Flow burst–induced large-scale plasma sheet oscillation

M. Volwerk,1,2 K.-H. Glassmeier,3 A. Runov,1 R. Nakamura,1 W. Baumjohann,1

B. Klecker,2 I. Richter,3 A. Balogh,4 H. Reme,5 and K. Yumoto6

Received 8 April 2004; revised 11 June 2004; accepted 11 August 2004; published 10 November 2004.

[1] On 12 August 2001 the Cluster spacecraft measured a rapid flux transport event,consisting of a strong perpendicular earthward flow burst combined with adipolarization of the magnetic field after a strong compression of the magnetotail.Combining the Cluster data with those from ground-based magnetometers, we find thatthis event is related to patchy reconnection taking place in the tail. After the eventthe magnetotail is locally evacuated of magnetic field, and an increased plasmapressure takes over from the magnetic pressure. This situation lasts for 15 min, afterwhich a new equilibrium is sought, resulting in an oscillating magnetic field with aperiod of 20 min. The rapid flux transport observed with Bz and vx is shown to be inagreement with Bx variation using Maxwell’s equations. The oscillation period agreeswell with what is theoretically predicted. Our results show how a dampedeigenoscillation of the magnetotail can be initiated by fast flows. INDEX TERMS: 2744

Magnetospheric Physics: Magnetotail; 2740 Magnetospheric Physics: Magnetospheric configuration and

dynamics; 2752 Magnetospheric Physics: MHD waves and instabilities; 2788 Magnetospheric Physics:

Storms and substorms; KEYWORDS: Cluster, magnetotail, ULF waves, plasma sheet oscillations, rapid flux

transport event

Citation: Volwerk, M., K.-H. Glassmeier, A. Runov, R. Nakamura, W. Baumjohann, B. Klecker, I. Richter, A. Balogh,

H. Reme, and K. Yumoto (2004), Flow burst–induced large-scale plasma sheet oscillation, J. Geophys. Res., 109, A11208,

doi:10.1029/2004JA010533.

1. Introduction

[2] The four Cluster spacecraft are in a geopolar orbitaround the Earth, locked in inertial space. This means thatthe apogee of the spacecraft rotates around the Earth in1 year and is located in the Earth’s magnetotail over theperiod of July–October at a radial distance of 19 RE. Theconfiguration of the spacecraft is such that at apogee theycreated a tetrahedron with sides of �1800 km length in2001. This setup was specifically chosen so that one can usethe data from the four spacecraft to discriminate betweenspatial and temporal variations of the magnetic field. Also,it gives the opportunity to determine gradients of themagnetic field.[3] In this paper we discuss Cluster magnetic field

[Balogh et al., 2001] and plasma [Reme et al., 2001]

observations of a dynamic magnetotail. The Clusterspacecraft and others (e.g., Active MagnetosphericParticle Tracer Explorers/IRM and Geotail) have alreadyshown a great diversity of magnetotail dynamics. Sergeevet al. [1998, 2003] have shown that the magnetotail as awhole can exhibit a large vertical-scale motion, calledflapping. Volwerk et al. [2003a] have shown evidence fora kink mode oscillation of the tail current sheet, andZhang et al. [2002] have shown that the current sheet canbe strongly warped.[4] Many of the dynamic processes in the magnetotail

include fast plasma flows [Baumjohann et al., 1990], theso-called bursty bulk flows (BBFs) [Angelopoulos et al.,1994]. These flows are often the driving force behindstrong wave activity in the magnetotail [Bauer et al.,1995a, 1995b; Volwerk et al., 2003b, 2004a, 2004b], thegeneration of Pi2 wave activity [Kepko and Kivelson,1999; Kepko et al., 2001], and the driving of turbulencein the magnetotail [Voros et al., 2003, 2004]. The BBFsare often associated with reconnection [Nagai et al.,1998; Runov et al., 2003] and dipolarization in the tail[Baumjohann et al., 1999; Nakamura et al., 2002a], andthey have a strong association with a thinning of thecurrent sheet [Nakamura et al., 2002b; Asano et al.,2003, 2004].[5] In this paper we discuss the events taking place on

12 August 2001, when the Earth’s magnetosphere wasstrongly compressed by a large increase in solar windpressure. Using a combination of Cluster observations,ACE, Wind, Geotail, and ground-based measurements, we

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, A11208, doi:10.1029/2004JA010533, 2004

1Institut fur Weltraumforschung, Osterreichischen Akademie derWissenschaften, Graz, Austria.

2Max-Planck-Institut fur Extraterrestrische Physik, Garching, Germany.3Institut fur Geophysik und Meteorologie, Technische Universitat,

Braunschweig, Germany.4Department of Space and Atmospheric Physics, Imperial College,

London, UK.5Centre d’Etude Spatiale des Rayonnements/Centre Nationale de la

Recherche Scientifique, Toulouse, France.6Department of Earth and Planetary Sciences, Kyushu University,

Fukuoka, Japan.

