High-latitude ionosphere convection and Birkeland current response for the 15 May 2005 magnetic...

High-latitude ionosphere convection and Birkeland

current response for the 15 May 2005 magnetic

storm recovery phase

S. Eriksson,1 M. R. Hairston,2 F. J. Rich,3,4 H. Korth,5 Y. Zhang,5 and B. J. Anderson5

Received 29 February 2008; revised 4 April 2008; accepted 10 June 2008; published 9 September 2008.

[1] The high-latitude response of sunward E � B flow and Birkeland field-alignedcurrents (FAC) is analyzed for the 15 May 2005 magnetic cloud that generated a greatmagnetic storm (SYM-H = �305 nT at 0820 UT). The interplanetary magnetic field (IMF)clock angle, q = arctan(By/Bz), gradually rotated from 65� to �80� during the 10-h longnorthward IMF period and the recovery of this storm. DMSP observations confirm adawnward migration of a Northern Hemisphere sunward E � B flow channel (FC)between a downward and upward FAC pair. This FAC system developed duringsouthward IMF (q = 109�) at the poleward edge of the duskside auroral oval as part of afour-sheet FAC system 23 min before the IMF became northward. TIMED/GUVIobservations show that the dawnward migration of the upward FAC coincides with adrifting transpolar auroral arc (TPA). IMAGE/WIC did not observe a TPA in the southern(winter) hemisphere. DMSP and Iridium observations are in good agreement with MHDsimulation predictions of a northward IMF reorientation of high-latitude FACs. Thenorthern FC migration was likely due to summer hemisphere conductances, a strongaverage IMF Bx = �35 nT and the sunward dipole tilt angle that favor a northern high-latitude reconnection mechanism for a well-organized sunward FC and FAC systemmigration. The storm recovery rate appeared to be related with the region 2 FAC. A fast11.4 nT/h rate was observed for a weak or nonexistent region 2 system during the high-latitude FAC redistribution. The SYM-H recovery slowed significantly to 0.9 nT/hfollowing the 1800 UT region 2 system recovery.

Citation: Eriksson, S., M. R. Hairston, F. J. Rich, H. Korth, Y. Zhang, and B. J. Anderson (2008), High-latitude ionosphere

convection and Birkeland current response for the 15 May 2005 magnetic storm recovery phase, J. Geophys. Res., 113, A00A08,

doi:10.1029/2008JA013139.

1. Introduction

[2] Northward interplanetary magnetic field (IMF) con-ditions typically result in high-latitude (HL) reconnectiontailward of the Earth’s cusps where the shear angle betweenthe magnetosheath and the geomagnetic fields may be large[e.g., Dungey, 1963; Song and Russell, 1992; Phan et al.,2003; Trattner et al., 2004]. Sunward E � B flow inthe high-latitude ionosphere is directly driven by the sun-ward j � B force generated by this process [e.g., Iijima,

2000; Eriksson et al., 2005]. Northward IMF conditions arealso favorable for the appearance of an additional high-latitude pair of upward and downward Birkeland field-aligned current (FAC) system which is known as the NBZcurrent system [e.g., Iijima et al., 1984; Zanetti et al., 1984;Iijima and Shibaji, 1987; Cowley, 2000; Iijima, 2000].[3] A dayside reverse two-cell convection system is

commonly observed at the noon-midnight meridian fordue northward IMF [e.g., Maezawa, 1976; Reiff and Burch,1985; Burch et al., 1985] as a result of the sunward E � Bflow. A downward (upward) NBZ current exists within thepostnoon counterclockwise (prenoon clockwise) lobe cell.The downward FAC is coupled via a dawnward ionosphericPedersen current (JP) to the upward FAC. The sunwardE � B flow channel exists within this Pedersen currentregion where JP � E > 0 [e.g., Iijima, 2000].[4] Southwood [1987] predicted an identical smaller-scale

electrodynamic system between two oppositely directedFACs, JP and E � B for the ionospheric signatures of fluxtransfer events at the magnetopause. The Southwood sche-matic of the localized E � B convection and thecorresponding FAC pair is reproduced in Figure 1.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, A00A08, doi:10.1029/2008JA013139, 2008

1Laboratory for Atmospheric and Space Physics, University ofColorado, Boulder, Colorado, USA.

2William B. Hanson Center for Space Sciences, University of Texas atDallas, Richardson, Texas, USA.

3Air Force Research Laboratory, Hanscom Air Force Base, Massachu-setts, USA.

4Now at Lincoln Laboratory, Massachusetts Institute of Technology,Lexington, Massachusetts, USA.

