A Substorm Event with Multiple Onsets on February 28 2007

A Substorm Event with Multiple Onsets on February 28 2007

H. Zhang, Y.S. Ge, T.-S. Hsu, M. G. Kivelson, V. Angelopoulos, K. K. Khurana, R.L. McPherron, and C. T. Russell

Institute of Geophysics and Planetary Physics, University of California, Los Angeles

Abstract

A substorm event with multiple onsets was observed by THEMIS spacecraft and ground-based observatories at a very early stage. The in situ observations in the near-Earth tail reveal that the tail reconfigured three times, and that the third event was more global and sustained than the first two. We will refer to this third reconfiguration as the major onset. The reconfigurationsderived fromthe variations both of BX and BZappear to link to different processes, with BXchanging in response to plasma sheet expansion and BZchanging in response to flux pileup. The initial formation of a field-aligned current was also diagnosed from the perturbations of BY. The conjugate observations in space and on the ground and the simultaneity of theintensification of aurora, the formation of field-aligned currents and theflux pileupprovide clues to a better understanding of the mechanism for the onset of substorm expansion.

Introduction

A prime objective in space physics research is to develop a detailed understanding of the interrelation of phenomena associated with a terrestrial substorm. Various signatures signal the onset of the expansion phase of a substorm, including auroral brightening followed by poleward expansionin the ionosphere, westward electrojets, magnetic field pulsations and tail magnetic field reconfiguration. The changes in space and on the ground can be, in part, understood as responses to a 3-D current system referred to as the substorm current wedge (SCW) [McPherronet al., 1973]. Many models and theories have been proposed to explain the substorm phenomenon, especially to explain how and where a substorm onset is initiated. But uncertainties remain because ofthe lack of extensive observations, including conjugate measurements of ground and in situ signatures. The ongoing THEMIS mission is now seeking to untangle the mystery of the substorm process, ultimately using 5 spacecraft in space supported by extensive coverage by ground magnetometers (GMAG) and all-sky imagers (ASI) over the North America.

An important signature in space during substorm onsets is dipolarization of tail magnetic field, i.e., the magnetic field changes from a tail-like configuration to a more dipolar configuration. There are many explanations for this reconfiguration of tail field. One possible explanation is the disruption of tail current in near-Earth tail region [Lui, 1991], while the effect has also been attributed to the piled-up fluxes that are brought into the near-Earth tail by fast flows from a near-Earth reconnection site [Angelopoulos et al., 1996; McPherron et al., 1973; Shiokawa et al., 1997, 1998; Baumjohann et al., 2002; Pu et al., 2001]. Thus the near-Earth tailis a key region in which observations can discriminate between the proposed mechanisms.

In both explanations, a pair of field-aligned currents (FACs) should be generated, flowing from the near-Earth tail to and from the ionosphere and connected by the substorm westward electrojet. These FACs in the SCW have been often observed passing above or below spacecraft. The location of FACs can be roughly inferred from such magnetic field observations but not accurately. During the initial phase of THEMIS mission, the five spacecraft were close to each other and their orbits passed through the near-Earth tail right in the pre-midnight sector, thus providing the opportunity to investigate this region with good spatial resolution.

In this paper, we present a multiple-onset substorm event observed by THEMIS only 13 days after its launch. The substorm onset signatures are well recorded on 4 THEMIS spacecraft and at the THEMIS ground-based observatories (GBO).

Data description

The substorm event was observed from 0400UT to 0500UT on Feb. 28th, 2007. All five THEMISspacecraft were inbound, following each other along anorbit with its apogee at 14.6 RE in the pre-midnight magnetotail. THEMIS C (THC) was the leading spacecraft; THD, THB and THA were very close to each other followed by THE. The NOAA/GOES -10 and -12 spacecraft were also in the midnight sector.Figure 1(a) displays the position of each spacecraft in GSM right at 0445UT, the major onset time.FGM data were available only from THA, THB, THC and THD, but not from THE.In this paper, FGM magnetic field data [Auster et al., 2008] are presented at3-second resolution in a coordinate system rotated around the Z axis of GSM so the new X axis points from the spacecraft to the Earth. MAG data from GOES-12 and -10 at 0.5-second resolution are given in the P-E-N coordinate system with P perpendicular to the spacecraft orbit plane, E pointing to the Earth and N to the east.