Copyright 2004 by the American Geophysical Union.0148-0227/04/2004JA010533$09.00

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Figure 1. (left) Interval 1000–1500 UT. Shown are the density and vx for Wind (shifted by 25 min) andACE (shifted by 45 min); the Bx components of Magnetsrode magnetometer, Bear Island (BJN), andBraunschweig, Kevo (KEV) International Monitor for Auroral Geomagnetic Effects (IMAGE) stations;and the BH components of Kaktovik (KAK) and Eagle (EAG) Geomagnetic Institute magnetometer array(GIMA) stations. There is a clear signature in all ground stations at the arrival of the increased rampressure of the solar wind. (right) Interval 1500–2000 UT. Shown are the density and vx for Wind (shifted15 min); the solar wind pressure and vx for Geotail; the Bx components of Cluster 1, BJN, and KEV; andthe BH components for Tixie (TIK) and Chokhurdakh (CHD). The IMAGE stations show a clear positivebay at 1630 UT and an increase of the electrojet, and the 210 meridian stations show a strong negativebay just before 1700 UT.

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try to analyze the processes taking place in the magnetotail.In this event we find strong magnetotail dynamics, includ-ing strong flux transfer resulting in a magnetic field–evacuated region in the tail, followed by a large-scaleoscillation of the magnetic field of the magnetotail.

2. Timeline

[6] On 12 August 2001 at �1050 UT the solar windobservations by ACE showed a strong increase in solarwind plasma density from �6 to �30 cm�3 and anincrease in velocity vx from �340 to �390 km s�1 andvz from 20 to 60 km s�1. This means an increase in rampressure by a factor >6. ACE was located at approxi-mately (242, �33, 21) RE in GSM coordinates. Thisincreased pressure is transported by the solar wind andshould reach the Earth in �60 min. Indeed, ground-basedmagnetometers show a large jump in the Bx component at�1135 UT. We show the solar wind and ground magne-tometer data in the left column of Figure 1, where wehave shifted the solar wind data in time as to be‘‘observed at the Earth’s magnetopause.’’ The data showthat the response of the ground magnetometers (Magnet-

srode and International Monitor for Auroral GeomagneticEffects (IMAGE)) occurs 45 min after ACE has seen thepressure pulse, indicating a small error in our determina-tion of the arrival time. The Wind spacecraft shows asimilar increase in density and velocity, although not asstrong as in ACE, while located at (53, 216, �100) RE inGSM coordinates. The Wind data are shifted by 25 minas to be observed at the Earth’s magnetopause.[7] A second pressure pulse shows up in the Wind data at

1615 UT, an increase in density N and vx. The Wind data arenow shifted by 15 min. This pressure pulse starts a strongpositive bay in the IMAGE magnetometer stations BearIsland (BJN) and Kevo (KEV) and is a precursor to strongnegative bays in the 210 meridian stations Tixie (TIK) andChokhurdakh (CHD), which are both near the foot points ofthe field lines of the Cluster spacecraft, shown in the rightcolumn of Figure 1. Cluster itself is located in the Earth’smagnetotail at (�17, �7, 6) RE. Also, Geotail, located at(11, 23, �5) RE, shows a gradual almost doubling of thesolar wind pressure around 1600 UT, which is maintaineduntil �1830 UT. With the start of this increase in solar windpressure the IMAGE stations BJN and KEV start a strongincrease in Bx, clearly visible in the right column of Figure 1.

Figure 2. Cluster (a) Bx, (b) Bz, (c) vx, and (d) Np as measured by the flux gate magnetometer andCluster ion spectrometry instruments for the interval 1500–2000 UT and BH components of (e) KAK and(f ) EAG GIMA stations. While the tail magnetic field increases, starting at 1500 UT, the GIMA stationsshow the start of a negative bay in Kaktovik station and a small decrease in Eagle station.

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Interestingly, we find that the strong negative bay at the footpoints of Cluster starts earlier than the strong drop in Bx

measured by Cluster.[8] A strong increase in the lobe magnetic field of the

magnetotail follows, as observed by Cluster. The lobe fieldincreases to �50 nT at 1650 UT, after which it slowlydecreases again. The data are shown in Figure 2. Thecompression of the Earth’s magnetosphere leads to a strongincrease in the eastward auroral electrojet around 1500 UTas can be seen in the IMAGE magnetometer data, shown inFigures 2e and 2f. Stations from the Geophysical Institutemagnetometer array show a different behavior; Eagle stationshows almost no change in BH, whereas Kaktovik stationshows a decrease in BH, leading into a strong negative bayaround 1700 UT.