5Johns Hopkins University Applied Physics Laboratory, Laurel, Mary-land, USA.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2008JA013139

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[5] A recent MHD simulation [Vennerstrøm et al., 2005]of high-latitude Northern Hemisphere FAC systems pole-ward of the region 2 current system [Iijima and Potemra,1976] suggests a gradual transition of a dayside NBZcurrent system (dusk-dawn aligned JP) into a cusp currentsystem (noon-midnight aligned JP) as the IMF clock angle,q = arctan(By/Bz), gradually rotates from q = 0� (duenorthward IMF) to q = 90� (positive IMF By). A dusksidepair of oppositely directed FACs extending to 1800 mag-netic local time (MLT) appeared for q = 45�. This simulatedduskside FAC pair, in particular, implies a relatively narrowsunward E � B flow channel (FC) between the oppositeFACs following the Southwood [1987] schematic in Figure 1[see also Iijima, 2000].[6] A northward IMF HL reconnection generation mech-

anism of this simulated IMF-dependent FAC pair predictsthat the sunward FC and its bounding FAC pair shouldmigrate dawnward across the polar cap as the antiparallelmerging region moves dawnward across the high-latitudemagnetopause in response to an IMF clock angle rotationfrom positive to negative IMF By [e.g., Luhmann et al.,1984; Reiff and Burch, 1985]. Such a migration of the jointsunward FC and its FAC system has yet to be clearlyconfirmed by observations.[7] We report a storm time ionospheric redistribution of

this pair of high-latitude Birkeland currents and its sunwardE� B FC in support of the given scenario during a 10-h longnorthward IMF period of the 15 May 2005 magnetic cloud[e.g., Burlaga et al., 1981; Gosling, 1990; Farrugia et al.,1993] that generated a very intense �305 nT geomagneticstorm during its initial southward IMF phase. We discussthe implications of the high-latitude FAC redistributionfor transpolar auroral arcs [e.g., Valladares et al., 1994;Zhu et al., 1997; Chang et al., 1998; Cumnock et al.,2002; Frey, 2007], the origin of the ‘‘four-sheet’’ FAC

structure [e.g., Ohtani et al., 1995; Eriksson et al., 2002;Ueno et al., 2007], and storm recovery rates.

2. IMF, Solar Wind, and SYM-H Observations

[8] Figure 2 shows ACE and Geotail magnetic field andplasma observations during the 0200–2300 UT period on15 May 2005. IMP-8 plasma data, when available, are alsoshown (IMP-8 magnetic field data were not available). Thisperiod was characterized by the passage of a magnetic cloudat 1 AU. The 0240 UT arrival of a forward shock at Geotailahead of the Earth-bound magnetic cloud resulted in Geotailmoving into the dayside magnetosheath (and a positiveSYM-H index increase). The ACE (IMP-8) data have beenshifted forward in time by 32 min (4 min) to match theoverall Geotail observations during this magnetic cloud.Figure 3 shows the GSE positions of the Geotail, IMP-8,and ACE solar wind monitors in the XY plane on 15 May2005. The 32 min ACE time delay corresponds to anaverage Vx = 820 km/s solar wind propagation speed ingeneral agreement with observations.[9] Geotail occasionally entered the solar wind (e.g.,

0604–0628 UT) and spent a considerable amount of timein the magnetosheath (450 < jVj < 750 km/s) with longerexcursions into the magnetosphere (jVj < 250 km/s; e.g.,0920–1009 UT and 1315–1510 UT). It is clear from anIMF clock angle comparison between the ACE and Geotail

Figure 1. Schematic ionospheric signatures of Birkelandcurrents (solid vertical lines) and their ionospheric foot-points (thick horizontal circle), Pedersen currents (dashedhorizontal lines), and the E � B flow (solid horizontal lines)for flux transfer events at the magnetopause. (Adapted fromSouthwood [1987].)

Figure 2. Geotail (red), IMP-8 (blue), and ACE (black)solar wind and IMF data are shown for the 0200–2300 UTperiod on 15 May 2005. ACE and IMP-8 data are shiftedforward in time as indicated.

