum, interme-diate, and maximum variance direction of the electric field. (top right) Their vectors in GSM coordinates aregiven in the upper right corner. (bottom right) Flow and magnetic field disturbance relative to the dipolar-ization front (solid line) is illustrated.

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the obtained velocity of the dipolarization front and the timescale of the dipolarization (BZ enhancement), we estimatethat the boundary scale size varies between 220 and 1800 km.The ion inertial length is about 300 km in the first event and600 km in the second event, which means that these bound-aries are between a fraction and 3 ion inertial lengths, asexpected in thin dipolarization fronts [Nakamura et al., 2002;Runov et al., 2009]. It should be noted that the first and thethird dipolarization fronts are consistent with fronts createdby Earthward flow (see also Figures 9a, 9c, 9d, and 9f),whereas the fast flow of the second event is nearly parallelto the dipolarization front for both spacecraft (see alsoFigures 9b and 9e). The normal component of the field isrelatively small in most cases, indicating that these frontsare tangential discontinuities.[34] The field and shear flow pattern near the dipolarization

front at 0222 UT observed by P3(THD) is given in Figure 10.The average magnetic field magnitude normal to the bound-ary is about 4 nT, about 30% of the horizontal component.The speed of the boundary obtained using the MFR method,Un = 109 km/s, is comparable to the normal component of theDeHofmann‐Teller velocity, 138 km/s. The normal compo-nent of the flow velocity, Vn, was relatively stable comparedto the other components and was on average 201 km/s,slightly higher than but still comparable to the boundaryspeed. Hence we believe that this boundary can be regardedas a stable structure at least over the time the boundary and itssurrounding structure take to pass over the spacecraft, i.e.,∼20 s. The flow eventually turned mainly along the normaldirection. Such a flow rotation has also been observed behinddipolarization fronts [Nakamura et al., 2002], indicating thatthe flow shear structure is created by high‐speed flow inter-acting with the ambient field. Clear shear structures observedin the intermediate component of the field and the flow,Vm and Bm, can be seen in the vector plots. The observedflow shear and magnetic perturbation pattern are illustratedFigure 10 (right) in GSM. The rotation shown in the l, m, ncoordinate corresponds to a clear magnetic (flow) shear pat-tern in a clockwise (anticlockwise) direction observed fromnorth and anticlockwise (clockwise) observed from the tail,indicating a structure associated with a current flowing outfrom the ionosphere. This is consistent with the dusksideedge of a BBF [Sergeev et al., 1996].[35] The foot point P3(THD) is predicted to be northeast

(using the T89 model) or slightly southeast (using the adap-tive model) of northwest to southeast directed slant auroralstructures (see 0223 UT panel in Figure 5). The foot point ofP4(THE), on the other hand, maps with the region of auroralprecipitation. The conjugate location of auroral precipitation

has been shown to correspond to the duskside edge of theBBF [Nakamura et al., 2001]. Hence, the tendency of theaurora to be duskside rather than dawnside of the foot pointsin this event indicates that the aurora relevant to the BBFobservations at P3‐4(THD‐E) corresponds to these slantedN‐S auroral structures.[36] We can obtain the spatial scale of the dipolarization

front using the speed of the boundary, 109∼138 km/s, and thetemporal scale of the dipolarization front, 4.5 s. The estimatedspatial scale of the dipolarization front was between 490 and620 km, which is 1–2 ion inertial scale lengths for this event.The Bl rotation suggests that this front is associated with astrong current jm ∼ 70 nA/m2. The observed shear flow regionlasted for ∼20 s, which then corresponded to a shear about3200 kmwide. From the Bm disturbance, we can also estimatethe possible field‐aligned current density, which was deter-mined to be ∼10 nA/m2 on average. The length of the dipo-larization boundary then would be about 3800 km, if thismagnetic shear produces a field‐aligned current intensity of∼0.1 MA, which corresponds to the value estimated fromflow vortices at 11 RE in a different THEMIS event analyzedby Keiling et al. [2009] that was associated with an auroralspiral.