[9] There are two earthward flow bursts at 1704 and1709 UT, the latter followed by a strong tailward burst at1711 UT, changing back to earthward at 1712 UT.Figures 3g–3i show that the plasma flow perpendicular tothe magnetic field is only earthward (the solid area underthe line). Also, one sees that these flows are mainly limitedto the strong variations in Bx. At 1713 UT the Bx stronglyand quickly decreases to Bx � 0 nT, whereas at the sametime, the Bz component increases to 20 nT and decreasesagain. This structure in Bz is accompanied by a fast flow inthe x direction and has the signature of flux transfer from areconnection event farther down the tail [Runov et al.,2003]. After �15 min, Bx recovers again, albeit to a fieldstrength of <40 nT, only to turn over again and once moredecrease to 0 nT in 20 min, with a quick recovery, after

Figure 3. (a–d) Magnetic field data from Cluster, showing three components and magnitude for theinterval 1700–1815 UT. Plasma data from Cluster show the (e) density and (f ) temperature (paralleltemperature indicated by solid line and perpendicular temperature indicated by solid area under the curve)and (g–i) three components of the velocity. In Figures 3g–3i the perpendicular flow (with respect to themagnetic field) is shown by the solid area under the curves.

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which the spacecraft truly start to enter the neutral sheet (seeFigures 2 and 3).[10] Figures 3e and 3f show that before the event at

1713 UT, there is a short period near 1709 UT when thespacecraft dip into the plasma sheet. This is probably causedby a small reconnection event in the distant tail. One can seethat the parallel and perpendicular temperature of theplasma starts to increase at this point, and strong field-aligned plasma flow starts. During the strong flows thedensity of the plasma decreases by a factor >3 but recoversagain after the strong flow. The strong increase in paralleltemperature is related to the flows.[11] We interpret the event at 1713 UT as a small

localized reconnection event in the tail that does notreconfigure/collapse the whole tail but creates a regionpartially evacuated of magnetic field yet leaving the large-scale structure of the magnetotail intact. In section 4 we willshow that the drop in Bx can be well described by the fluxtransport shown by the Bz and vx components. After the fluxtransport has taken place, part of the lobe is evacuated ofmagnetic field, and the plasma takes over the role ofpressure carrier. This causes an imbalance of the pressureat the lobe–plasma sheet boundary and leads to a rapidexpansion of the plasma sheet.[12] Thus the reconnection heats up the plasma, and it

flows into the evacuated region. One can see that there isstill some perpendicular flow after the magnetic field Bx hasdropped to near zero, i.e., between 1715 and 1716 UT,which fills up the evacuated region with hot plasma.[13] In Figure 4 we show the magnetic and plasma

pressure as measured by Cluster 1 (C1). The pressure alsogives an indication of where the spacecraft are. The total

pressure before 1713 UT is �1.1 nPa, which means that thelobe magnetic field strength, in absence of any plasmapressure, is 53 nT. The total pressure decreases after1700 UT to reach a value around 0.5 nPa.[14] This evacuated lobe region is unstable, and the

magnetotail reacts by starting to oscillate, returning the fieldto almost preevent values, decreasing the magnetic field toBx = 0 again shortly and returning to Bx � 30 nT. After thisthe field decreases, and only small oscillations are present inBx, indicating that the oscillation is strongly damped. Also,the fact that the spacecraft have moved into the current sheetby then makes observing the oscillation more difficult. Thisoscillation of the tail has a period of �20 min. This behavioris different from the ‘‘flapping motion’’ observed bySergeev et al. [2003, 2004] in that the timescales involvedare much longer, e.g., the 15 min of Bx � 0, and theoscillation is apparently limited to one side of the neutralsheet as there are no Bx = 0 crossings. A schematic view ofthe timeline is shown in Figure 5.[15] Starting at 1838 UT, there is a strong flow in the

current sheet, with simultaneous thinning of the currentsheet from �1 RE to �400 km. This event is discussed byNakamura et al. [2002b] and could be related to theoscillation of the tail, possibly triggering another reconnec-tion event, shown by the intensification of the aurora, asshown by Nakamura et al. [2002b]. At apogee the space-craft move in z with vz � 1 RE hr�1 � 1.5 km s�1.

3. A Tilted Magnetotail

[16] The Cluster mission was set up such that spatial andtemporal variations of the magnetic field could be distin-

Figure 4. Magnetic (thin solid line), plasma (dotted line), and total pressure (thick solid line) for theinterval 1500–2000 UT, as measured by Cluster 1.