A00A08 ERIKSSON ET AL.: POLAR ELECTRODYNAMICS AT STORM RECOVERY

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data in Figure 2 that the 32 min time shift of the ACE solarwind data is accurate to within minutes for most of thecloud passage.[10] Figure 4 displays the time shifted ACE IMF and

solar wind conditions during the 18.5-h-long 0430–2300 UT period on 15 May 2005. This was a time of severegeomagnetic activity as indicated by the 1-min resolutionSYM-H index [Wanliss and Showalter, 2006]. A veryintense geomagnetic storm initiated 21.5 min after the0602 UT southward turning of the IMF that settled aroundIMF Bz = �45 nT for �30 min. The SYM-H index reacheda �305 nT minimum at 0820 UT. A storm recovery periodwas maintained at or below SYM-H = �100 nT for theremainder of this day following a�0858 UT IMF northwardturning to Bz = 15 nT. The IMF stayed northward until�1913 UT. This �10-h long northward IMF phase wascharacterized by a gradual IMF clock angle rotation from q= 65� (By > 0) to q = �80� (By < 0) and an intermediate 3-h long 1342–1642 UT period at qavg = 10�.

3. Convection and Birkeland CurrentObservations

[11] We analyzed the ion drift meter and magnetometerinstrument data [Rich and Hairston, 1994] from consecutiveDMSP F13 and F15 satellite orbits during the 0800–2100UTperiod on 15 May 2005 for any evidence of sunward FCs.The DMSP satellites have a 1 h 42 min orbital period at acircular �840 km altitude. The fixed Sun-synchronousDMSP orbital planes allow for a clear identification of thesunward FC feature, if present.[12] The DMSP F13 and F15 satellites confirmed a

migration of a sunward FC across the polar cap from duskto dawn during the recovery phase of this magnetic storm.Eight consecutive F13 northern polar passes of the cross-track component (Vy) of the ion drift speed are shown inFigure 5. The maximum sunward Vy speed (m/s) for eachpass above 50� MLat (solid dot) is indicated as well as thetime of maximum sunward flow observation. We alsoindicate the average q (C in �) and SYM-H index (S innT) at the time of the sunward flow interval. A sunward FCis clearly present on the duskside at 1021 UT for q = 43�and also at 1204 UT (q = 48�). The FC eventually reachedits maximum MLat along the noon-midnight meridian by1530 UT (q = 3�). At this time, a reverse double lobe cellsystem dominated the polar region poleward of 73� MLatalong the F13 dusk-dawn trajectory.[13] It is unclear from the F13 polar plots alone whether

the FC moved beyond the noon-midnight meridian after1530 UT. However, DMSP F15 ion drift observations at1702 UT (q = �17�) and 1844 UT (q = �92�) shown in

Figure 6 confirmed that the sunward FC indeed moved intothe dawnside MLT sector for negative IMF By. A compar-ison with the DMSP F13 observations at 1713 UT (seeFigure 5) suggests that the sunward speed within the FCslowed significantly between 0900 and 0600 MLT or thatthe FC did not extend to 0600 MLT.[14] The light blue interval in Figure 4 indicates the

0841–1206 UT time period when F13 confirmed a dusksidesunward FC. Evidence for its presence by 0841 UT on the

Figure 3. Geotail, IMP-8, and ACE GSE positions are shown in the XYplane during the passage of themagnetic cloud on 15 May 2005.

Figure 4. ACE IMF, solar wind density, speed, and theSYM-H index are displayed during the 0430–2300 UTperiod on 15 May 2005. The ACE data were shifted forwardin time by 32 min on the basis of the good match withGeotail and IMP-8 observations. Vertical colored intervalscorrespond to Northern Hemisphere DMSP F13 observa-tions of a high-latitude sunward E � B flow channel at dusk(blue), noon (red), and dawn (green).


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basis of F13 magnetometer data is shown below. The lightred interval in Figure 4 likewise corresponds to the F131348–1529 UT period of a sunward FC at noon, while theF13 1710–2032 UT light green interval marks the finaltransition into a dawnside sunward FC as supported by both

F13 and F15 observations. The times given in this para-graph refer to the times of maximum F13 and F15 MLatpositions.[15] Figure 7 displays individual �20-min long time

series of the F13 cross-track components of the E � B driftvelocity (Vy) and the horizontal disturbance magnetic fielddB above 50� MLat for the eight consecutive dusk-to-dawnpasses shown in Figure 5. A sunward drift corresponds toVy > 0. It is clear from the slopes of the cross-track dBcomponent (black line) on either side of the sunward FC thatthis Vy > 2.3 km/s flow was bounded by a well-defineddownward FAC on its duskside and an upward FAC on itsdawnside. This dynamic FAC pair exists poleward of theregion 1/2 FAC system and it moves dawnward with thesunward FC across the polar region. When it reaches its mostpoleward latitude we commonly refer to it as an NBZ currentsystem.[16] Figure 7 shows that the low-latitude region 2 current

