Geomagnetic signatures of current wedge produced by fast flowsin a plasma sheet

Jin‐Bin Cao,1 Chunxiao Yan,2,3 Malcolm Dunlop,4 Henri Reme,5 Iannis Dandouras,5

Tielong Zhang,6 Dongmei Yang,7 Alexey Moiseyev,8 Stepan I. Solovyev,8 Z. Q. Wang,2,3

A. Leonoviche,9 N. Zolotukhina,9 and V. Mishin9

Received 10 September 2009; revised 1 February 2010; accepted 24 February 2010; published 7 August 2010.

[1] This paper uses the plasma data from Cluster and TC‐1 and geomagnetic data to studythe geomagnetic signatures of the current wedge produced by fast‐flow braking in theplasma sheet. The three fast flows studied here occurred in a very quiet background andwere accompanied by no (or weak) particle injections, thus avoiding the influences fromother disturbances. All the geomagnetic signatures of a substorm current wedge can befound in the geomagnetic signatures of a current system produced by the braking of fastflows, indicating that the fast flows can produce a complete current wedge which containspostmidnight downward and premidnight upward field‐aligned currents, as well as awestward electrojet. The Pi2 precursors exist not only at high latitudes but also atmidlatitudes. The starting times of midlatitude Pi2 precursors can be identified moreprecisely than those of high‐latitude Pi2 precursors, providing a possible method todetermine the starting time of fast flows in their source regions. The AL drop that a burstybulk flow produces is proportional to its velocity and duration. In three cases, the AL dropsare <100 nT. Because the AE increase of a typical substorm is >200 nT, whether asubstorm can be triggered depends mainly on the conditions of the braking regions beforefast flows. The observations of solar wind before the three fast flows suggest that it isdifficult for the fast flows to trigger a substorm when the interplanetary magnetic field Bz

of solar wind is weakly southward.

Citation: Cao, J.‐B., et al. (2010), Geomagnetic signatures of current wedge produced by fast flows in a plasma sheet,J. Geophys. Res., 115, A08205, doi:10.1029/2009JA014891.

1. Introduction

[2] Near‐Earth reconnection in the Earth’s magnetotailcan produce intermittent, bursty earthward fast flows in theplasma sheet, which are considered to be important phe-nomena closely related to magnetospheric activity andtransport of energy and magnetic flux [Baumjohann et al.,1990; Angelopoulos et al., 1992, 1994; Slavin et al., 1997,2002]. The bursty fast flows in the inner central plasma

sheet (ICPS) are termed bursty bulk flows (BBFs)[Angelopoulos et al., 1992]. BBFs contain many short‐lived(<60 s) high‐velocity (400 km/s or more) flow bursts [Ma etal., 2009] and occur mainly during magnetospheric activeperiods [Angelopoulos et al., 1994; Ma et al., 2006]. Theaverage duration of BBFs is ∼20 min [Cao et al., 2006].[3] When the earthward fast bursty flows in the ICPS

propagate to the Earth, they soon encounter the stronglydipolar field of the inner magnetosphere, where the plasma bbecomes smaller and smaller [Baumjohann, 2002]. The out-ward pressure gradient strongly decelerates and brakes thisflow [Haerendel, 1992; Shiokawa et al., 1997], with some ofit possibly diverted duskward. Computer simulations showthat the braking itself, and in particular the plasma pressuregradients built up by the plasma transported inward with thebursty flows, lead to the generation of a dawnward cross‐tailcurrent, diversion of the originally duskward neutral sheetcurrent through the auroral ionosphere, dipolarization of thefield, and thus the establishment of the substorm currentwedge (SCW) [Birn et al., 1999]. The buildup of the SCW istypically associated with Pi2 pulsations, auroras, and auroralkilometric radiation [Shiokawa et al., 1998; Fairfield et al.,1999; Nakamura et al., 2001; Hsu and McPherron, 2007].Shiokawa et al. [1998] studied the braking effects of flowbursts and found that although the inertial currents contribute

1Space Science Institute, School of Astronautics, Beijing University ofAeronautics and Astronautics, Beijing, China.

2Key Laboratory for Space Weather, Center for Space Science andApplied Research, Beijing, China.

3Also at Graduate University, Chinese Academy of Sciences, Beijing,China.

4Rutherford Appleton Laboratory, Didcot, UK.5Centre d’Etude Spatiale des Rayonnements, Toulouse, France.6Space Research Institute, Austrian Academy of Sciences, Graz, Austria.7Institute of Geophysics, China Earthquake Administration, Beijing,

China.8Institute of Cosmophysical Research and Aeronomy, Siberian Branch,

Russian Academy of Sciences, Yakutsk, Russia.9Institute of Solar‐Terrestrial Physics, Irkutsk, Russia.