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guished from each other. The perfect tetrahedral constella-tion at apogee makes it possible to determine the magneticfield gradients within the constellation by linear extrapola-tion of the field [Harvey, 1998]. The smaller the distancebetween the different spacecraft, the better the estimate ofthe gradients. However, at strongly active times the fieldbecomes nonlinear, and the gradient determination breaksdown. Indications of this effect are large values of thenumerically derived divergence of the magnetic field, as isthe case in this event.[17] To obtain some information about the tilting of the

magnetotail, we have performed a minimum variance anal-

ysis [see, e.g., Song and Russell, 1999] on the magneticfield data for three intervals around the sharp drop in Bx.The minimum variance direction is the direction of thenormal to the magnetic field of the tail. We show the resultfor each spacecraft in Table 1. It is clear that the threespacecraft that are at roughly the same zGSM (C1, C2, andC4) show the same minimum variance direction, whereasthe spacecraft much farther south (C3) shows a differentdirection.[18] In the interval just at the start of our event, 1712–

1714 UT, the field is strongly tilted in the xz plane; duringthe event, 1714–1715 UT, the field is strongly tilted in the

Figure 5. A schematic view of the timeline. (a) Reconnection takes place in the far tail, and Cluster islocated in a compressed magnetotail. (b) Cluster observes the flux transfer event. (c) Cluster is located ina magnetic field-evacuated tail region, and magnetic pressure is replaced by plasma pressure. (d) Magneticfield moves back into the tail region, and Cluster observes an oscillating magnetic field.

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yz plane, as one would expect. When the magnetic field–evacuated period is reached, the field is tilted in alldirections. It is not uncommon to have such stronglyvarying field directions, as was shown by Zhang et al.[2002] and Runov et al. [2004].[19] We have determined the velocity of the oscillating

field, using a timing procedure for the first two oscil-lations on low-pass filtered data (t > 2.5 min filter). Thefiltering is done to reduce the effect of false timing bythe high-frequency oscillations. The mean velocity for thefirst two oscillations is shown in Table 2.[20] Clearly, we see that for three of the four intervals

the mean velocity is in the xz plane; only for the rapid returnof the magnetic field to lobe values during 1755:43–1756:21 UT, is there a significant deviation. The maindifference for this interval is that the plasma flow changesfrom earthward (vx > 0) to tailward (vx < 0); all otheroscillations have earthward flow. The perpendicular plasmaflow, v?; x, remains small and earthward during the first halfof this interval.

4. Localized Reconnection

[21] As mentioned in section 2, the event at 1713 LThas the signature of a rapid flux transport event [Schodelet al., 2001] connected to a reconnection event fartherdown the tail. We will now look in detail at this event,zooming in on the interval 1700–1800 UT, as shown inFigure 3. The magnetotail is rather tilted in the yz plane,as shown in section 3, and we can rotate in this planesuch that the direction is in the new z direction (notshown).

4.1. Flux Transport in the Tail

[22] We see that the inclination of the magnetic fieldincreases (Bz gets stronger, and Bx gets weaker) and thendecreases again (Bz gets weaker), and over that sameinterval, there is strong earthward plasma flow. This isreminiscent of reconnection [see, e.g., Runov et al., 2003].Figure 3 shows that the largest component of the flow isfield aligned, and only during the strong decrease of Bx, isthere significant perpendicular flow (the solid region belowthe curves), which is the flow that transports magnetic flux.Using Maxwell’s equations and frozen-in condition, we canwrite

@B

@t¼ �rrrr ^ E; ð1Þ

E ¼ �v ^ B; ð2Þ

@B

@t¼ r ^ v ^ Bð Þ: ð3Þ

Assuming an invariant direction in y, we can write for thetime variation of Bx

@Bx

@t¼ @

@zvxBz � vzBxð Þ: ð4Þ

With this starting point we can model the behavior of Bx,assuming a flux transport/reconnection event, where we usea simplified model for the observed Bz and v variation:

Bz tð Þ ¼ Bz;0 exp � t2

2s2

� �ð5Þ

v tð Þ ¼ v0 exp � t2

2s2

� �; ð6Þ

where both the magnetic field and the flow are described bythe same Gaussian variation in time. Substituting inequation (4),

ZdBx ¼

@

@z

Zv0Bz;0 cosa� v0Bx sina� �

exp � t2

s2

� �dt; ð7Þ

where a is the angle the velocity makes with the x axis. Tosolve for Bx, one needs values for the quantities in equation(7). Most we can simply read off the data from Cluster; onlythe vertical scale of the whole magnetotail L (for @/@z)cannot be directly measured and needs to be assumed: L =50,000 km. A justification for this assumption can be foundin the location of Cluster at the start of the event, as given insection 2. In GSM coordinates the spacecraft are located atzGSM � 6 RE � 38,000 km. The dipole tilt will, in this case,reduce the distance to the neutral sheet, but we need notassume that Cluster is located right at the boundary of theevacuated region. We use the following measured values:Bz,0 = 30 nT, Bx = 40 nT, v0 = 500 km s�1, a = 15�, DT =3 min, and s � 0.5 min. A simple estimate of DBx can befound, assuming that the error function contributes a factorof order O(1), and with the values given above one finds