was very weak during the 1015–1720 UT storm recoveryphase when the average SYM-H recovery rate was 11.4 nT/h.That was 12 times faster than the average 0.9 nT/h rate thatpersisted until the SYM-H had fully recovered by the end of19 May. The duskside region 2 current rebound significantlybetween 1720 and 1845 UT. This appears to have had astabilizing effect on SYM-H which leveled off at �90 nTaround 1800 UT. There were even indications of a SYM-Hintensification in the 1800–2130 UT period to�110 nT (seeFigure 4).[17] The dawnside FAC systems were likely dominated

by the narrow high-speed sunward FC (cf. 1844 UT F15observations in Figure 6 and 2037 UT F13 observations inFigure 5) and its intense FAC pair poleward of a muchweaker (cf. 2038–2040 UT dB observations in Figure 7) ornonexistent region 1/2 system in its usual sense. Themaximum dawnside Vy = 2.5 km/s speed was twice as largeas the duskside return flow at this time and it was compa-rable to the maximum speed within the dawnward migratingsunward FC during its entire northern polar crossing. Thissuggests that it should be identified as a dawnside sunwardFC driven by HL reconnection rather than as the usualDungey cycle return flow [Dungey, 1961].[18] A region 1-type current was clearly observed equa-

torward of the high-latitude FAC pair throughout the FCmigration. Indirect evidence for the first 0835 UT observa-tion of the sunward FC (Vy = 2906 m/s) at 62.8� MLat and16.7 MLT in the auroral region (see auroral data in section 4)

Figure 5. Cross-track E � B (Vy) vectors are displayedalong the �20 min long dawn-to-dusk Northern Hemi-sphere DMSP F13 passes above 50� MLat. The maximumsunward Vy speed above 50� MLat (solid dot) and the timesof maximum sunward flow observations are indicated in theupper right hand section of each MLT-MLat plot. Theaverage IMF clock angle (C in �) and SYM-H index (S innT) during the sunward FC are shown in the lower leftsection.

Figure 6. Cross-track E � B (Vy) vectors are displayedalong the �20 min long Northern Hemisphere DMSP F15passes above 50� MLat. Same format as Figure 5.


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is given by means of its bounding FAC system as obtainedby the DMSP F13 magnetometer at 0835:30 UT (approx-imate center of the FAC pair in Figure 7a). The mostequatorward pair of strong FACs in Figure 7a is interpretedas a region 1/2 system. The dB signatures observed duringthe 0832–0837 UT interval in this duskside sector havepreviously been reported as ‘‘four-sheet’’ FAC structures[Ohtani et al., 1995; Ueno et al., 2007].

4. TIMED/GUVI Auroral Observations

[19] The Global Ultraviolet Imager (GUVI) instrument onthe TIMED satellite [Christensen et al., 2003] regularly

provides far UV measurements of the auroral flux. TheTIMED satellite was launched on 7 December 2001 into a1 h and 38 min orbit. GUVI performs 15-s cross-track scansalong the TIMED orbit and provides auroral coverage in arelatively narrow sector as compared with the IMAGEsatellite [Burch, 2000]. One high-latitude auroral GUVIimage above 50� MLat is obtained in about 20 min andrepresents an accumulated auroral observation. This issufficient, however, to capture the motion of any large-scaleauroral feature during the gradual IMF clock angle rotationof a magnetic cloud such as the one on 15 May 2005.[20] The TIMED/GUVI instrument captured the initial

phase of a northern duskside detachment of a large-scaletranspolar auroral arc segment from the auroral oval asshown in Figure 8. The two solid dots along the DMSP F13pass closest in time to each consecutive TIMED observationmark the locations of maximum sunward flow and theconvection reversal boundary to antisunward flow on itspoleward side. This interval corresponds to a major section

Figure 7. Time series of eight consecutive DMSP F13cross-track E � B drift (positive Vy in sunward direction)and magnetic field disturbance vector (dB) observations(cross-track in black, along-track in gray) are shown for thecorresponding polar plots in Figure 5. A positive (negative)slope of the cross-track dB component indicates a down-ward (upward) FAC.

Figure 8. Northern Hemisphere duskside sector TIMED/GUVI energy flux observations accumulated along theTIMED orbit above 45� MLat on 15 May 2005. Times inblack indicate the maximum MLat TIMED position. Apurple solid line shows the corresponding dusk-to-dawnDMSP F13 orbit (see Figure 5). Equatorward solid dotsindicate the location of maximum sunward flow (timesindicated in purple) while the poleward dots mark theconvection reversal boundary to antisunward flow.


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of the upward Birkeland current (see Figure 7) on thedawnside of the migrating sunward FC. The auroral re-sponse thus coincides spatially and temporally with thedawnward migration of the upward current of the jointsunward FC and FAC system (see Figures 5–7).