Copyright 2010 by the American Geophysical Union.0148‐0227/10/2009JA014891

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A08205, doi:10.1029/2009JA014891, 2010

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to the initial current reduction and diversion, the dominantand more permanent contribution stems from the pressuregradient terms. Until now, however, the geomagnetic sig-natures relating to the electrical current system producedexclusively by the braking of fast flows have rarely beenreported. The reason for this lack is that the fast flows usuallyoccur during magnetospherically active times, and it is dif-ficult to extract those geomagnetic signatures of the electricalcurrent system produced by the braking of fast flows from adisturbed geomagnetic field background.[4] The purpose of the work presented here is to select three

fast flows which occurred in a very quiet background and tostudy quantitatively net geomagnetic signatures associatedwith the electrical current system produced exclusively byfast flows in the ICPS. All the fast‐flow events studied hereoccur during quiet times of the magnetosphere (before fastflows: AL less than −35 nT, Dst greater than −7 nT, Kp =0+ to 2−). As a result, the influences from other disturbancesources (e.g., preexisting substorm current systems) are verysmall and can be ignored.[5] The ground‐based magnetometric data used here

come from 17 stations: 7 stations with data at 1 s timeresolution, 2 stations with data at 10 s time resolution, and8 stations with data at 1 min time resolution. The geographicand geomagnetic coordinates of these 17 stations are givenin Table 1, where the stations are ordered by geographiclongitude. The fast‐flow data and the magnetic field datacome from the Hot‐Ion Analyzers (HIAs) and the FluxgateMagnetomers (FGMs) of Cluster [Reme et al., 2001; Baloghet al., 2001] and TC‐1 [Reme et al., 2005; Carr et al.,2005]. The detailed information about Cluster and TC‐1can be found in Escoubet et al. [2001] and Liu et al. [2005].The fast‐flow data and the magnetic field data are all at atime resolution of 4 s (spin resolution). The satellite positionsand ion flow velocities are given in the GSE coordinatesthroughout this paper.

2. Observations

[6] The three fast‐flow events studied here occurred on22 October 2004 (observed by Cluster), 29 September 2004

(observed by TC‐1) and 7 October 2004 (observed by TC‐1),respectively. We first analyzed the BBFs on 22 October 2004because for this event, more nightside geomagnetic fieldobservations were accessible.

2.1. Observations From 22 October 2004

[7] Figure 1 shows the ionic and magnetic field mea-surements made on Clusters C1 (black) and C3 (green) and onthe electrojet indexes (AE, AU, and AL) during the interval1700–1830 UT on 22 October 2004. The Cluster satellitesobserved two BBFs during the interval 1708 –1750 UT,which are marked by A and B, respectively, in Figure 1b.Before the onset of fast flows, there was almost no eastwardelectrojet (AU <30); the westward electrojet was very weak(AL is greater than −35 nT), and consequently the substormelectrojet index AEwas <70 nT (see Figure 1g). TheKp indexwas 2− and Dst index was −6 nT. The data of energeticparticles on board Los Alamos National Laboratory (LANL)satellites show that the first BBF (BBF A) was accompaniedby a weak energetic injection which started at ∼1712 UT,but there was no energetic particle injection for the secondBBF (BBF B; see http://leadbelly.lanl.gov/lanl_ep_data/).

Table 1. Ground Stations

Ground Station(Code)

Geographic Geomagnetic

ResolutionLatitude(°N)

Longitude(°E)

Latitude(°N)

Longitude(°E)

Manhay (MAB) 50.3 5.7 51.4 89.6 1′Surlari (SUA) 44.6 26.3 42.4 106.7 1′Qsaybeh (QSB) 33.9 35.6 30.3 112.2 1′KaShi (KAS) 39.5 76.0 30.4 151.3 1′Jayuguan (JYG) 39.7 98.0 28.8 169.8 1″Taian (TAA) 36.3 117.1 25.9 192.8 1″Tixie Bay (TIX) 71.6 129.0 65.7 196.9 1″Kanoya (KNY) 31.4 130.9 21.9 198.6 1′Kakioka (KAK) 36.2 140.2 26.2 206.7 1″Memambe‐Tsu

(MMB)43.9 144.2 34.2 209.1 1″

Chokurdakh (CHD) 70.6 147.9 64.7 212.2 1″Honolulu (HON) 21.3 202.0 21.6 267.2 1′Barrow (BRW) 71.3 203.4 69.6 242.0 1′Fort Sim Pson (FSI) 61.8 238.8 67.2 287.3 10″Fresno (FRN) 37.1 240.3 43.5 302.0 1′Newport (NEW) 48.3 242.9 54.9 300.9 1′Fort Smith (FSM) 58.0 246.0 64.7 298.8 10″

Figure 1. (a) The ion flow velocity V, (b) earthward of per-pendicular ion flow velocity Vperpx, (c–e) three components(Bx, By, and Bz) of magnetic field, (f) plasma b recorded bytwo Cluster satellites (C1, black; C3, green), and (g) the AE,AU, and AL indexes during the interval of 1700–1830 UTon 22 October 2004.