DBx / L�1 v0Bz;0 cosa� v0Bx sina� �

DT � 33 nT: ð8Þ

Indeed, this simple estimate shows that with the observedsignatures in Bz and v it is possible to ‘‘deplete’’ Bx from itsstarting value at �40 nT.[23] However, with the four Cluster spacecraft, spatial

gradients in the magnetic field and the velocity can be

Table 1. Minimum Variance Directionsa

Spacecraft 1712–1714 UT 1714–1715 UT 1715–1717 UT

C1 (�0.9, 0.1, �0.4) (0.1, 0.7, �0.8) (0.7, 0.6, �0.5)C2 (�0.7, �0.3, �0.7) (0.1, �0.6, 0.8) (0.7, 0.4, �0.5)C3 (0.0, 0.7, �0.7) (0.0, 0.9, �0.4) (0.8, 0.6, 0.1)C4 (�0.9, 0.1, �0.5) (0.0, �0.6, 0.8) (0.3, 0.8, �0.4)

aC1, C2, C3, and C4 are Clusters 1, 2, 3, and 4, respectively.

Table 2. Mean Velocities of the Magnetotail Oscillation for the

First Two Oscillationsa

Interval, UT vx vy vz

1730:34–1733:20 �9 �2 �171749:29–1752:17 �11 6 171755:43–1756:21 15 �30 �301801:17–1804:39 8 �3 10

aVelocities are given in km s�1.

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determined. As already mentioned in section 3, the lineargradient estimator [Harvey, 1998] is not accurate in thisevent. Therefore we use two spacecraft (C3 and C4),separated well in z and only slightly in x, to determine thegradients in Bx, Bz, vx, and vz with respect to z. These can bedetermined for each data point, where we have used 4 sresolution magnetic field data and 8 s resolution plasmadata. The plasma data were interpolated to have the sameresolution as the magnetic field data. Expanding the right-hand side of equation (4),

@Bx

@t¼ Bz

@vx@z

þ vx@Bz

@z� Bx

@vz@z

� vz@Bx

@z; ð9Þ

we can substitute the observed quantities in equation (9),where we have used the average value of the magnetic fieldstrength and velocity measured by C3 and C4 for thenonderivative terms. The time derivative of the magneticfield, the left-hand side of equation (9), @Bx/@t, can theneasily be integrated over time to find DBx � �41 nT,agreeing well with what we observed.

4.2. Eigenoscillation of the Tail

[24] Eigenoscillations of the plasma sheet have beendiscussed in various papers [see, e.g., Roberts, 1981a,1981b; Lee et al., 1988; Seboldt, 1990; Smith et al., 1997;Louarn et al., 2004; Fruit et al., 2004]. For the casediscussed in section 4.1 we apply the results obtained bySeboldt [1990], who calculated the low-frequency wavemodes in the Earth’s plasma sheet. Using the basic MHDequations with a polytropic pressure law for the plasma andassuming an invariant direction (in this case the y direction),Seboldt [1990] calculates the spectrum of a generalizedeigenvalue problem, among others, for a two-dimensionalperturbation, kx 6¼ 0 and ky = 0, where k is the wave vectorof the perturbation. For a polytropic index g = 5/3 severalsolutions w2 / nA

2 for the eigenmodes are found, where nA2 =

v02/L0

2 is the square of the typical Alfven frequency, with v02 =

B02/m0r. The frequencies f1 � f4 (see Table 3) are given for

typical measured values of the current sheet: B0 � 5 nT, r �1 cm�3, and L0 � 50,000 km (see also section 4.1), with theAlfven frequency nA � 3 � 10�3 Hz. The first foureigenfrequencies (two symmetric and two antisymmetric)are shown in Table 3 in units of nA and mHz. A symmetricmode is characterized by a mirror symmetry in the neutral

sheet of the oscillating field at both sides of the neutralsheet, e.g., a sausage mode oscillation. In this mode, there isa periodic exchange between magnetic and plasma pressure.An antisymmetric mode is characterized by velocities of theoscillation in the same direction at both sides of the neutralsheet, e.g., a kink mode oscillation. In this mode themagnetic and plasma pressure remain constant, but themagnetic tension varies periodically.[25] From the symmetry of our event, a magnetic field–

evacuated region in the tail, we expect that the magnetotailwill oscillate in a symmetric mode. First, the pressure ofthe plasma sheet is balancing the pressure of the magneticfield in the lobes, both north and south of the neutral sheet,and then gives way to the magnetic pressure. Indeed,Figure 4 shows that the plasma and magnetic pressure areoscillating in antiphase, indicative of a sausage-likesymmetric mode. The frequencies listed in Table 3 showthat the period for the oscillation that we observed insection 2, Tosc � 20 min ! f0 � 0.8 mHz, is close to thefrequency of the first harmonic f1 of the symmetric mode.Indeed, when we assume that equations (8) and (9) insection 4.1 should show the same DBx, our assumed L(being the only free parameter in our problem) should besmaller by a factor of �1.24, which would change the firstharmonic frequency to f1 � 0.8 mHz.[26] In section 3 we have determined the velocity of the

magnetic field over the Cluster spacecraft for four intervals(see Table 2). Three of the intervals show velocities inwhich vy is the minor component, which we expect as weapply Seboldt’s [1990] solution for kx 6¼ 0 and ky = 0.

