Flow burst induced large-scale plasma sheet oscillation

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2004ja010533, 2004 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. Rème, 5 and K. Yumoto 6 Received 8 April 2004; revised 11 June 2004; accepted 11 August 2004; published 10 November [1] On 12 August 2001 the Cluster spacecraft measured a rapid flux transport event, consisting of a strong perpendicular earthward flow burst combined with a dipolarization of the magnetic field after a strong compression of the magnetotail. Combining the Cluster data with those from ground-based magnetometers, we find that this event is related to patchy reconnection taking place in the tail. After the event the magnetotail is locally evacuated of magnetic field, and an increased plasma pressure takes over from the magnetic pressure. This situation lasts for 15 min, after which a new equilibrium is sought, resulting in an oscillating magnetic field with a period of 20 min. The rapid flux transport observed with B z and v x is shown to be in agreement with B x variation using Maxwell s equations. The oscillation period agrees well with what is theoretically predicted. Our results show how a damped eigenoscillation 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. Rème, and K. Yumoto (2004), Flow burst induced large-scale plasma sheet oscillation, J. Geophys. Res., 109,, doi: /2004ja Introduction [2] The four Cluster spacecraft are in a geopolar orbit around the Earth, locked in inertial space. This means that the apogee of the spacecraft rotates around the Earth in 1 year and is located in the Earth s magnetotail over the period of July October at a radial distance of 19 R E. The configuration of the spacecraft is such that at apogee they created a tetrahedron with sides of 1800 km length in This setup was specifically chosen so that one can use the data from the four spacecraft to discriminate between spatial and temporal variations of the magnetic field. Also, it gives the opportunity to determine gradients of the magnetic field. [3] In this paper we discuss Cluster magnetic field [Balogh et al., 2001] and plasma [Rème et al., 2001] 1 Institut für Weltraumforschung, Österreichischen Akademie der Wissenschaften, Graz, Austria. 2 Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany. 3 Institut für Geophysik und Meteorologie, Technische Universität, Braunschweig, Germany. 4 Department of Space and Atmospheric Physics, Imperial College, London, UK. 5 Centre d Etude Spatiale des Rayonnements/Centre Nationale de la Recherche Scientifique, Toulouse, France. 6 Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan. Copyright 2004 by the American Geophysical Union /04/2004JA010533$09.00 observations of a dynamic magnetotail. The Cluster spacecraft and others (e.g., Active Magnetospheric Particle Tracer Explorers/IRM and Geotail) have already shown a great diversity of magnetotail dynamics. Sergeev et al. [1998, 2003] have shown that the magnetotail as a whole can exhibit a large vertical-scale motion, called flapping. Volwerk et al. [2003a] have shown evidence for a kink mode oscillation of the tail current sheet, and Zhang et al. [2002] have shown that the current sheet can be strongly warped. [4] Many of the dynamic processes in the magnetotail include fast plasma flows [Baumjohann et al., 1990], the so-called bursty bulk flows (BBFs) [Angelopoulos et al., 1994]. These flows are often the driving force behind strong wave activity in the magnetotail [Bauer et al., 1995a, 1995b; Volwerk et al., 2003b, 2004a, 2004b], the generation of Pi2 wave activity [Kepko and Kivelson, 1999; Kepko et al., 2001], and the driving of turbulence in the magnetotail [Vörös et al., 2003, 2004]. The BBFs are 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], and they have a strong association with a thinning of the current 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 was strongly compressed by a large increase in solar wind pressure. Using a combination of Cluster observations, ACE, Wind, Geotail, and ground-based measurements, we 1of10

2 Figure 1. (left) Interval UT. Shown are the density and v x for Wind (shifted by 25 min) and ACE (shifted by 45 min); the B x components of Magnetsrode magnetometer, Bear Island (BJN), and Braunschweig, Kevo (KEV) International Monitor for Auroral Geomagnetic Effects (IMAGE) stations; and the B H 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 ram pressure of the solar wind. (right) Interval UT. Shown are the density and v x for Wind (shifted 15 min); the solar wind pressure and v x for Geotail; the B x components of Cluster 1, BJN, and KEV; and the B H components for Tixie (TIK) and Chokhurdakh (CHD). The IMAGE stations show a clear positive bay at 1630 UT and an increase of the electrojet, and the 210 meridian stations show a strong negative bay just before 1700 UT. 2of10