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Therefore, the magnetospheric activity was very weakbefore the onset of fast flows, which is also evidenced bythe geomagnetic data at midlatitude stations shown inFigure 2.[8] During the interval 1700–1800 UT on 22 October

2004, Cluster was basically located in the near‐Earth ICPSand C1 moved from GSE coordinates (−11.75, 9.05, 3.88)RE to (−12.09, 9.72, 3.32) RE. The positive value of Bx

indicates that the two satellites were located in the northernICPS. During this interval, two BBFs with large earthwardcomponents (xgse) of flow velocity perpendicular to themagnetic field are evident (see Figure 1b). Both of themare cross‐field flow bursts in the proper ICPS (b > 0.3according to Slavin et al. [2002] and Bamjohann et al.[1990]). The BBFs A and B appeared at 1707:58 and

Figure 2. (a) The x component of perpendicular ion flow velocity Vperpx recorded by two Cluster satel-lites (C1, black; C3, green), (b) unfiltered H (black) and D (blue) components recorded by two nightsidehigh‐latitude stations (TIX and CHD), (c) unfiltered H (black) and D (blue) components recorded byseven nightside midlatitude stations (MAB, SUA, QSB, KAS, JYG, TAA, and KAK) during the interval1700–1830 UT on 22 October 2004. The AL index is plotted in Figure 2b for comparison. The two ver-tical dashed lines in Figure 2a mark the starting times of two fast flows. The two vertical dotted lines inFigure 2b mark the starting times of two Pi2s. The two vertical solid lines in Figure 2c mark the startingtimes of two midlatitude Pi2 precursors (MLPP). In each graph, the stations are ordered from (top) pre-midnight to (bottom) postmidnight.

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1733:00 UT. Their peak velocities were 600 km/s and520 km/s, respectively.[9] Figure 2 shows the x component of the perpendicular

ion flow velocity Vperpx (a), unfiltered H (black) and D(blue) components recorded by nine stations on nightside(TIX, CHD, MAB, SUA, QSB, KAS, JYG, TAA, andKAK) during the interval 1700–1830 UT on 22 October2004. For the purpose of comparison, the AL index is alsoplotted in red in Figure 2b. Before the first flow burst, thegeomagnetic fields at nine stations were very quiet. Partic-ularly at seven midlatitude stations, there was almost nodisturbance in the geomagnetic fields. After 1710 UT,however, the H and D components at the nine stationsdisplay the features similar to those of SCW. The visualcheck shows that there are two negative H bays at the high‐latitude stations TIX and CHD, which are indicated by 1 and2, respectively. The total decrease of the first negative H bayis 31 nT. The start of the first H decrease at TIX occurredroughly at 1709:30 UT, delayed by 92 s with respect to thestart of the first BBF. This time delay is approximately equalto the propagation time of an Alfven wave from each sat-ellite to the Earth if we assume that the Alfven velocity is1000 km/s and the distance between the satellite and theEarth is ∼15.2 RE. The first negative bay is accompanied bytwo Pi2s. The first Pi2 pulsation at TIX and CHD started at1711:04 UT. The second Pi2 pulsation started at 1713:35 UT.According to Kepko et al. [2004], these Pi2s are calledinertial current (IC) Pi2 because they are associated with theICs produced by braking of BBFs. It is worth noting that thefirst Pi2 seems to be more like a pulse than a damped Pi2. Areport of the geomagnetic pulse produced by such flowbursts can also be found in the work of Kepko and Kivelson[1999]. In their paper, the periodic flow bursts directly led toa Pi2 pulsation on the ground. Kepko et al. [2001] show thatlow‐latitude Pi2 pulsations can be directly driven by com-pressional pulses associated with the braking of oscillatoryearthward flows. The periodic flow bursts may originatefrom repeated reconnections in the tail [Sergeev et al.,2007]. In fact, if each flow burst of the periodic sequenceexcites a geomagnetic pulse, the geomagnetic disturbanceproduced by periodic flow bursts will look like a Pi2. Butthis kind of Pi2, which is directly driven by periodic flowbursts, is not the same as the classical damping Pi2 associ-ated with the onset of substorm, and sometimes it canbecome enhanced, depending on the characteristics of flowbursts in the tail. Kim et al. [2007] found that the BBFsdetected close to the midplane of the ICPS have morechance to excite ground Pi2s.[10] Before high‐latitude Pi2s, there is a gradual decrease