5. Global Birkeland Current Distributions

[21] The 15 May 2005 DMSP F13 and F15 ion drift andmagnetometer observations of the sunward FC and itsbounding FAC pair are in good agreement with MHDpredictions of high-latitude FAC redistributions reportedby Vennerstrøm et al. [2005]. Strong supporting evidenceof the observed and predicted redistribution of the high-

latitude Birkeland currents during northward IMF andchanging By conditions was obtained from observationsby the Iridium satellite constellation on this day.[22] The Iridium constellation consists of over 70 satellites

located in six approximately equally spaced circular polarorbits at 780 km altitude. Each satellite has a magnetometerwhich measures the magnetic perturbation due to the FACabove the ionosphere [Anderson et al., 2000]. The magneticfield data are routinely used to derive global distributions ofthe large-scale Birkeland currents from spherical harmonicfits to these perturbations [Waters et al., 2001; Anderson etal., 2002]. To improve the quality of the spherical harmonicanalysis, the Iridium data set may be complemented bymagnetic perturbation data obtained by other satellites. Thewidth of the time window required to sufficiently resolve thelarge-scale Birkeland currents is 1 h or longer and varies withthe specific interval.[23] Figure 9 shows the Iridium observations for three

distinct intervals during the storm recovery phase thatreflect the three average IMF clock angles of q = 47�(Figure 9a), 8� (Figure 9b), and�44� (Figure 9c), respectively.Figure 9 (left) shows the Iridium cross-track magneticperturbations (red) and the CHAMP [Reigber et al., 2002]vector magnetic perturbations (blue). Figure 9 (right) showsthe distributions of the Birkeland currents obtained byapplying Ampere’s law to the global fits of the magneticperturbations. Downward and upward directed FACs areshown in blue and red, respectively. The northern DMSPF13 and F15 passes during these Iridium periods are shownas well as the location of maximum drift speed within thesunward FC (cf. Figures 5–6). This location should appearbetween a downward FAC toward dusk and an upwardFAC toward dawn as was generally the case.[24] The Iridium observations show the reorientation of

the high-latitude Birkeland currents during northward IMFconditions. For q > 0 (Figure 9a), a cusp current systemhaving an upward current toward the geomagnetic pole anda downward current at lower latitude is observed. Thesecurrents transform into an NBZ current system [Zanetti etal., 1984] for q�0 (Figure 9b), where the upward anddownward currents are located in the prenoon and postnoonsectors, respectively. Finally, the Birkeland currents transitioninto a cusp system of the opposite sense for q < 0 (Figure 9c)with a downward current at higher latitude. The Birkelandcurrent distributions in Figure 9 are consistent with thestatistical analysis by Anderson et al. [2008] such that thepolarity of the most poleward Birkeland current is upward forBy > 0 (Figure 9a) and downward for By < 0 (Figure 9c). Fordue northward IMF, these currents are located side-by-side,forming the NBZ current system.[25] The sense of the neighboring high-latitude upward

and downward FACs in Figure 9 is consistent with theSouthwood schematic in Figure 1 and a sunward flow beingdiverted through the cusp region, westward for q > 0 andeastward for q < 0 [e.g., Friis-Christensen et al., 1972;Cowley et al., 1991]. Dusk-dawn meridian plane cuts of thethree FAC maps in Figure 9 would result in the samedawnward migration of a sunward FC as observed byDMSP F13. It is also clear from Figure 9c that the twooppositely directed cusp currents gradually separate andweaken as they extend from 0900 toward 0600 MLT. This isconsistent with the observed slower sunward drift speed at

Figure 9. (left) Northern Hemisphere global CHAMP andIridium constellation maps of the along-track horizontal dBmagnetic field disturbance vector and (right) the derivedFAC maps during the three indicated time intervals. Red(blue) color corresponds to an upward (downward) FAC.Corresponding DMSP F13 and F15 passes with locations ofmaximum sunward flow (solid dot) are also shown.


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the 1713 UT DMSP F13 0600 MLT location as comparedwith the stronger sunward speed that DMSP F15 observednear 0900 MLT at 1702 UT (see Figures 5–6).