3 Figure 2. Cluster (a) B x,(b)b z,(c)v x, and (d) N p as measured by the flux gate magnetometer and Cluster ion spectrometry instruments for the interval UT and B H components of (e) KAK and (f ) EAG GIMA stations. While the tail magnetic field increases, starting at 1500 UT, the GIMA stations show the start of a negative bay in Kaktovik station and a small decrease in Eagle station. try to analyze the processes taking place in the magnetotail. In this event we find strong magnetotail dynamics, including strong flux transfer resulting in a magnetic field evacuated region in the tail, followed by a large-scale oscillation of the magnetic field of the magnetotail. 2. Timeline [6] On 12 August 2001 at 1050 UT the solar wind observations by ACE showed a strong increase in solar wind plasma density from 6 to 30 cm 3 and an increase in velocity v x from 340 to 390 km s 1 and v z from20to60kms 1. This means an increase in ram pressure by a factor >6. ACE was located at approximately (242, 33, 21) R E in GSM coordinates. This increased pressure is transported by the solar wind and should reach the Earth in 60 min. Indeed, ground-based magnetometers show a large jump in the B x component at 1135 UT. We show the solar wind and ground magnetometer data in the left column of Figure 1, where we have shifted the solar wind data in time as to be observed at the Earth s magnetopause. The data show that the response of the ground magnetometers (Magnetsrode and International Monitor for Auroral Geomagnetic Effects (IMAGE)) occurs 45 min after ACE has seen the pressure pulse, indicating a small error in our determination of the arrival time. The Wind spacecraft shows a similar increase in density and velocity, although not as strong as in ACE, while located at (53, 216, 100) R E in GSM coordinates. The Wind data are shifted by 25 min as 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 v x. The Wind data are now shifted by 15 min. This pressure pulse starts a strong positive bay in the IMAGE magnetometer stations Bear Island (BJN) and Kevo (KEV) and is a precursor to strong negative bays in the 210 meridian stations Tixie (TIK) and Chokhurdakh (CHD), which are both near the foot points of the field lines of the Cluster spacecraft, shown in the right column of Figure 1. Cluster itself is located in the Earth s magnetotail at ( 17, 7, 6) R E. Also, Geotail, located at (11, 23, 5) R E, shows a gradual almost doubling of the solar wind pressure around 1600 UT, which is maintained until 1830 UT. With the start of this increase in solar wind pressure the IMAGE stations BJN and KEV start a strong increase in B x, clearly visible in the right column of Figure 1. 3of10

4 Figure 3. (a d) Magnetic field data from Cluster, showing three components and magnitude for the interval UT. Plasma data from Cluster show the (e) density and (f ) temperature (parallel temperature 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 the magnetic field) is shown by the solid area under the curves. Interestingly, we find that the strong negative bay at the foot points of Cluster starts earlier than the strong drop in B x measured by Cluster. [8] A strong increase in the lobe magnetic field of the magnetotail follows, as observed by Cluster. The lobe field increases to 50 nt at 1650 UT, after which it slowly decreases again. The data are shown in Figure 2. The compression of the Earth s magnetosphere leads to a strong increase in the eastward auroral electrojet around 1500 UT as can be seen in the IMAGE magnetometer data, shown in Figures 2e and 2f. Stations from the Geophysical Institute magnetometer array show a different behavior; Eagle station shows almost no change in B H, whereas Kaktovik station shows a decrease in B H, leading into a strong negative bay around 1700 UT. [9] There are two earthward flow bursts at 1704 and 1709 UT, the latter followed by a strong tailward burst at 1711 UT, changing back to earthward at 1712 UT. Figures 3g 3i show that the plasma flow perpendicular to the magnetic field is only earthward (the solid area under the line). Also, one sees that these flows are mainly limited to the strong variations in B x. At 1713 UT the B x strongly and quickly decreases to B x 0 nt, whereas at the same time, the B z component increases to 20 nt and decreases again. This structure in B z is accompanied by a fast flow in the x direction and has the signature of flux transfer from a reconnection event farther down the tail [Runov et al., 2003]. After 15 min, B x recovers again, albeit to a field strength of <40 nt, only to turn over again and once more decrease to 0 nt in 20 min, with a quick recovery, after 4of10