in H components, which is termed the Pi2 precursor byKepko et al. [2004]. This high‐latitude Pi2 precursor to thePi2 was related to the motion of the flow burst through themagnetotail. The movement of fast flows in the ICPS couldgenerate ionospheric currents that produce the magneticperturbations.[11] The H component at TIX reached the bottom of the

first negative H bay at 1721 UT. Then it started to increase.At 1733 UT, the second negative H bay emerged due to theappearance of BBF B. The H component decreased againuntil it reached the bottom of the second negative bay at1751 UT. The H decrease of the second negative H bay is∼45 nT.

[12] Figure 2 also shows that the durations of negativehigh‐latitude H bays are much larger than the durations ofBBF. The duration of BBF B is 9 min, whereas the durationof H bay 2 is 50 min. The duration of high‐latitude bays isabout five times that of BBFs. This is because, although theICs contribute to the initial current reduction and diversion,the dominant and more permanent contribution stems fromthe pressure gradient terms [Shiokawa et al., 1998; Birn etal., 1999]. Furthermore, the pressure gradients last longerthan IC effects and can persist even after the flow burst ends[Birn et al., 1999]. Therefore, it can be anticipated that thenegative high‐latitude H bay can last longer than the BBF.Here we do not use BBF A for comparison because H bay 1is not completed.[13] At seven midlatitude stations, two positive bays were

detected in H components. The starting times of the twopositive H bays correspond very well to the start times of thefirst and second negative H bays at high latitudes, and aswas true at the high latitudes, two midlatitude Pi2 pulsationsassociated with two BBFs are clearly seen. In addition, ashort period (∼50 s) Pi2 appears at almost the same time asthe first IC Pi2. This Pi2 is a cavity mode and has beenstudied in detail by Cao et al. [2008]. This cavity mode Pi2is strongest at midlatitudes but can spread to higher andlower latitudes, suggesting that the source of the cavitymode Pi2 does not originate from high latitudes. The cavitymode Pi2 is also a possible ground signature of fast‐flowbursts, although not all fast flows can drive the cavity modePi2s. Previous studies have shown that the braking of theflow burst serves as a source of broadband power that drivesan inner magnetospheric oscillator (e.g., the plasmasphere),and if the compressional signal contains power at the reso-nant frequency of the oscillator (the cavity), cavity modepulsations can be generated [Keiling et al., 2001]. Li et al.[1998] suggested that magnetospheric cavity is a source oflow‐latitude Pi2 pulsations. Lee [1998] also made a theo-retic analysis to show that arbitrary incoming impulses fromthe source in the magnetotail might produce a low‐latitudePi2 inside the plasmasphere. Lee also suggested that theflows in the magnetotail determine the properties of the low‐latitude Pi2 pulsations.[14] For the D components at seven mid‐ to low‐latitude

stations, the situation is a little complicated. At MAB, SUA,and QSB, the D variations displayed two positive bayswhich corresponded to the two positive midlatitude H bays.According to Lester et al. [1984], A midlatitude station isconsidered inside (or outside) the wedge if the H componentbay is positive (or negative), and furthermore, it is to thewest (or east) of the wedge if the D component bay ispositive (or negative). The positive D bays mean that MAB,SUA, and QSB are located westward of the center of thefield‐aligned system. However, at KAS, JAG, TAA, andKAK, there are two negative D bays. The negative D bays atKAS, JAG, TAA, and KAK suggest that the four stationsare located eastward of the center of the field‐aligned cur-rent (FAC) system. Therefore, the center of current wedgeshould be located between QSB (LT = 1943 and MLT =2012 at 1710 UT) and KAS (LT = 2212 and MLT = 2242 at1710 UT), where the midlatitude D variations change sign.At 1710 UT, the Cluster satellite was located at LT = 2132or MLT = 2133. Therefore, the flow‐braking region in spacecorresponds to the center of wedge current on the ground.