5. Discussion

[27] In this paper we have discussed the rapid fluxtransport event measured by Cluster, set on by a reconnec-tion event farther down the tail. We have shown that thesignatures of the flow vx and the magnetic field Bz are inagreement with flux transport calculated with Maxwell’sequations and with the drop in Bx resulting from it. After theflux transfer event, Cluster is located in a magnetic field–evacuated region of the magnetotail, where the surroundingmagnetic field is held off by the large plasma pressure. Thistransient situation of the tail, in which the plasma pressurekeeps off the magnetic field of the lobe, is maintained for15 min, after which the magnetic field returns to theevacuated region and tries to establish a new stable config-uration, which results in a damped oscillating motion of themagnetic field. The period of this oscillating motion fitswell with the periods obtained in theory by Seboldt [1990].[28] We can now put together the observations which lead

to the following model for this event of patchy reconnectionin the magnetotail. There is an onset of reconnection in thedistant tail just before 1700 UT, which is registered by the210 meridian stations TIK and CHD. There is no signatureobserved in the Cluster ion spectrometry (CIS) data. Then at1704 UT, CIS data show a strong, field-aligned earthwardflow (see vx in Figure 3). At 1709 UT, there is again strongfield-aligned earthward flow, with a signature in Bx mea-sured by Cluster, the flow reverses to tailward at 1711 UT,and the Bx recovers again until strong earthward field-aligned and perpendicular flows start at 1713 UT, andCluster shows the rapid flux transfer event.

Table 3. First Four Eigenfrequencies for the Current Sheet as

Determined by Seboldt [1990]a

Eigenfrequencies Value

Symmetricw1, nA

ffiffiffiffiffiffiffiffiffi1:22

p

f1, mHz 0.5w3, nA

ffiffiffiffiffiffiffiffiffiffiffi10:45

p

f3, mHz 1.4Antisymmetric

w2, nAffiffiffiffiffiffiffiffiffi5:00

p

f2, mHz 1.0w4, nA

ffiffiffiffiffiffiffiffiffiffiffi17:75

p

f4, mHz 1.9aEigenfrequencies are given in units of the Alfven frequency nA

2 = v02/L0

2

and in mHz using the observed quantities B0 � 5 nT, r � 1 cm�3, and anestimated vertical scale of the magnetotail L0 � 50,000 km.

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[29] During this event, the Cluster spacecraft remain inthe northern lobe of the magnetotail. Reconnection isassumed to start in the center of the neutral sheet, and fromthe X point or the diffusion region, there is an outflowregion, where plasma is accelerated along the magnetic fieldlines. Nagai et al. [1998] studied the dynamics of substormonsets, with their associated plasma flows, using Geotaildata. They summarize the magnetic field and plasma flowstructure in their Figure 12. Not only is there strongperpendicular plasma flow (together with magnetic fluxtransport) near the magnetic X line, where reconnectionoccurs, but ions are also accelerated away from the X lineregion along the magnetic field lines. We now propose,from our observations, that a large outflow region existsnear the X line with field-aligned ion flows onto field linesnot yet involved in the reconnection process.

6. Conclusions

[30] This event has shown us important evidence that fastflows, related to reconnection farther down the tail, canresult in a large-scale, damped oscillation of the plasmasheet. Importantly, a significant part of the flow is perpen-dicular to the magnetic field, as this constitutes flux trans-port in the tail.[31] There seems to be patchy reconnection in the tail,

evidenced by the flow bursts measured by Cluster, relatedto the observed rapid flux transport. Evidence of flowbursts related to the substorm onset measured by groundstations is not present. We attribute this lack of evidenceto Cluster being on field lines too far away from thereconnection region. There has to be a sizable ionoutflow region around the reconnection site, which cre-ates the magnetic field-aligned ion flow bursts measuredby Cluster with no signature of flux transport present inthe data.