Data from ASI and GMAGof THEMIS/GBO are available to provide supporting ground-based signatures of the substorm [Mende et al., 2008]. Figure 1(b) gives the locations of the stations used in this paper. The central meridian of substorm current wedge is inferred from the D components of the magnetic field (0.5-second resolution) measured by the mid-latitude GMAG stations[Clauer et al., 1974]. High resolution auroral images (3 seconds and 256×256 pixels) are available from the GILL (66° MLat) and FSMI (68° MLat), and lower resolution aurora data (6 seconds and 32×32 pixels) are also available from SNKQ (66° MLAT). All the auroral data are given in this paperby videos. We should emphasize that the footprints of THEMIS are very close to GILL based on the estimates from the T96 model (solid circles in Figure 1(b)) and from 0400UT to 0500UT they movedlittle[Tsyganenko et al., 1996].

Observations

Figure 2 shows an overview of the substorm event. THEMIS observed three magnetic field disturbances. The first disturbance began at 0401:30 UT whenBX, BZandBT, and the elevation angle of the local magnetic field at all 4 spacecraft began to increase. These traces attained their maxima at 0406UT at THD, THB and THA andthen immediately began to decrease. The magnitude of enhancement of BZat THC, the innermost spacecraft, is not so large as those at other spacecraft. Drops of BX, BZand BTwerealso observed at THC but 1 minute earlier than the drops at other spacecraft. The decreasesofBXwere all associated with a bipolar perturbationinBY(from positive to negative). After 0415 UT, BT and BX at THEMIS gradually increased; BZ and the elevation angle continued to decrease until 0422:30 UTat THD, THB and THA, and until 0424:30 UT at THC. Throughout this event, IMF conditions were monitored by Cluster-II which was located at (11.23, 3.05, -12.30)REin GSM,just in front of magnetopause. Figure 2 shows that from0340 UTIMF BZwas primarilypositive but small with amplitudeless than3 nT. The IMF turned southward at 0418 UT and basically kept -5 nT in this event. In fact, from 0040 UT to 0220 UT, IMF was also southward with magnitude of -5 nT (not shown).

After 0422:30UT, a second disturbance similar to the first was initiatedby an enhancement of BX and a slight increase in BZat THD, THB and THA (not seen at THC). After 0424UT BX, BZand BTdropped slightly atall spacecraft, and0.5 minutes later (0424:30UT) an impulsive enhancement of BZ, BXand BTwas detected at THA, B, D and then at THC. At 0425:10UT, BX, BZand BT again decreased significantly, first at THC and then at the others. Almost simultaneously with the increase in BZ, BYbecame dynamic and predominantly positive. The magnetic disturbance at THC appeared different from thoseat the other spacecraft. The initial gradual enhancement of BZwas absent at THC. After this disturbance, BZat all spacecraft recovered; and BX and BT continuously increased till 0445UT. [I1][MSOffice2] IMF remained negative during and after the second perturbation.