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[15] All three Pi2s can be observed in D components atmidlatitudes, but with smaller amplitudes. Figure 3 showsthat the short‐period cavity mode Pi2s are not clear in Dcomponents. This fact is consistent with cavity mode theorybecause generally the compressional component of the fast‐mode waves maps to the H (horizontal, northward) com-ponent on the ground [Takahashi et al., 1999; Han et al.,2004].[16] Figure 2c shows that before midlatitude Pi2s, there

was a gradual increase in the H components and a gradualdecrease in the D components at KAS, JAG, TAA, andKAK. In a similar manner to high‐latitude Pi2 precursor,this gradual H increase and D decrease before Pi2s can betermed the midlatitude Pi2 precursor. The starting time ofmidlatitude Pi2 precursor (marked by the vertical solid linein Figure 2c) is exactly the same as that at high latitudes.This means that Pi2 precursors exist not only at high lati-tudes but also at midlatitudes.[17] It is interesting to find that the durations of increasing

phase of the two midlatitude positive H bays are very closeto the durations of BBFs. Here the increasing phase ofpositive bay is defined as beginning at the starting time of Hincrease and ending at the time of H maximum. In fact, theincreasing phase of the midlatitude positive bays corre-

sponds very well to the decreasing phase of the high‐latitudenegative bay. Nevertheless, because there are more magneticfield disturbances at high latitudes, it is difficult to identifythe starting point of the high‐latitude negative H bay pre-cisely. The increasing phase of positive H bay 2 starts at1733 UT and ends at 1744 UT. Thus, the duration ofincreasing phase of positive H bay 2 is 11 min, very closeto that of BBF B. This observation suggests that the ICdominates during the increasing phase of the H bay. Whenfast flows end, the FAC stops increasing and the midlati-tude H component reaches its maximum. Then the mid-latitude H component starts to recover to its normal level,During the recovery phase, the current of pressure gradientwhich is accumulated in the braking process of fast flowsdominates.[18] Figure 2 also shows that the H components at both

high and midlatitudes are highly correlated with the west-ward electrojet index, AL (see Figures 2b and 2c). The Hcomponents at high latitudes (TIX and CHD) and thewestward electrojet index, AL, started to decrease, reachedthe minimum, and returned to their normal level almost allat the same times. The H components at midlatitudes alsohave also a perfect negative correlation with the westwardelectrojet index AL. These observations indicate that thedecreases of the AL index are related to fast flows. There-fore, it can be concluded that fast flows can produce stronggeomagnetic disturbances and cause a remarkable increaseof the AE index (or a decrease of the AL index). In addition,this good correlation also shows that there is no other largesource of electrical current besides flow braking currentwedge (FBCW) and that the background is considerablyquiet.[19] The above observations show clearly that the ground

signatures of the current system produced by flow brakingare very similar to those of the SCW. Almost all the groundsignatures of the SCW, including: Pi2s, negative H bays athigh latitudes, positive H bays at mid‐ to low latitudes (thestations are inside the FAC system), and positive or negativeD bays at mid‐ to low latitudes (depending positions of thestations relative to the center of FAC system), can be foundin Figure 2. This indicates that the fast flows can produce acomplete current wedge which contains postmidnightdownward FAC, westward electrojet, and premidnightupward FAC.

2.2. Observations From 29 September 2004

[20] The second example occurred on 29 September 2004.Figure 3 shows the ionic and magnetic field parametersrecorded by TC‐1. During the interval 0930–1030 UT, TC‐1was basically located in the near‐Earth ICPS and movedfrom GSE coordinates (−12.61, 3.28, −1.26) RE to (−12.42,2.45, 0.97) RE. During this interval, the LANL 1981–080was on the nightside at ∼2300 LT. The energetic electronflux with energies 50–315 keV recorded at LANL 1981–080shows that there was no obvious particle injection (seehttp://leadbelly.lanl.gov/lanl_ep_data/). Before the fast flow,the AL index was about −10 nT, the Dst index was −5 nT,and the Kp index was ∼1−. Thus, the magnetosphere wasvery quiet before the fast flow. A fast flow with largeearthward x components of flow velocity perpendicular tothe magnetic field was detected. The Bx component andplasma b in Figure 3 indicate that the satellite was crossing

Figure 3. (a) The ion flow velocity V, (b) x component ofperpendicular ion flow velocity Vperpx, (c–e) three compo-nents (Bx, By, and Bz) of magnetic field, (f) plasma b re-corded by TC‐1, and (g) AE , AU, and AL indexesbetween 0930 and 1030 UT on 29 September 2004.