5 Figure 4. Magnetic (thin solid line), plasma (dotted line), and total pressure (thick solid line) for the interval UT, as measured by Cluster 1. which the spacecraft truly start to enter the neutral sheet (see Figures 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 the spacecraft dip into the plasma sheet. This is probably caused by a small reconnection event in the distant tail. One can see that the parallel and perpendicular temperature of the plasma starts to increase at this point, and strong fieldaligned plasma flow starts. During the strong flows the density of the plasma decreases by a factor >3 but recovers again after the strong flow. The strong increase in parallel temperature is related to the flows. [11] We interpret the event at 1713 UT as a small localized reconnection event in the tail that does not reconfigure/collapse the whole tail but creates a region partially evacuated of magnetic field yet leaving the largescale structure of the magnetotail intact. In section 4 we will show that the drop in B x can be well described by the flux transport shown by the B z and v x components. After the flux transport has taken place, part of the lobe is evacuated of magnetic field, and the plasma takes over the role of pressure carrier. This causes an imbalance of the pressure at the lobe plasma sheet boundary and leads to a rapid expansion of the plasma sheet. [12] Thus the reconnection heats up the plasma, and it flows into the evacuated region. One can see that there is still some perpendicular flow after the magnetic field B x has dropped 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 also gives an indication of where the spacecraft are. The total pressure before 1713 UT is 1.1 npa, which means that the lobe magnetic field strength, in absence of any plasma pressure, is 53 nt. The total pressure decreases after 1700 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 field to almost preevent values, decreasing the magnetic field to B x = 0 again shortly and returning to B x 30 nt. After this the field decreases, and only small oscillations are present in B x, indicating that the oscillation is strongly damped. Also, the fact that the spacecraft have moved into the current sheet by then makes observing the oscillation more difficult. This oscillation of the tail has a period of 20 min. This behavior is different from the flapping motion observed by Sergeev et al. [2003, 2004] in that the timescales involved are much longer, e.g., the 15 min of B x 0, and the oscillation is apparently limited to one side of the neutral sheet as there are no B x = 0 crossings. A schematic view of the 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 current sheet from 1 R E to 400 km. This event is discussed by Nakamura et al. [2002b] and could be related to the oscillation of the tail, possibly triggering another reconnection event, shown by the intensification of the aurora, as shown by Nakamura et al. [2002b]. At apogee the spacecraft move in z with v z 1 R E hr km s A Tilted Magnetotail [16] The Cluster mission was set up such that spatial and temporal variations of the magnetic field could be distin- 5of10

6 Figure 5. A schematic view of the timeline. (a) Reconnection takes place in the far tail, and Cluster is located in a compressed magnetotail. (b) Cluster observes the flux transfer event. (c) Cluster is located in a magnetic field-evacuated tail region, and magnetic pressure is replaced by plasma pressure. (d) Magnetic field moves back into the tail region, and Cluster observes an oscillating magnetic field. guished from each other. The perfect tetrahedral constellation at apogee makes it possible to determine the magnetic field gradients within the constellation by linear extrapolation of the field [Harvey, 1998]. The smaller the distance between the different spacecraft, the better the estimate of the gradients. However, at strongly active times the field becomes nonlinear, and the gradient determination breaks down. Indications of this effect are large values of the numerically derived divergence of the magnetic field, as is the case in this event. [17] To obtain some information about the tilting of the magnetotail, we have performed a minimum variance analysis [see, e.g., Song and Russell, 1999] on the magnetic field data for three intervals around the sharp drop in B x. The minimum variance direction is the direction of the normal to the magnetic field of the tail. We show the result for each spacecraft in Table 1. It is clear that the three spacecraft that are at roughly the same z GSM (C1, C2, and C4) show the same minimum variance direction, whereas the spacecraft much farther south (C3) shows a different direction. [18] In the interval just at the start of our event, UT, the field is strongly tilted in the xz plane; during the event, UT, the field is strongly tilted in the 6of10