4. Discussion

[37] In this study we examined the ground signatures,midtail, and near‐Earth plasma sheet during a double‐onsetsubstorm when a weakly southward IMF BZ gradually turnednorthward (+4 nT). The center of the electrojet and the aurorawere located at higher latitudes than normal (above 71° ingeomagnetic latitude) beyond the standard AE stations, whichdetected only a small AL decrease of −250 nT. An intensewestward electrojet with a peak magnitude of 500 nT wasobserved at the high‐latitude station Iqaluit during the secondsubstorm. The higher‐latitude distribution of the disturbancerecorded by magnetometer arrays and auroral images hasbeen identified as characteristic of northward IMF BZ sub-storms [Akasofu et al., 1973]. Interestingly, even during suchhigh‐latitude substorms, when the IMF input was relativelysmall, active plasma sheet signatures (fast flows) have beenreported [Petrukovich et al., 2000]. It was suggested thatduring such small substorms, magnetotail coherency in theinner and midtail regions across the tail is lost [Petrukovichet al., 2000]. The double‐onset substorm event discussed inthis study also shows typical signatures of a substorm withinthe plasma sheet, including clear dipolarization, fast flows,current wedge features at geosynchronous orbit, and midtailreconnection signatures despite the contracted oval. A par-ticularly interesting feature of this event is the apparent dif-

Table 1. Orientation and Motion of the Dipolarization Front Obtained From Maximum Variance Analysis of the Electric Field

Spacecraft Time nX nY nZ lE1 /lE2Un

(km/s)hVni(km/s)

hBni(nT)

VHT · n(km/s)

db

(km)

THD 02:21:55–02:22:05 0.71 0.56 −0.40 23 109 201 −3.9 138 490 (620)THE 02:22:15–02:22:25 0.76 0.15 −0.62 7 143 145 −0.3 145 790 (800)THD 02:44:10–02:44:24 0.93 0.01 −0.35 55 −220 33 −1.1 −54 880 (220)THE 02:44:40–02:45:05 0.67 −0.44 −0.58 15 −27 46 −0.3 ..a 220 (..a)THD 02:45:38–02:45:52 0.86 0.50 −0.03 80 188 93 0.3 ..a 1320 (..a)THE 02:46:12–02:46:20 0.72 0.68 0.01 14 395 99 2.6 ..a 1780(..a)

aVHT failed to be determined.bObtained using dipolarization time scale and Un or normal component of VHT (in parentheses).

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ference in the strength and the location of the two onsets,which is rather unexpected, given the relationships betweenweak and strong substorms examined statistically in the studybyMiyashita et al. [2004] or the relationship between growthphase, pseudobreakup, and the major onset [Ohtani et al.,1993; Nakamura et al., 1994]. In other words, the secondonset, which took place during a weaker IMF BZ period, was

more disturbed in the ionosphere as well as in the magneto-sphere than the first onset, and the location of the secondonset was determined to be further tailward in the magneto-sphere and poleward in the ionosphere than the first onset.[38] We suggest that these seemingly inconsistent features

can be explained by the reconfiguration of the current sheetthat took place between these two onsets (illustrated in

Figure 11. Sketch of the magnetotail configuration and possible location of the reconnection in the thincurrent sheet based on the THEMIS spacecraft observed around the 0220 UT onset (first panel), the0230 and the 0238 UT activation events (second and third panels), and the 0243 UT onset (fourth panel).

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Figure 11). Figure 11 shows the possible magnetic fieldconfiguration and the location of some THEMIS spacecraftat four times (around the 0220 UT onset, the 0230 UT and the0238 UT reconnection events, and the 0243 UT onset). Herewe illustrate the possible location of the reconnection andmidtail current sheet configuration based mainly on obser-vations from THC(P2) at 18 RE. Magnetic field configurationnear Earth (10–11 RE) is illustrated based on the innerTHEMIS spacecraft observation. Since the dipolarization/current wedge disturbance at geosynchronous orbit was onlyobserved after the 0243 UT onset, later than any THEMISobservations, and the earliest signature of the substormdisturbance was observed at THC(P2) for both onsets, webelieve that the onset disturbance would be tailward of 11 RE,closest to THC(P2) for both onsets. The illustrated onsetdisturbance can be therefore interpreted also as activationof reconnection.[39] The first panel in Figure 11 shows that at around onset