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the current sheet from the northern to the southern ICPSsand that the fast flow was in the proper ICPS. The peakvelocity of this fast flow reached 800 km/s. The earthward xcomponent of perpendicular flow velocity was ∼670 km/s.The starting time of fast flow was 0953:30 UT, and theending time was 1008:00 UT. The geomagnetic data fromtwo nightside high‐latitude stations (FSI and FSM) and fournightside midlatitude stations (KAK, MMB, FRN, andNEW) are shown in Figure 4. Figure 4b shows that there is anegative H bay at the high‐latitude stations. The negative Hbay began at 0951 UT and ended at 1033 UT.

[21] The negative bay was accompanied by a Pi2 pulsa-tion which appeared around 0957 UT. The negative bay athigh latitudes and associated Pi2s are the geomagnetic sig-natures of a FAC system. The negative bay corresponded toan AL decrease of 60 nT. The AU index also displayed somevariation with a magnitude of 30 nT. Correspondinglyduring this event, the AE index increased from 37 to 109 nT.It is worth noting that at FRN and NEW, the midlatitude Pi2precursor (marked by a vertical solid line in Figure 4c)started at 0949 UT, earlier than Pi2 and even fast flows. TheH components (D components) from the midlatitude stationshave positive (negative) bays, indicating that these two

Figure 4. (a) The x component of perpendicular ion flow velocity Vperpx recorded by TC‐1, (b) unfil-tered H (black) and D (blue) components recorded by three nightside high‐latitude stations (BRW,FSI, and FSM), and (c) unfiltered H (black) and D (blue) components recorded by four nightside midlat-itude stations (KAK, MMB, FRN, and NEW) between 0940 and 1040 UT on 29 September 2004. Thevertical solid lines in Figure 4c mark the starting time of the midlatitude Pi2 precursor (MLPP). The ALindex is plotted in Figure 4b for comparison.

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stations are located eastward of the center of FAC systemproduced by braking of the BBFs.

2.3. Observations From 7 October 2004

[22] Figure 5 shows another example of fast flow, whichoccurred on 7 October 2004. During the interval 1230–1330 UT, TC‐1 was located in the near‐Earth ICPS (b >0.3)and moved from GSE coordinates (−11.36, 3.09, 0.84) RE to(−10.87, 2.18, 0.84) RE. The fast flow detected by TC‐1comprised several flow bursts which were separated by2–4 min. Its maximum velocity reached 800 km/s. Themaximum earthward x component of perpendicular velocitywas ∼620 km/s. The starting time of fast flow was 1249 UT ifwe neglect an earlier, small, short‐lived flow burst, and theending time was 1304 UT (see Figure 5b). Thus, the durationof the BBF was ∼15 min. The data from LANL‐97A showsthat this fast flow was not accompanied by obvious particleinjection (see http://leadbelly.lanl.gov/lanl_ep_data/). Beforethe fast flow, the AL index was about −25 nT, the Dst indexwas −6 nT, and the Kp index was about 0+.[23] The geomagnetic data at high and midlatitudes is

plotted in Figure 6. There is a negative bay at the high‐latitude station BRW, which started at 1253 UT. The H

variations at BRW are highly correlated with AL variations.The negative high‐latitude bay started at 1253 UT and endedat 1334 UT. Thus, the duration of H bay was ∼41 min. Atmidlatitude stations, the positive H bays are clearly seen in Hcomponents at KNY, KAK, MMB, and HON. The Pi2sappear in the increasing phase of positive H bay at KNY,KAK, and MMB where 1 s resolution data are available.There is also a midlatitude Pi2 precursor which started at1248 UT. Because the data for this event are limited, it isdifficult for us to give a more detailed analysis of thecharacteristics of the FAC system. However, the above‐mentioned observations of midlatitude positive H and Dbays, Pi2s and high‐latitude negative bay are sufficient toconfirm that the fast flow in the ICPS produced a FACsystem. The AL decrease for this event was ∼92 nT. Cor-respondingly, during this event, the AE index increased from64 to 160 nT.[24] The SCW consisted of earthward (downward) FACs

on the eastern side of the wedge, a westward auroral elec-trojet in the ionosphere and tailward (upward) FACs on thewestern side of the wedge. AL, as an index of the westwardauroral electrojet, to some extent represents the strength ofFACs. The AL indexes that decreased in the above‐mentioned three cases indicate that the braking of fast flowsin the ICPS can cause current disruption in the near‐EarthICPS, resulting in the formation of a FAC system. The ALindex decrease and corresponding nightside high‐latitudenegative bay were strongly dependent on the peak velocityof fast flows. The fast flow on 22 October 2004, which had apeak velocity of ∼600 km/s, caused an AL decrease of∼40 nT, whereas the fast flow on 7 October 2004, whichhas a peak velocity of ∼800 km/s, caused an AL decreaseof ∼92 nT.