7 Table 1. Minimum Variance Directions a Spacecraft UT UT 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) a C1, C2, C3, and C4 are Clusters 1, 2, 3, and 4, respectively. yz plane, as one would expect. When the magnetic field evacuated period is reached, the field is tilted in all directions. It is not uncommon to have such strongly varying 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 oscillations on low-pass filtered data (t > 2.5 min filter). The filtering is done to reduce the effect of false timing by the high-frequency oscillations. The mean velocity for the first 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 return of the magnetic field to lobe values during 1755: :21 UT, is there a significant deviation. The main difference for this interval is that the plasma flow changes from earthward (v x > 0) to tailward (v x < 0); all other oscillations have earthward flow. The perpendicular plasma flow, v?; x, remains small and earthward during the first half of this interval. 4. Localized Reconnection [21] As mentioned in section 2, the event at 1713 LT has the signature of a rapid flux transport event [Schödel et al., 2001] connected to a reconnection event farther down the tail. We will now look in detail at this event, zooming in on the interval UT, as shown in Figure 3. The magnetotail is rather tilted in the yz plane, as shown in section 3, and we can rotate in this plane such that the direction is in the new z direction (not shown) Flux Transport in the Tail [22] We see that the inclination of the magnetic field increases (B z gets stronger, and B x gets weaker) and then decreases again (B z gets weaker), and over that same interval, there is strong earthward plasma flow. This is reminiscent of reconnection [see, e.g., Runov et al., 2003]. Figure 3 shows that the largest component of the flow is field aligned, and only during the strong decrease of B x,is there significant perpendicular flow (the solid region below the curves), which is the flow that transports magnetic flux. Using Maxwell s equations and frozen-in condition, we ¼r^ ð v ^ B Þ: ð3þ Assuming an invariant direction in y, we can write for the time variation of B v xb z v z B x Þ: ð4þ With this starting point we can model the behavior of B x, assuming a flux transport/reconnection event, where we use a simplified model for the observed B z and v variation: B z ðþ¼b t z;0 exp t2 2s 2 ð5þ vt ðþ¼v 0 exp t2 2s 2 ; ð6þ where both the magnetic field and the flow are described by the same Gaussian variation in time. Substituting in equation (4), Z db x Z t 2 v 0 B z;0 cos a v 0 B x sin a s 2 dt; where a is the angle the velocity makes with the x axis. To solve for B x, one needs values for the quantities in equation (7). Most we can simply read off the data from Cluster; only the vertical scale of the whole magnetotail L cannot be directly measured and needs to be assumed: L = 50,000 km. A justification for this assumption can be found in the location of Cluster at the start of the event, as given in section 2. In GSM coordinates the spacecraft are located at z GSM 6 R E 38,000 km. The dipole tilt will, in this case, reduce the distance to the neutral sheet, but we need not assume that Cluster is located right at the boundary of the evacuated region. We use the following measured values: B z,0 =30nT,B x =40nT,v 0 = 500 km s 1, a =15, DT = 3 min, and s 0.5 min. A simple estimate of DB x can be found, assuming that the error function contributes a factor of order O(1), and with the values given above one finds ð7þ DB x / L 1 v 0 B z;0 cos a v 0 B x sin a DT 33 nt: ð8þ Indeed, this simple estimate shows that with the observed signatures in B z and v it is possible to deplete B x from its starting value at 40 nt. [23] However, with the four Cluster spacecraft, spatial gradients in the magnetic field and the velocity can be Table 2. Mean Velocities of the Magnetotail Oscillation for the First Two ¼ r^e; E ¼ v ^ B; ð1þ ð2þ Interval, UT v x v y v z 1730: : : : : : : : a Velocities are given in km s 1. 7of10