of the first small substorm expansion (0220 UT), a thin cur-rent sheet was detected at the inner probes when the enhancedflux transport associated with negative BZ excursion startedin the midtail region. Following the 0220 UT onset, themagnetic flux accumulated in the near‐Earth tail region. At0230 UT (second panel), the next flux transport enhance-ments started. This flux transport was associated with nega-tive BZ and tailward flows in the midtail, indicating that theX line formed Earthward of P2(THC). The continuous fluxtransport from the midtail caused a flux pileup and com-pressed configuration in the near‐Earth region and led toformation of a thin current sheet with smaller BZ in themidtail region, making conditions favorable for anotheronset of reconnection tailward of the first location at around0238 UT (third panel). At 0243 UT, when additional enhance-ment in the flux transport was observed, the midtail recon-nection eventually involved lobe flux, which led to globalreconfiguration of the tail, and the second onset was observed(fourth panel). Hence, the second onset took place tailwardof the first one, since the thin current sheet is expected todevelop tailward.[40] If a continuous solar wind driver exists during a double

onset substorm, the tail can be further loaded. Loading ofthe magnetotail has been observed as an increase in pressureand stretching and thinning of the current sheet, as shown inrecent studies on two‐onset substorms [Pu et al., 2010; Tanget al., 2010]. When a pseudobreakup (or the first onset)occurred during such a loading phase, the auroral locationwas poleward [Nakamura et al., 1994] or the signatures in themagnetosphere were confined to the tailward region [Ohtaniet al., 1993] of the location where subsequent major onsetdisturbances took place. For the event in this study, however,the IMF BZ was small so that the magnetotail could not beloaded significantly before the second onset. Most likely,closed‐field reconnection associated with the first onsetredistributed the closed flux (or BZ in the current sheet) andproduced a favorable configuration for a major onset insta-bility, i.e., a thin current sheet with a small BZ and thicknessless than an ion scale.[41] It has been suggested that the northward turning of

the IMF and the subsequent change in the reconnection ratein the distant tail are important factors in development ofa double‐onset substorm [Pulkkinen et al., 1998; Russell,2000]. According to these studies, near‐Earth reconnection

begins on closed field lines during the first onset when theIMF is southward and remains on the closed field lines as longas closed flux accumulates due to the distant tail reconnec-tion. When the IMF turns northward, distant tail reconnectionstops, allowing near‐Earth reconnection to reach the openflux region and initiate a second onset [Russell, 2000]. Inthe double‐onset substorm in this study, the IMF also turnednorthward but gradually. The first onset seemed to be asso-ciated with closed field line reconnection, while the secondonset appeared to extend to the lobe. For the substorm in thisstudy, the initially southward IMF turned north in a two‐stepmanner with an interval of weak IMF between the two onsets.This gradual development possibly allowed enough time forreconfiguration of the current sheet in the midtail and achange in the location of the reconnection site for the secondonset, indicating that the internal processes can contributeto the formation and location of a second onset.[42] According to Petrukovich et al. [2000], out of several

tens of small, contracted‐oval substorms in their study, afew had a peculiar onset sequence. The sequence consistedof a high‐latitude geomagnetic onset recorded on the groundabout 40 min after a clear magnetotail onset, including apressure maximum and plasmoid extraction. These observa-tions fit the above‐described double‐onset sequence, becausesolar wind driving for those onsets was also weak. The firstplasma sheet activation was so weak that it resulted in only apseudobreakup. The second plasma sheet intensification wasconsiderably delayed because the tail required time to recre-ate the threshold state partially removed by the first activa-tion. It might need even more time because of the weak input.The observed event in this study fits this logical scheme, sinceit has an amplitude intermediate between those of very weakcontracted‐oval substorms [Petrukovich et al., 2000] andrelatively large double‐onset examples [Pulkkinen et al.,1998; Russell, 2000]. The first onset was large enough toproduce a measurable geomagnetic bay, but the second onsetrequired time to accumulate more energy. An importantaspect of the double onset discussed in this paper is plasmasheet preconditioning. The initial plasma sheet reconnectionwas rather slow for the first onset. In a situation with weakdriving, such as our double onset event, the plasma sheetshould be sufficiently thick that reconnection associated withthe first onset would be capable of removing only a part of theclosed magnetic flux. Still, this allowed the plasma sheetto thin in the midtail, where lobe reconnection could moreeasily occur for the second onset.[43] Clear dipolarization was observed for both onsets.