3. Discussion and Conclusions

[25] This paper uses the plasma data from satellites andthe geomagnetic stations at mid‐ and high latitudes to studythe geomagnetic signatures produced by the braking of fastflows. All of the three fast flows that we examined occurredin a very quiet background (AL greater than −30 and Dstgreater than −7) and were accompanied by no particle in-jections or weak particle injections, thus avoiding influencesfrom other substorm current systems. The results show thatall the geomagnetic signatures of the SCW can be found inthe geomagnetic signatures of the current system producedby the braking of fast flows. These include the H decrease athigh latitudes, H increases at midlatitudes, and Pi2s (orpulse) at both high and midlatitudes. This indicates that thebraking of fast flows can produce a complete current wedgewhich contains postmidnight downward FAC, westwardelectrojet, and premidnight upward FAC.[26] The Pi2 precursors exist not only at high latitudes but

also in midlatitudes. At high latitudes, the Pi2 precursorstarts at the time that the H component starts to decrease[Kepko et al., 2004]. However, in most cases, it is difficultto apply this criterion to a high‐latitude H componentbecause of variations in disturbances from other sources. Atthe midlatitude, the Pi2 precursor starts at the time when Hand D components deviate (H increases and D decreases).Because there are much fewer disturbances in the midlati-tude geomagnetic field than in the high‐latitude geomag-

Figure 5. (a) The ion flow velocity V, (b) x component ofperpendicular ion flow velocity Vperpx, (c–e) three compo-nents (Bx, By, and Bz) of magnetic field, (f) plasma b recordedby TC‐1, and (g) theAE, AU, andAL indexes during the inter-val of 1230–1330 UT on 7 October 2004.

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netic field, the starting times of midlatitude Pi2 precursorscan be identified more precisely than those of high‐latitudePi2 precursor (see Figures 2, 4, and 6). According to Kepkoet al. [2004], the high‐latitude precursor to Pi2 pulsations isrelated to the currents expected from the passage of a flowburst through the ICPS before the SCW onset and is aresponse to processes that occur before the flow reaches theinner magnetosphere. But the expected currents are toosmall to generate the intense parallel electric fields necessaryfor discrete auroral arc formation. Taking into considerationthe similarity between high‐latitude Pi2 precursors and

midlatitude precursors, we argue that it is possible to use themidlatitude precursor to determine the starting time of fastflows in their source regions.[27] The high correlation between H components at high‐

and midlatitudes and the AL index means that the fast flowitself can cause a drop in the AL index or, in other words, anincrease in the AE index. In the 22 September 2004 case, thefirst BBF with a peak velocity of 600 km/s produced a31 nT drop in the AL index. However, on 7 October 2004,the BBF with a peak velocity of ∼800 km/s caused a 92 nTdrop in the AL index. The AL decrease and corresponding

Figure 6. (a) The x component of perpendicular ion flow velocity Vperpx recorded by TC‐1, (b) unfil-tered H (black) and D (blue) components recorded by the nightside high‐latitude stations (BRW), and(c) unfiltered H (black) and D (blue) components recorded by five nightside midlatitude stations(KNY, KAK, MMB, HON, and SIT) during the interval 1240–1340UT on 7 October 2004. The verticalsolid lines in Figure 6c mark the starting time of the midlatitude Pi2 precursor (MLPP). The AL index isplotted in Figure 6b for comparison.

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nightside high‐latitude negative bay are dependent on thepeak velocity of fast flows. It is noted that on 22 September,the second BBF which appeared at the recovery phase ofthe first negative H bay 25 min later than the first BBF alsocaused a 39 nT drop in the AL index. The two BBFs pro-duced a total drop in the AL index of 50 nT. Therefore, theAL drop that a BBF can produce is proportional to itsvelocity and duration.[28] Although the above‐mentioned observations show

that FBCW is very similar to a typical SCW, there existsome differences between FBCW and SCW. First, theduration of negative H bay at high latitudes of FBCW isusually 1–2 times that of BBF, much shorter than theduration of the negative H bay at high latitudes of SCW.Second, although the flow burst can produce an H decreaseat high latitudes, once it is without new flow bursts, the Hcomponent will return to a preflow level within a timeequivalent to that of the H decrease. Third, besides the long

period TR or IC Pi2, the ground signatures of FBCW mayinclude short‐period cavity mode Pi2s at midlatitudes.However, the SCW cannot produce cavity mode Pi2s at themidlatitude stations.[29] A typical substorm with auroral breakup, particle