The third magnetic field disturbance began with the sudden jump of BZ around 0445:15UT first observed by THD, B and then by THA at 0445:30 UT.25 seconds later at 0445:40UT this sudden enhancement of BZ was detected closer to the Earth by THC. Figure 3 shows the details of this perturbation and we notice that all of the enhancements of BZ are accompanied by sudden increases in BXandBT. It is noticed that BX began to decrease slightly at THCat 0444:45UT (seen in Figure 2),butdid not drop significantlyuntil 0445:25UT, several seconds later than the increase of BZ at the outside spacecraft and 15 seconds before the abrupt jump of BZ at THC. Thesharp drops of BX were also detected by THD, THB and THA around 0446:30UT which is 50 seconds later than the initial enhancement of BZ at these spacecraft. It is worth emphasizing that the initial BYperturbations at THD, THB and THA werepositive and began at almost the same time as or a little bit later than the jumpof BZ. During the decrease in BX, the perturbation of BY changedits sign from positive to negative. However, at THC, the perturbation of BYwhich started at 0445:25UT remained negative when BX suddenly dropped. The perturbation of BZin this event waslarger and more sustained than those in the former two events. The 5thpanel of Figure 2shows that the elevation angle rosein association with every magnetic perturbation.During the first two perturbations the elevation angle increased and then recovered soon,whereas after the third perturbation, the magnetic field became more dipolar with the elevation angle increase of more than 40 degree and the dipolar configuration did not recover soon. Just ahead of this disturbance IMF had a short-time northward turning,thereafter it remained southward.

The provisional AL index and the power in mid-latitude Pi2 pulsations are shown in panels 7 and 8 of Figure 2. During this period, the provisional AL index decreased gradually after the first two onsets and abruptly, associated with the strongermid-latitude Pi2 pulsations,following the third onset. For the first two onsets, the decreases in AL were tiny with magnitudes of 50 nT and 200 nT, respectively; while for the third onset, AL suddenly dropped 350 nT within 2 minutes. Intensification was also seen in AL at 0455 UT.

Auroral activity was recorded by all sky imagers at the ASI/THEMIS-GBO of GILL, SNKQ and FSMI, and the observations are available asvideos. A weak auroral brightening first appeared at the common boundary of the field of view of GILL and SNKQ at 0402:00UT. From 0402:30UT distinct arcs were continuously present at the poleward boundary of the bright region; these bright arcs weretraveling westward and equatorward. At the same time, the poleward boundary of the bright region expanded further poleward. About 7 minutes later (0409:30UT) the bright region stopped expanding and thenthe luminosity died away. After 0413:00UT, there was just a stationary faint auroral arcover GILL and SNKQ, and THEMIS observed a quiet tail.The second auroral intensification began with a weak brighteningarc in the western field of view of SNKQ at 0423:00UT. After 0424UT the brightened aurora expandedpoleward rapidly with auroral arcs traveling equatorward. The aurora over GILL and SNKQ continued to be dynamic until 0431UT and after that time, the near-Earth magnetotail became quiet again. At 0445:09UT, a bright point of aurora appeared in the northeast of the field of view of GILL. This brightened region expanded poleward and azimuthally and several equatorward traveling arcs appeared in this region. After 0445:39UT, a vortex structure appeared and the aurora became more dynamic. The aurora fully expanded and filled all of the field of view of GILL and SNKQafter 0450UT. Some auroral brightening was also detected by FSMI which is located to the west of GILL.Correspondingly, the near-Earth tailwas highly disturbed during this period.

Figure 4 displays the D components of the geomagnetic field at different mid-latitude THEMIS-GBO stations. Around 0404UT, the perturbations of the D components of almost all stations were positive but very weak. The magnetic perturbations in the N direction at GOES-10 and GOES-12 were both positive (eastward) (see in Figure 2). After 0424UT, the negative (positive) changes of the D components were recorded by the stations eastward (westward) of SWNO (PINE). The footprint of GOES-12 was located ~1 hour east of SWNO at this time. A negative (westward) perturbation was detected at GOES-12. GOES-10 was further eastward and detected Pi2 pulsations in the HN component. After the third onset at 0445UT, it was found that the perturbations of geomagnetic D components change signs between BMLS and RMUS. Immediately after 0445 UT, a negative perturbation appeared in the HN component both at GOES-12 and GOES-10. These perturbations reached their peaks at GOES-12 at 0449:10UT and then at GOES-10 at 0450:20UT.