8 Table 3. First Four Eigenfrequencies for the Current Sheet as Determined by Seboldt [1990] a Eigenfrequencies Value Symmetric pffiffiffiffiffiffiffiffiffi w 1, n A 1:22 f 1, mhz pffiffiffiffiffiffiffiffiffiffiffi 0.5 w 3, n A 10:45 f 3, mhz 1.4 Antisymmetric pffiffiffiffiffiffiffiffiffi w 2, n A 5:00 f 2, mhz pffiffiffiffiffiffiffiffiffiffiffi 1.0 w 4, n A 17:75 f 4, mhz 1.9 a Eigenfrequencies are given in units of the Alfvén frequency n 2 A = v /L 0 and in mhz using the observed quantities B 0 5nT,r 1cm 3, and an estimated vertical scale of the magnetotail L 0 50,000 km. determined. As already mentioned in section 3, the linear gradient estimator [Harvey, 1998] is not accurate in this event. Therefore we use two spacecraft (C3 and C4), separated well in z and only slightly in x, to determine the gradients in B x, B z, v x, and v z with respect to z. These can be determined for each data point, where we have used 4 s resolution magnetic field data and 8 s resolution plasma data. The plasma data were interpolated to have the same resolution as the magnetic field data. Expanding the righthand side of equation x x ¼ B þ z z x ; we can substitute the observed quantities in equation (9), where we have used the average value of the magnetic field strength and velocity measured by C3 and C4 for the nonderivative terms. The time derivative of the magnetic field, the left-hand side of equation x /@t, can then easily be integrated over time to find DB x 41 nt, agreeing well with what we observed Eigenoscillation of the Tail [24] Eigenoscillations of the plasma sheet have been discussed 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 case discussed in section 4.1 we apply the results obtained by Seboldt [1990], who calculated the low-frequency wave modes in the Earth s plasma sheet. Using the basic MHD equations with a polytropic pressure law for the plasma and assuming an invariant direction (in this case the y direction), Seboldt [1990] calculates the spectrum of a generalized eigenvalue problem, among others, for a two-dimensional perturbation, k x 6¼ 0 and k y = 0, where k is the wave vector of the perturbation. For a polytropic index g = 5/3 several solutions w 2 / n 2 A for the eigenmodes are found, where n 2 A = v 2 0 /L 2 0 is the square of the typical Alfvén frequency, with v 2 0 = B 2 0 /m 0 r. The frequencies f 1 f 4 (see Table 3) are given for typical measured values of the current sheet: B 0 5nT,r 1cm 3, and L 0 50,000 km (see also section 4.1), with the Alfvén frequency n A Hz. The first four eigenfrequencies (two symmetric and two antisymmetric) are shown in Table 3 in units of n A and mhz. A symmetric mode is characterized by a mirror symmetry in the neutral ð9þ sheet of the oscillating field at both sides of the neutral sheet, e.g., a sausage mode oscillation. In this mode, there is a periodic exchange between magnetic and plasma pressure. An antisymmetric mode is characterized by velocities of the oscillation in the same direction at both sides of the neutral sheet, e.g., a kink mode oscillation. In this mode the magnetic and plasma pressure remain constant, but the magnetic tension varies periodically. [25] From the symmetry of our event, a magnetic field evacuated region in the tail, we expect that the magnetotail will oscillate in a symmetric mode. First, the pressure of the plasma sheet is balancing the pressure of the magnetic field 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 are oscillating in antiphase, indicative of a sausage-like symmetric mode. The frequencies listed in Table 3 show that the period for the oscillation that we observed in section 2, T osc 20 min! f mhz, is close to the frequency of the first harmonic f 1 of the symmetric mode. Indeed, when we assume that equations (8) and (9) in section 4.1 should show the same DB x, our assumed L (being the only free parameter in our problem) should be smaller by a factor of 1.24, which would change the first harmonic frequency to f 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 in which v y is the minor component, which we expect as we apply Seboldt s [1990] solution for k x 6¼ 0 and k y =0. 