6. Discussion

[26] The first 0835 UT sunward FC observation occurredduring southward IMF conditions (Bx = �42 nT, By = 33 nT,Bz = �11 nT, q = 109�) 23 min before the northward IMFphase that we estimate began at �0858 UT. We identifiedthis initial sunward FC indirectly by means of the mostpoleward FAC pair of a four-sheet FAC system [e.g., Ohtaniet al., 1995; Eriksson et al., 2002; Ueno et al., 2007] thatlater separated from the duskside region 1/2 system andmoved across the polar region with the sunward FC (seeFigure 7). This poleward section of a low-latitude sunwardflow region would otherwise have been mistaken for a high-speed sunward return flow of the Dungey cycle [Dungey,1961]. Similar observations of sunward FCs during south-ward IMF and IMF By dominant conditions have beenreported [e.g., Eriksson et al., 2003; Sandholt and Farrugia,2006]. The 0858 UT northward turning and the resulting 23min estimate are based on a 32 min time shift of the ACEdata to Geotail. This time shift was confirmed as accuratewithin minutes by the good match between the ACE andGeotail IMF clock angle observations whenever Geotailwas in the dayside magnetosheath or in the solar wind (seeFigure 2) during the magnetic cloud passage at Earth.[27] High-latitude transpolar auroral arc (TPA) observations

generally display the same IMF clock angle dependence[e.g., Valladares et al., 1994; Kullen et al., 2002] as clearlyobserved here for the migrating northern sunward FC and itsupward and downward FAC pair. Indeed, the TIMED/GUVIobservations on 15May 2005 showed the formation of a TPAthat separated from the duskside auroral oval during the0653–0829 UT interval as an oval-aligned TPA [Kullen etal., 2002]. The dayside part of the TPA then drifted polewardtoward the noon-midnight meridian as the duskside auroraloval faded. The nightside TPA section appeared to beanchored in the premidnight oval during this time. Thedayside TPA coincided with the upward FAC region of theoppositely directed downward and upward FACs that DMSPF13 observed on either side of the migrating sunward FC.[28] The IMF was southward during the 0653–0829 UT

period. However, the trend consisted of a gradual northwardturning. The IMF By was steady positive throughout thisTPA formation period (average By = 34 nT) without anylarge By variations. This contradicts the Chang et al. [1998]expectations of IMF conditions for the TPA formation. Theyalso stated geomagnetic quite conditions for TPA formation.The 15 May 2005 TPA of course developed during a greatgeomagnetic storm.[29] The dayside TPA drifted dawnward toward the noon-

midnight meridian between 0829 and 1142 UT togetherwith the upward FAC of the sunward FC. This periodcorresponded to a strictly positive IMF By. No By signchange occurred until after 1700 UT. What did changeduring the consecutive TIMED/GUVI observations shownin Figure 8 was the IMF clock angle that gradually rotateddue northward. Chang et al. [1998], however, state that theTPA should move from its duskside formation region anddrift dawnward for a negative IMF By. Cumnock et al.

[2002] support this conclusion. The 15 May 2005 TPAevent shows that an IMF By sign change is not required for adrift toward higher latitude, at least for the dayside part ofthe TPA. This observation supports the Eriksson et al.[2005] conclusion of separate dayside and nightside drivingmechanisms of the entire TPA. A coupled dawnward drift ofthe dayside TPA and the upward high-latitude FAC is inagreement with a gradually moving HL reconnection regionfrom dusk to dawn and tailward of the cusp.[30] DMSP F13 ion precipitation observations (not

shown) displayed an apparent ion energy-time of flightdispersion signature in the 1–10 keV energy range. Ahigher 10 keV energy was observed toward the equatorwardside of the FC while lower 1 keV magnetosheath-likeenergies were observed toward the location of maximumsunward flow at the center of the FC. This was independentof the FC location relative to noon. The ion dispersionsuggests a gradual draining of 10 keV energy 107–108

energy flux plasma sheet-like ions toward the FC centerwhere low-energy magnetosheath ions dominate the popu-lation since DMSP F13 traversed the FC nearly perpendic-ularly. A northward IMF reverse energy-latitude dispersionof 1 keV cusp ions is rather expected along the sunward FC[e.g., Woch and Lundin, 1992; Eriksson et al., 2005].Magnetosheath-like ions at the center of the sunward FCeither suggest that these field lines were open at the daysidelocation of DMSP F13 or that they had been open in arecent past before becoming newly closed via a double HLreconnection process tailward of both cusps [Song andRussell, 1992]. The magnetosheath ions at the center ofthe FC contradict TPA models that invoke a bifurcation ofthe lobe by closed plasma sheet field lines [e.g., Chang etal., 1998].[31] The agreement between the migrating upward FAC