[32] Acknowledgments. We would like to thank H.-U. Eichelbergerfor preparing the Cluster MAG data. We would like to acknowledgeCDAweb (http://cdaweb.gsfc.nasa.gov) for providing the publicly availabledata from ACE, Wind, and Geotail. Furthermore, we would like to thankJ. Olson at the UAF Geophysical Institute for making the GIMA magne-tometer data available on the web. We thank the institutes who maintain theIMAGE magnetometer array and the Finnish Meteorological Institute forproviding the data. The work by M.V. and K.H.G./I.R. was financiallysupported by the German Bundesministerium fur Bildung und Forschungand the Zentrum fur Luft- und Raumfahrt under contracts 50 OC 0104 and50 OC 0103, respectively.[33] Lou-Chuang Lee thanks Rudolf Treumann and the other reviewer

for their assistance in evaluating this paper.

ReferencesAngelopoulos, V., C. F. Kennel, F. V. Coroniti, R. Pellat, M. G. Kivelson,R. J. Walker, C. T. Russell, W. Baumjohann, W. C. Feldman, and J. T.Gosling (1994), Statistical characteristics of bursty bulk flow events,J. Geophys. Res., 99, 21,257–21,280.

Asano, Y., T. Mukai, M. Hoshino, Y. Saito, H. Hayakawa, and T. Nagai(2003), Evolution of the thin current sheet in a substorm observed byGeotail, J. Geophys. Res., 108(A5), 1189, doi:10.1029/2002JA009785.

Asano, Y., T. Mukai, M. Hoshino, Y. Saito, H. Hayakawa, and T. Nagai(2004), Current sheet structure around the near-Earth neutral lineobserved by Geotail, J. Geophys. Res., 109, A02212, doi:10.1029/2003JA010114.

Balogh, A., et al. (2001), The Cluster magnetic field investigation: Over-view of in-flight performance and initial results, Ann. Geophys., 19,1207–1217.

Bauer, T. M., W. Baumjohann, R. A. Treumann, N. Sckopke, and H. Luhr(1995a), Low-frequency waves in the near-Earth plasma sheet, J. Geo-phys. Res., 100, 9605–9617.

Bauer, T. M., W. Baumjohann, and R. A. Treumann (1995b), Neutral sheetoscillations at substorm onset, J. Geophys. Res., 100, 23,737–23,743.

Baumjohann, W., G. Paschmann, and H. Luhr (1990), Characteristics ofhigh-speed flows in the plasma sheet, J. Geophys. Res., 95, 3801–3809.

Baumjohann, W., M. Hesse, S. Kokubun, T. Mukai, T. Nagai, and A. A.Petrukovich (1999), Substorm dipolarization and recovery, J. Geophys.Res., 104, 24,995–25,000.

Fruit, G., P. Louarn, E. Budnik, J. A. Sauvaud, C. Jacquey, D. Le Queau,H. Reme, E. Lucek, A. Balogh, and N. Cornilleau-Wehrlin (2004), On thepropagation of low-frequency fluctuations in the plasma sheet: 2. Char-acterization of the MHD eigenmodes and physical implications, J. Geo-phys. Res., 109, A03217, doi:10.1029/2003JA010229.

Harvey, C. C. (1998), Spatial gradients and the volumetric tensor, inAnalysis Methods for Multi-Spacecraft Data, edited by G. Paschmannand P. Daly, ISSI Sci. Rep. SR-001, pp. 307–322, Int. Space Sci. Inst.,Bern, Switzerland.

Kepko, L., and M. Kivelson (1999), Generation of Pi2 pulsations by burstybulk flows, J. Geophys. Res., 104, 25,021–25,034.

Kepko, L., M. G. Kivelson, and K. Yumoto (2001), Flow bursts, braking,and Pi2 pulsations, J. Geophys. Res., 106, 1903–1915.

Lee, L. C., S. Wang, C. Q. Wei, and B. T. Tsurutani (1988), Streamingsausage, kink and tearing instabilities in a current sheet with applicationsto the Earth’s magnetotail, J. Geophys. Res., 93, 7354–7365.

Louarn, P., G. Fruit, E. Budnik, J. A. Sauvaud, C. Jacquey, D. LeQueau,H. Reme, E. Lucek, and A. Balogh (2004), On the propagation of low-frequency fluctuations in the plasma sheet: 1. Cluster observations andmagnetohydrodynamic analysis, J. Geophys. Res., 109, A03216,doi:10.1029/2003JA010228.

Nagai, T., et al. (1998), Structure and dynamics of magnetic reconnectionfor substorm onsets with Geotail observations, J. Geophys. Res., 103,4419–4440.

Nakamura, R., et al. (2002a), Motion of the dipolarization front during aflow burst event observed by Cluster, Geophys. Res. Lett., 29(20), 1942,doi:10.1029/2002GL015763.

Nakamura, R., et al. (2002b), Fast flow during current sheet thinning,Geophys. Res. Lett., 29(23), 2140, doi:10.1029/2002GL016200.

Reme, H., et al. (2001), First multi-spacecraft ion measurements in and nearthe Earth’s magnetosphere with the identical Cluster ion spectrometry(CIS) experiment, Ann. Geophys., 19, 1303–1354.