injections, and westward auroral electrojet has an AE indexin excess of ∼200 nT. The three cases we presented hereshow that a BBF generally can hardly produce an AEincrease >150 nT. According to Ma et al. [2009], in thegrowth phase of the substorm, only 10% of BBFs have apeak velocity >750 km/s. Even in the expansion phase of thesubstorm, ∼20% of BBFs have a peak velocity >750 km/s.Therefore, the majority of BBFs cannot produce an AEincrease >200 nT themselves. The small AL drop means thatthe FAC flowing into the ionosphere produced by braking offast flows is very small compared to typical SCW currentsand cannot generate the intense parallel electric fields nec-essary for discrete auroral arc formation. Whether a sub-storm can be triggered and whether a current systemproduced by the braking of fast flows can evolve into aSCW depend mainly on the conditions of the current dis-ruption regions (i.e., braking regions) before the onset of thesubstorm expansion phase or the onset of fast flows.[30] Because it is the solar wind condition that mainly

determines the state of the magnetosphere, it is meaningfulto check the solar wind conditions before the three BBFsstudied here. Figure 7 shows the interplanetary magneticfield (IMF) Bz and Vx of solar wind before the three BBFs. Itis interesting to find that for all three BBF events, the solarwind magnetic fields were weakly southward and the IMFBzs were between −3 and +2 nT. The solar wind velocitieswere <400 km/s. This kind of solar wind condition does notallow enough energy accumulation in the magnetosphere forsubstorm development, because when IMF Bz is weaklysouthward (i.e., southward but greater than −5 nT), themagnetosphere is in a state of continuous magnetosphericdissipation, where the dayside and nightside flux transferrates balance each other in a quasi‐continuous manner[Tanskanen et al., 2005].[31] Angelopoulos et al. [1992] reported that the occur-

rence of BBF increases with increasing AE index. Our ob-servations suggest that this might not be because BBFs canbe generated more easily during magnetospherically activetimes, but because they may arise when the BBFs decelerateand brake close to the inner magnetosphere, so that the AEindex can increase to a relatively high level. It is also easierto understand previous reports that very few BBFs occurwhen the AE is <100 nT [Angelopoulos, 1996]. The reasonfor this phenomenon is that the earthward BBF itself caneasily make the AE index >100 nT.

[32] Acknowledgments. This work was supported by the NationalScience Foundation of China grants 40931054 and 40523006, 973 program2006CB806305, 863 program 2008AA12A216, and the SpecializedResearch Fund for State Key Laboratories. The work was partially sup-ported by the Siberian Branch, Russian Academy, project (69) and byRussian Foundation for Basic Research grant 09‐05‐98546. We are gratefulto the World Data Center for Geomagnetism in Kyoto, Institut RoyalMeteorologique de Belgique, Kakioka Magnetic Observatory of JapanMeteorological Agency, and the U.S. Geological Survey (Geomagnetism).We thank I. R. Mann and the Canadian Array for Realtime Investigations ofMagnetic Activity (CARISMA) team for data. CARISMA is operated by theUniversity of Alberta and funded by the Canadian Space Agency. We thank

Figure 7. The interplanetary magnetic field Bz and earth-ward velocity of solar wind at 1 AU for the three bursty bulkflows on 29 September, 7 October, and 22 October 2004.

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the LANL Energetic Particle Team for the data of energetic particles andGoddard Space Flight Center for the data of solar wind on OMNIWEB.[33] Zuyin Pu thanks Wen‐yao Xu and another reviewer for their assis-

tance in evaluating this paper.

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J.‐B. Cao, Beijing University of Aeronautics and Astronautics, Beijing100190, China. (jbcao@cssar.ac.cn)I. Dandouras and H. Reme, Centre d’Etude Spatiale des Rayonnements,

Toulouse F‐31028, France.M. Dunlop, Rutherford Appleton Laboratory, Didcot OX11 0QX, United

Kingdom.A. Leonoviche, V. Mishin, and N. Zolotukhina, Institute of Solar‐

Terrestrial Physics, Irkutsk P/B 4026, Russia.A. Moiseyev and S. I. Solovyev, Institute of Cosmophysical Research

and Aeronomy, Siberian Branch of the Russian Academy of Sciences,Lenin Ave. 31 Yakutsk 677891, Russia.Z. Q. Wang and C. Yan, Key Laboratory for Space Weather, Center for

Space Science and Applied Research, Beijing 100190, China.D. Yang, Institute of Geophysics, China Earthquake Administration, 5,

Minzudaxuenanlu 100081, Beijing, China.T. Zhang, Space Research Institute, Austrian Academy of Sciences, Graz

A‐8042, Austria.

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