and the TPA as well as the observation of 1 keV magneto-sheath ions at the center of the sunward FC favor a HLreconnection generation mechanism of that section of theTPA where sunward flow is observed if we assume that thesunward E � B flow and its bounding FAC pair were drivenby the sunward j � B force from a dawnward migrating HLreconnection region [e.g., Maezawa, 1976; Iijima, 2000;Eriksson et al., 2005].[32] IMAGE WIC observed the entire Southern Hemi-

sphere auroral oval at the time of the northern TPAevolution. Interestingly, there was no evidence of a large-scale TPA detachment from the dawnside southern auroraloval nor any evidence of a duskward drifting TPA (notshown). This suggests an interhemispherically independentresponse of the high-latitude electrodynamics and a NorthernHemisphere preference of the HL reconnection process forthe 15 May 2005 conditions. The very strong average IMFBx = �35 nT during the 0858–1913 UT northward IMFperiod of the magnetic cloud and the sunward dipole tiltappear consistent with an active HL reconnection regiontailward of the northern cusp only to explain the lack of alarge-scale Southern Hemisphere TPA on this day [Songand Russell, 1992; Østgaard et al., 2003].[33] However, the Southern Hemisphere DMSP F13 ion

drift observations (not shown) confirmed an irregular butlarge-scale sunward flow region centered along the noon-midnight meridian during the 1438–1618 UT period (qavg = 7�)as expected for HL dual-point reconnection tailward of


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both cusps. There are even indications of a duskwardmigration of a sunward FC during the 0935–1939 UTinterval. This identification is less definitive due to sub-stantial small-scale flow irregularities related to SouthernHemisphere low-conductance conditions.[34] The evolution of the Birkeland currents in the

Southern Hemisphere could not be faithfully determinedfrom the Iridium observations during the storm recoverysince the magnetic perturbations are much lower in thewinter hemisphere due to the lower ionospheric conduc-tance so that the derived currents have magnitudes near orbelow the 2-s confidence threshold [Korth et al., 2004].[35] An active HL reconnection region is likely required

tailward of the southern cusp on 15 May 2005 to explain thesunward flow in the 1438–1618 UT period for qavg = 7�.This may have been caused by a draping effect of the IMFover the magnetopause that resulted in a less negative oreven positive IMF Bx in the vicinity of the southern cuspregion. However, an additional factor is necessary to explainthe lack of a southern TPA for a HL reconnection scenario.It may be that the very irregular southern flow structure dueto lower conductance conditions as expected for a sunwarddipole tilt prevented the formation of a stable high-latitudeBirkeland current system and a large-scale southern TPAfeature. The higher conductance of the northern ionospherelikely promoted the intense and well-organized oppositelydirected Birkeland current system on either side of thesunward FC [Freeman et al., 1993; Korth et al., 2004].This suggests that the interhemispheric nonconjugacy oftheta aurora [Østgaard et al., 2003] may be related to acombination of HL reconnection (sign of IMF Bx and afavorable dipole tilt) and a higher conductance backgroundfor the closure of the FAC system via ionospheric Pedersencurrents.[36] Daglis [1997] and Jordanova [2003] found that O+

often dominates the total energy density of the ring currentduring intense magnetic storms and that charge exchange isa predominant ring current energy loss mechanism. Daglis[1997] noted that the O+ lifetime is much shorter than thatof H+. This suggests that the ring current during a greatstorm is strongly affected by changes to the O+ populationand its energy density (gains and losses).[37] An initial strong 63.3 nT/h geomagnetic storm SYM-H

recovery rate occurred during the 0820–1010 UT northwardIMF turning for the great storm on 15 May as expected whenthe southward IMF driver is turned off [e.g., Daglis, 1997].The SYM-H recovery after 1010UTwas directly related to thestrength of the low-latitude region 2 current system asindicated by DMSP F13.[38] The 1015–1720 UT period of weak to nonexistent

region 2 currents during HL reconnection and the polar capreorientation of the joint sunward FC and FAC pair coin-cided with a relatively fast 11.4 nT/h recovery rate. Therestrengthening of the region 2 system that occurred duringthe 1720–1845 UT period as the IMF clock angle rotatedtoward q = �80� was reflected by a clear slowing down ofthe storm SYM-H index recovery to 0.9 nT/h. The 1800 UTSYM-H = �90 nT stabilization and the later intensificationto SYM-H = �110 nT did not appear to be caused by anexternal solar wind speed or density enhancement (seeFigure 4).