From the observations by P3(THD) and P4(THE), whichwere separated by less than 1 RE, we identified similaritiesand differences in dipolarization and flow signatures andobtained clues about the temporal evolution and spatial struc-ture of the dipolarization. Detailed analysis of the boundariesusing high‐resolution field measurements enabled us to quan-tify dipolarization front characteristics. An important aspectis resolving the relationship between an Earthward propa-gating dipolarization front [Nakamura et al., 2002; Runovet al., 2009] and the classical tailward propagation of dipo-larization [Baumjohann et al., 1999]. During the first onset,the first dipolarization front was identified at both at P3(THD)and P4(THE), and it had similar flow pattern at each space-craft, suggesting Earthward propagation of an extendeddipolarization front. From the flow pattern as well as the

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dipolarization signature, it can be seen that the magnetic fieldconfiguration at P4(THE) changed earlier toward a morestable dipolar configuration and the fast flow ceased earlierthan at P3(THD). This suggests effects of magnetic field fluxpileup ahead of the flows. It is then expected that spacecraftcloser to the Earth will observe a larger magnetic flux pileupassociated with the dipolarization. Similar evolution of anEarthward and tailward dipolarization event has been observedby Nakamura et al. [2009] with the Cluster spacecraft around10 RE.[44] The second onset, however, did not display the same

systematic development of the flow and dipolarization at bothP3(THD) and P4(THE) spacecraft. But interestingly the firstdipolarization after the second onset was associated with adawnward flow. In that dipolarization, the flow direction wasnearly parallel to the dipolarization front. In the subsequentdipolarization, the fast flow was mainly Earthward but morelocalized, so the correlation between the two spacecraft waspoor. Furthermore, the flow pattern seemed to have a vortexstructure rather than a clear, steady boundary. During thesecond onset when the two spacecraft were located in astronger dipolar field region, the fast flow had a localizedshear structure and appeared to be associated with filamentaryauroral structures.[45] Despite the unstructured flow shear pattern observed

during the second onset at P4(THE) and P3(THD), geosyn-chronous satellites GOES 10 and GOES 12 observed aclassical current wedge signature [Nagai, 1982] propagatingwestward during the second event. This suggests that theoverall disturbance reached closer to the Earth for the secondonset. The transport of flux closer to the Earth was notexpected in the second event since the near‐Earth magneticfield was already in a relatively dipolar configuration, if weassume a constant flux transport rate. While there was noflow‐braking signature and the dipolarization front showedless compression than in the first onset, the frequent flowreversals suggested a localized vortex pattern. Either the fluxtransport mode changed with the dipolar configuration orsome processes other than flux transport from the tail governplasma motion in this region. It is interesting that the currentwedge started at GOES 10 just after the dawnward flow‐associated dipolarization at P4(THE) and P3(THD), butbefore the Earthward propagating dipolarization front. Thissuggests that the current wedge signature cannot be relateddirectly to the Earthward moving dipolarization front observedby P4(THE) and P3(THD), but instead is related to thedipolarization front associated with the dawnward flowobserved by P4(THE) and P3(THD). This dawnward flowcould be part of a vortex pattern created by a fast flow dusk-side of the two THEMIS spacecraft near the dipolar region,where the flow has rebounded off the inner magnetosphere,which was observed [Panov et al., 2010] and modeled in anMHD simulation by Birn et al. [2011].[46] Double‐onset substorm mechanisms have been also

discussed from near‐Earth observations of a substorm byCombined Release and Radiation Effects Satellite (CRRES)[Erickson et al., 2000] and substorm observations by Geotail[Saito et al., 2010]. These mechanisms were developed basedon local propagation direction properties of the onset dis-turbances. For these substorms, it was concluded that the firstonset was due to the ballooning instability that created a

rarefaction wave that propagated tailward and initiatedreconnection for the second onset further tailward. Theessential differences between these Geotail and CRESS near‐Earth, double‐onset substorms and the substorm in this studyare the initial configuration of the current sheet (as well as thelatitude of the electrojet) and the way of activating recon-nection for the second onset, i.e., either tailward propagationof a rarefaction wave or enhanced flux pile up and com-pression due to the first reconnection. We suggest in thisstudy that the large‐scale reconfiguration of the magnetotailassociated with the second onset depends on the two factors:the preconditioning of the plasma sheet by the IMF driverand by the internal tail current sheet processes precedingthe onsets. Multistep processes could therefore be a naturalscheme of the substorm expansion.