Discussion

In the Near-Earth Neutral Line (NENL) model, both pseudo-breakups and major onsets are thought to have common features [Nakamura et al., 1994] and both are believed to be caused bythe magnetic reconnections while at different stages, i.e., the closed field line reconnection and the open lobe field line reconnection [Russell et al., 2000, Baumjohann, 2002]. Since open field line reconnections release the lobe magnetic energy accumulatedduring the growth phase, disturbances atmajor onsets can proceed to a full expansion phase development while disturbances at pseudo-breakups are subject to subside quickly and be tightly localized. In this substorm eventthe plasma sheet thinning, a typical signature of growth phase, has been seen before the second and third onsets when IMF was southward, and the elevation angle and BZ remained large and persistently disturbed after the third dipolarization while recovered soon after the first two. The sudden decrease in AL index associated with the strongest Pi2 pulsations at mid-latitude alsosuggests the third perturbation wasthe major onset [Hsu et al., 2008].During this onset the aurora expanded wide and remained dynamic for more than 30 minutes while the aurora were localized and lasted for less than 10 minutes during the pseudo-breakups. A northward turning of IMF is clearly seen before the major onset, just as predicted by Russell et al. [2000].

The updated NENL model has associated the dipolarization and the reduction and diversion of cross-tail current with the flow braking in the near-Earth tail. Shiokawa et al. [1997] pointed out that the flow braking process attributed to the earthward pressure gradientandinferred that the magnetic flux carried by the earthward convective flows should be piled-up at the braking point. Zhang et al. [2007] suggests that the enhancements of BX, BZ and BT can be a direct evidencefor flux pileup. In this substorm event, these flux pileup signatures were detected by all spacecraft duringeach disturbance, but appeared stronger and sustained longer in the major onset. The pileup signatures at the major onset were first observed at the middle spacecraft THB and THD, and later at the innermost and the outmost spacecraft, suggesting that the pileup region expanded both earthward and tailward.The inner compressional front moves radially inward at a speed of about 300 km/s, while the speed of tailward front is hard to estimate due to the close separation of spacecraft.

The inertial current can be generated during the flow braking and partially contributes to the reduction of cross-tail current[Shiokawa et al.,1997]. Simulations done by Birn et al. [1999] shows that the reduction of the cross-tail current is mostly resulted from the reduction of the curvature drift current. However, the inertia current might still play a role as introducing the initial reduction of magnetic field line curvature inside of the braking point. The significant reduction of the curvature drift current is consistent withthe expansion of the plasma sheet which can be signaled by the sudden decreases in BX and BT [Zhang et al., 2007; Ge et al., 2007]. As the pileup front moves tailward, the significant current-reduction region and the plasma sheet expansion region move tailward, too.Thetailward motionof expansion regionwas seen in all the three onsets of the substorm studied in this paper.

The cross-tail current is believed to be reduced and diverted to field-aligned directionswhich are downward at dawnside and upward at duskside. For the three onsets of this substorm, BY perturbationscaused by FACs are detected after the beginning of each flux pileup. During the first onset, the geomagnetic signatures show that THEMIS is close to the upward FAC. The reversal of BY component in space suggests thatthis FACcrossed all the THEMIS spacecraft during the plasma sheet expansion. For the second onset, THEMIS was located near the central meridian of SCW while it’s clear that GOES-12 was located inside the downward FAC.During the major onset, the negative perturbation of BY component at THC and the positive changes at the outer spacecraft indicates that anupward FAC initially formed between THC and the others. This is also confirmed by the geomagnetic signatures that the central meridian of SCW was located eastward of THEMIS spacecraft. The formation of FAC is found right after the beginning of pileup, i.e., after the initial enhancements of BX, BZ and BT at THD, B and A. It is consistent with the argument that the flow braking (flux pileup) leads to the initial reduction and diversion of the cross-tail current. Then the bipolar signatures ofBYassociated with the plasma sheet expansion indicate that the FAC moved tailward across the three outer spacecraft. On the other side of the central meridian, GOES-10 and -12 detected a downward FAC formed outside of the synchronous orbit. The time delay of negative perturbations on east-west components between these two spacecraft suggests that this downward FAC appears to move azimuthally.