[39] O+ outflow is mainly correlated with the auroral oval[e.g., Yau et al., 1984; Andersson et al., 2004] and it occursat all times, from very quiet to geomagnetically very activeperiods [e.g., Yau and Andre, 1997]. This indicates that thetotal O+ content of the magnetosphere and its energy densityduring great storms are less dependent on changes in the O+

outflow source rate than on changes in O+ energization andloss. We propose that the nonexistent region 2 systemreflects a very weak convection electric field of the returnflow in the magnetotail and hence a lower energization rate[e.g., Jordanova, 2003] of the O+ already present in themagnetosphere. This is consistent with a high 11.4 nT/h SYM-Hrecovery rate due to O+ charge exchange losses if the ringcurrent was dominated by O+ during the 15 May 2005great magnetic storm. The slow 0.9 nT/h recovery rateafter 1800 UT was likely caused by the recovery of themagnetotail convection electric field as an O+ energizationmechanism and less so the region 2 system as an O+

source. This was probably caused by the relocation of themagnetopause merging region toward the dayside andprenoon flank magnetopause for dominant IMF By < 0that likely initiated a restrengthening of the region 1/2system and a stronger convection electric field.

7. Summary and Conclusions

[40] Northern Hemisphere DMSP F13 and F15 ion driftand magnetometer observations during the northward IMFstorm recovery phase of the 15 May 2005 magnetic cloudconfirm an IMF clock angle dependent dawnward migrationof a sunward E � B flow channel (Vy > 2.3 km/s) in thedawn-dusk meridian plane that directly reflects the observeddayside drift of large-scale polar auroral arcs as corroboratedon this day by TIMED/GUVI.[41] It is confirmed that the sunward FC is bounded by a

pair of oppositely directed NBZ Birkeland currents inagreement with the Southwood [1987] schematic whichwas originally proposed for flux transfer events and theirionospheric mesoscale signatures.[42] We conclude on the basis of the observed

magnetosheath-like 1 keV ion precipitation at the center ofthe sunward FC, the IMF clock angle dependence and thestrong quasi-steady sunward Vy > 2.3 km/s flow within themigrating FC during these gradual magnetic cloud conditionsthat the joint sunward E � B FC and its FAC system weregenerated at a HL reconnection region that gradually movedacross the magnetopause from a duskside flank location viathe expected tailward of the cusp location and finally to adawnside flank magnetopause region. The sunward j � Bforce of the newly reconnected fields likely corresponds tothe sunward E � B flow observed in the ionosphere [Iijima,2000; Eriksson et al., 2005]. The Southwood [1987] scenariofor flux transfer events is thus shown to apply to quasi-steadyHL reconnection tailward of the cusp and its large-scaleionospheric signatures of a sunward FC and an NBZBirkeland current pair.[43] Essentially the same high-latitude FAC pair reorien-

tation as observed on 15 May 2005 by DMSP F13 and theIridium constellation was predicted by Vennerstrøm et al.[2005] on the basis of an MHD simulation during a 90�clock angle rotation from q = 0� to q = 90�.


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[44] The rate of the geomagnetic storm recovery appearedto be directly related to the strength of the region 2 Birkelandsystem. A faster 11.4 nT/h SYM-H recovery occurred duringa very weak to nonexistent region 2 system. This recoveryrate slowed down significantly to 0.9 nT/h as the region 2system began its recovery from the prior HL reconnectionperiod.

[45] Acknowledgments. We thank Hermann Luhr at Geoforschungs-zentrum Potsdam, Germany for use of CHAMP data. We thank the ACEMAG and SWEPAM instrument teams and the ACE Science Center forproviding the ACE Level 2 data via http://www.srl.caltech.edu/ACE/ASC/level2/. We also thank the CDAWeb at http://cdaweb.gsfc.nasa.gov/ and S.Kokubun at STELAB, Nagoya University, Japan (Geotail MGF), T. Mukaiat ISAS, Japan (Geotail LEP), and A. Lazarus at M.I.T. (IMP-8 Plasma).Work by MH was supported by NSF grant R1WH5700065 at the Univer-sity of Texas at Dallas. Work by SE was supported by NASA grantNNX07AH72G at the University of Colorado at Boulder.[46] Wolfgang Baumjohann thanks James Burch and another reviewer

for their assistance in evaluating this paper.

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��B. J. Anderson, H. Korth, and Y. Zhang, Johns Hopkins University

Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD20723-6099, USA.S. Eriksson, Laboratory for Atmospheric and Space Physics, University

of Colorado, 1234 Innovation Drive, Boulder, CO 80303-7814, USA.([email protected])M. R. Hairston, William B. Hanson Center for Space Sciences,

University of Texas at Dallas, P. O. Box 830688 WT15, Richardson, TX75083-0688, USA.F. J. Rich, Lincoln Laboratory, Massachusetts Institute of Technology,

244 Wood Street, Lexington, MA 02420-9108, USA.


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