5. Discussion [27] In this paper we have discussed the rapid flux transport event measured by Cluster, set on by a reconnection event farther down the tail. We have shown that the signatures of the flow v x and the magnetic field B z are in agreement with flux transport calculated with Maxwell s equations and with the drop in B x resulting from it. After the flux transfer event, Cluster is located in a magnetic field evacuated region of the magnetotail, where the surrounding magnetic field is held off by the large plasma pressure. This transient situation of the tail, in which the plasma pressure keeps off the magnetic field of the lobe, is maintained for 15 min, after which the magnetic field returns to the evacuated region and tries to establish a new stable configuration, which results in a damped oscillating motion of the magnetic field. The period of this oscillating motion fits well 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 reconnection in the magnetotail. There is an onset of reconnection in the distant tail just before 1700 UT, which is registered by the 210 meridian stations TIK and CHD. There is no signature observed in the Cluster ion spectrometry (CIS) data. Then at 1704 UT, CIS data show a strong, field-aligned earthward flow (see v x in Figure 3). At 1709 UT, there is again strong field-aligned earthward flow, with a signature in B x measured by Cluster, the flow reverses to tailward at 1711 UT, and the B x recovers again until strong earthward fieldaligned and perpendicular flows start at 1713 UT, and Cluster shows the rapid flux transfer event. 8of10

9 [29] During this event, the Cluster spacecraft remain in the northern lobe of the magnetotail. Reconnection is assumed to start in the center of the neutral sheet, and from the X point or the diffusion region, there is an outflow region, where plasma is accelerated along the magnetic field lines. Nagai et al. [1998] studied the dynamics of substorm onsets, with their associated plasma flows, using Geotail data. They summarize the magnetic field and plasma flow structure in their Figure 12. Not only is there strong perpendicular plasma flow (together with magnetic flux transport) near the magnetic X line, where reconnection occurs, but ions are also accelerated away from the X line region along the magnetic field lines. We now propose, from our observations, that a large outflow region exists near the X line with field-aligned ion flows onto field lines not yet involved in the reconnection process. 6. Conclusions [30] This event has shown us important evidence that fast flows, related to reconnection farther down the tail, can result in a large-scale, damped oscillation of the plasma sheet. Importantly, a significant part of the flow is perpendicular to the magnetic field, as this constitutes flux transport in the tail. [31] There seems to be patchy reconnection in the tail, evidenced by the flow bursts measured by Cluster, related to the observed rapid flux transport. Evidence of flow bursts related to the substorm onset measured by ground stations is not present. We attribute this lack of evidence to Cluster being on field lines too far away from the reconnection region. There has to be a sizable ion outflow region around the reconnection site, which creates the magnetic field-aligned ion flow bursts measured by Cluster with no signature of flux transport present in the data. [32] Acknowledgments. We would like to thank H.-U. Eichelberger for preparing the Cluster MAG data. We would like to acknowledge CDAweb ( for providing the publicly available data from ACE, Wind, and Geotail. Furthermore, we would like to thank J. Olson at the UAF Geophysical Institute for making the GIMA magnetometer data available on the web. We thank the institutes who maintain the IMAGE magnetometer array and the Finnish Meteorological Institute for providing the data. The work by M.V. and K.H.G./I.R. was financially supported by the German Bundesministerium für Bildung und Forschung and the Zentrum für Luft- und Raumfahrt under contracts 50 OC 0104 and 50 OC 0103, respectively. [33] Lou-Chuang Lee thanks Rudolf Treumann and the other reviewer for their assistance in evaluating this paper. References Angelopoulos, 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 by Geotail, J. Geophys. Res., 108(A5), 1189, doi: /2002ja Asano, Y., T. Mukai, M. Hoshino, Y. Saito, H. Hayakawa, and T. Nagai (2004), Current sheet structure around the near-earth neutral line observed by Geotail, J. Geophys. Res., 109, A02212, doi: / 2003JA Balogh, A., et al. (2001), The Cluster magnetic field investigation: Overview of in-flight performance and initial results, Ann. Geophys., 19, Bauer, T. M., W. Baumjohann, R. A. Treumann, N. Sckopke, and H. Lühr (1995a), Low-frequency waves in the near-earth plasma sheet, J. Geophys. Res., 100, Bauer, T. M., W. Baumjohann, and R. A. Treumann (1995b), Neutral sheet oscillations at substorm onset, J. Geophys. Res., 100, 23,737 23,743. Baumjohann, W., G. Paschmann, and H. Lühr (1990), Characteristics of high-speed flows in the plasma sheet, J. Geophys. Res., 95, Baumjohann, W., M. Hesse, S. Kokubun, T. Mukai, T. 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