Roberts, B. (1981a), Wave propagation in a magnetically structured atmo-sphere I: Surface waves at a magnetic interface, Sol. Phys., 69, 27–38.

Roberts, B. (1981b), Wave propagation in a magnetically structured atmo-sphere II: Waves in a magnetic slab, Sol. Phys., 69, 39–56.

Runov, A., et al. (2003), Current sheet structure near magnetic X-lineobserved by Cluster, Geophys. Res. Lett., 30(11), 1579, doi:10.1029/2002GL016730.

Runov, A., V. A. Sergeev, R. Nakamura, W. Baumjohann, T. L. Zhang,Y. Asano, M. Volwerk, Z. Voros, A. Balogh, and H. Reme (2004),Reconstruction of the magnetotail current sheet structure using multi-point Cluster measurements, Planet. Space Sci, in press.

Schodel, R., W. Baumjohann, R. Nakamura, V. A. Sergeev, and T. Mukai(2001), Rapid flux transport in the central plasma sheet, J. Geophys. Res.,106, 301–313.

Seboldt, W. (1990), Nonlocal analysis of low-frequency waves in theplasma sheet, J. Geophys. Res., 95, 10,471–10,479.

Sergeev, V., V. Angelopoulos, C. Carlson, and P. Sutcliffe (1998), Currentsheet measurements within a flapping plasma sheet, J. Geophys. Res.,103, 9177–9187.

Sergeev, V., et al. (2003), Current sheet flapping motion and structureobserved by Cluster, Geophys. Res. Lett., 30(6), 1327, doi:10.1029/2002GL016500.

Sergeev, V., A. Runov, W. Baumjohann, R. Nakamura, T. L. Zhang,A. Balogh, P. Louarn, J.-A. Sauvaud, and H. Reme (2004), Orientationand propagation of current sheet oscillations, Geophys. Res. Lett., 31,L05807, doi:10.1029/2003GL019346.

Smith, J. M., B. Roberts, and R. Oliver (1997), Magnetoacoustic wavepropagation in current sheets, Astron. Astrophys., 327, 377–387.

Song, P., and C. T. Russell (1999), Time series data analysis in spacephysics, Space Sci. Rev., 87, 387–463.

Volwerk, M., K.-H. Glassmeier, A. Runov, W. Baumjohann, R. Nakamura,T. L. Zhang, B. Klecker, A. Balogh, and H. Reme (2003a), Kink modeoscillation of the current sheet, Geophys. Res. Lett., 30(6), 1320,doi:10.1029/2002GL016467.

Volwerk, M., et al. (2003b), A statistical study of compressional waves inthe tail current sheet, J. Geophys. Res., 108(A12), 1429, doi:10.1029/2003JA010155.

Volwerk, M., et al. (2004a), Compressional waves in the Earth’s neutralsheet, Ann. Geophys., 22, 303–315.

Volwerk, M., et al. (2004b), Multi-scale analysis of turbulence in the Earth’scurrent sheet, Ann. Geophys., 22, 2525–2533.

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9 of 10

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Voros, Z., et al. (2003), Multi-scale magnetic field intermittence in theplasma sheet, Ann. Geophys., 21, 1955–1964.

Voros, Z., W. Baumjohann, R. Nakamura, A. Runov, M. Volwerk, and T. L.Zhang (2004), Wavelet analysis of magnetic turbulence in the Earth’splasma sheet, Phys. Plasmas, 11, 1333–1338.

Zhang, T. L., W. Baumjohann, R. Nakamura, A. Balogh, and K.-H.Glassmeier (2002), A wavy twisted neutral sheet observed by Cluster,Geophys. Res. Lett., 29(19), 1899, doi:10.1029/2002GL015544.

�����������������������A. Balogh, Blackett Laboratory, Department of Space and Atmospheric

Physics, Imperial College, Prince Consort Road, London SW7 2BZ, UK.

W. Baumjohann, R. Nakamura, A. Runov, and M. Volwerk, Institut furWeltraumforschung der OAW, Schmiedlstr. 6, 8042 Graz, Austria.(martin.volwerk@oeaw.ac.at)K.-H. Glassmeier and I. Richter, Institut fur Geophysik und Meteo-

rologie, Technische Universitat, Mendelssohnstr. 3, BraunschweigD-38106, Germany.B. Klecker, Max-Planck-Institut fur Extraterrestrische Physik, Karl-

Schwarzschild Str. 1, Postfach 1312, D-85741 Garching, Germany.H. Reme, Centre d’Etude Spatiale des Rayonnements, CNRS, 9 Avenue

Colonel-Roche, Toulouse F-31028, France.K. Yumoto, Department of Earth and Planetary Sciences, Kyushu

University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan.

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