5. Summary and Conclusion

[47] Based on simultaneous multipoint observations by theTHEMIS and GOES spacecraft and ground‐based observa-tions, we inferred the current sheet evolution during a double‐onset substorm. Details of onset disturbance timing cannot bepursued because none of the THEMIS spacecraft foot pointsare exactly at the local time of the auroral onset. Nonetheless,large‐scale magnetospheric configuration and local observa-tion of the magnetic field and plasma flow suggest that bothonsets begin in the thin current sheet region.[48] We found that the first onset, which took place when

the thin current sheet was extended relatively Earthward(reaching the 11 RE region), initiated effective Earthwardflux transport of closed flux. As a result, the closed flux wascarried away from the midtail region and piled up in thenear‐Earth region. The flux pileup caused compression andthickening of the near‐Earth current sheet, while the thincurrent sheet regionwas shifted tailward of 18RE.We suggestthat this internal redistribution of magnetic flux results inproduction of a thin current sheet with a smaller BZ com-ponent in the midtail region. This reconfiguration leads tofavorable conditions for lobe reconnection. Lobe reconnec-tion associated with the second onset caused a global recon-figuration of the magnetotail. Since the flux pileup leads totailward shift of the thin current sheet region, the second onsetoccurred tailward of the first onset region. All‐sky imagessupport this conclusion. The first auroral onset was localized,but the second onset took place at higher latitudes and expandedover a larger area.[49] For both onsets, dipolarization in the near‐Earth

(11 RE) plasma sheet took place about 1–2 min after onsetof the midtail unloading signature and had about the sameamplitude (DBZ). Yet there were also significant differencesin flow and field characteristics in this region. Before the firstonset, a relatively thin current sheet developed at the near‐Earth region, and about 2 min after that onset, the Earthwardpropagating dipolarization front and resulting flux pileupwere observed. During the second onset, on the other hand,the initial dipolarization signature was associated not withEarthward flow but rather with dawnward flow and tailwardpropagation. After this period of dawnward flow, fast butfluctuating Earthward flow with an Earthward propagatingdipolarization front was observed. Auroral signatures alsovaried with these flow and field characteristics. The dipo-

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larization front observed after the first onset was associatedwith north‐south auroral development, while the dipolariza-tion after the second onset was associated with fine structuredaurora extending farther equatorward. These observationsindicate that dipolarization signatures differ during substormevolution depending on the distance from the source regionand on the ambient plasma/field condition.[50] Based on the boundary analysis, it was found that

the spatial scales of the dipolarization fronts were a fractionto 3 ion inertial length, i.e., few hundred to about 2000 km.The first dipolarization front associated with the current sheetthinning could be identified as a stable discontinuity struc-ture over a time scale of a few minutes that was propagatingEarthward with Earthward flow. A localized (∼3200 km)flow/magnetic shear structure formed around this thin dipo-larization front, which had a scale size of 1–2 ion inertiallengths. This shear corresponded to a current density of70 nA/m2 associated with a dawn‐to‐dusk current and con-tributed to local thinning of the current sheet. This structurewas shown to create a strong, upward field‐aligned current(∼10 nA/m2), most likely producing the elongated thin north‐south aurora observed at the conjugate ionosphere. To betterunderstand the generation of these dipolarization fronts, it isessential to do a systematic study of the local characteristicsof these boundaries (i.e., orientation, motion, and scale size),while also taking into account the large‐scale context of thesubstorm activity and plasma sheet configuration.

[51] Acknowledgments. The authors gratefully acknowledgeINTERMAGNET, GIMA, USGS, CARISMA, MACCS, DMI, CSA,I. Mann, S. Mende, and E. Donovan for providing ground‐based magneticfield and/or auroral data, N. Ness (CalTech) for use of ACE data, H. Singer(NOAA) for use of GOES data, and M. Kubyshkina and V. Sergeev forproviding foot point calculation results. This research was supported bythe Austrian Science Fund FWF I429‐N16, by NASA grant NAS5‐02009,and by the Russian Foundation for Basic Research (project 10‐05‐01001).We thank J. Hohl for proofreading the manuscript.[52] Masaki Fujimoto thanks Kyoung‐Joo Hwang and another reviewer

for their assistance in evaluating this paper.

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