Geotail encounter with reconnection diffusion region in the Earth s magnetotail: Evidence of multiple X lines collisionless reconnection?

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003ja010031, 2004 Geotail encounter with reconnection diffusion region in the Earth s magnetotail: Evidence of multiple X lines collisionless reconnection? X. H. Deng, 1,2 H. Matsumoto, 3 H. Kojima, 3 T. Mukai, 4 R. R. Anderson, 5 W. Baumjohann, 6 and R. Nakamura 6 Received 11 May 2003; revised 11 January 2004; accepted 11 March 2004; published 11 May [1] On 11 December 1994 the Geotail spacecraft encountered an active reconnection diffusion region around the X line in the Earth s magnetotail. Three interesting features were observed. One is quadrupole pattern of the out-of-plane B y magnetic field component during the passage of magnetic islands and the crossing of the neutral sheet. The second is a direction reversal of the electron beams in the vicinity of the separatrix of the magnetic topology of reconnection. The third is a clear plasma flow reversal. By combining the observations of plasma, magnetic field, particles, and waves, this report may provide evidence of multiple X lines collisionless reconnection in the magnetotail at the microscopic level. INDEX TERMS: 7835 Space Plasma Physics: Magnetic reconnection; 2748 Magnetospheric Physics: Magnetotail boundary layers; 7807 Space Plasma Physics: Charged particle motion and acceleration; 7867 Space Plasma Physics: Wave/particle interactions; 7815 Space Plasma Physics: Electrostatic structures Citation: Deng, X. H., H. Matsumoto, H. Kojima, T. Mukai, R. R. Anderson, W. Baumjohann, and R. Nakamura (2004), Geotail encounter with reconnection diffusion region in the Earth s magnetotail: Evidence of multiple X lines collisionless reconnection?, J. Geophys. Res., 109,, doi: /2003ja Introduction [2] Magnetic reconnection is one of the most important energy conversion processes in space physics. It is widely believed that critical explosive events occur in the regions where the magnetic field reverses, resulting in a magnetic X line configuration [Parker, 1957; Dungey, 1961; Petschek, 1964] as schematically shown in Figure 1. Understanding how magnetic energy can be released so quickly has perplexed scientists for nearly four decades. The breaking of the magnetic field lines actually takes place in a narrow region around the X line. In this narrow region the magnetohydrodynamic description fails and the diffusion region is found to develop a multiscale structure based on the electron and ion scale lengths [Drake et al., 1997; Otto, 2001]. The relative motion of electrons and ions results in the Hall currents which produce the quadrupole out-ofplane magnetic field pattern B y near the reconnection region as shown in Figure 1 [Sonnerup, 1979]. As a result, the reconnection rate is high and no macroscopic nozzle exists. Though there are reports of some evidence of collisionless 1 Department of Space Physics, Wuhan University, Wuhan, China. 2 Now at Radio Science Center for Space and Atmosphere, Kyoto University, Kyoto, Japan. 3 Radio Science Center for Space and Atmosphere, Kyoto University, Kyoto, Japan. 4 Institute of Space and Astronautical Science, Kanagawa, Japan. 5 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA. 6 Space Research Institute, Austrian Academy of Sciences, Graz, Austria. Copyright 2004 by the American Geophysical Union /04/2003JA010031$09.00 reconnection at the dayside magnetopause and in the magnetotail [Deng and Matsumoto, 2001; Mozer et al., 2002; Oieroset et al., 2001; Nagai et al., 2001; Runov et al., 2003], more detailed measurements of the microscopic structure of the reconnection layer, especially the electron and ion flows around the X line, are rarely reported. Here we describe an encounter of the Geotail spacecraft with an active reconnection diffusion region around the X line in the Earth s magnetotail which provides direct and strong evidence for collisionless reconnection by combining the observations of plasma, the magnetic field, particles, and waves. 2. Observations 2.1. Encounter With an Active Reconnection Diffusion Region Around the X Line [3] Figure 2 shows a summary of the observations by the Geotail satellite for the interval from 15:05 to 18:20 UT on 11 December Figure 2a contains schematic diagrams of the collisionless reconnection configuration and the relative location of Geotail during the observations. The GSM coordinate system is used with x toward the Sun, and the x z plane contains the Earth s dipole axis. Figure 2b is a plot of the plasma flow speed component V x [Mukai et al., 1994]. Figures 2c and 2d are plots of the magnetic field components B x and B z [Kokubun et al., 1994]. The key result is there appear certain different signatures of magnetic field and flow velocity when the satellite is located on the earthward or tailward side of the X line, which are identified as L 1 to L 8 and bounded by the green dashed lines. As Geotail approached the neutral sheet (identified by the decreasing of B x toward zero in Figure 2c and confirmed 1of9

2 Figure 1. Multiple X lines collisionless reconnection configuration in the magnetotail for 11 December The Geocentric-Solar-Magnetospheric (GSM) coordinate system is used with x toward the Sun, and the x-z plane contains the Earth s dipole axis. The neutral sheet is marked with the pink dashed line, and the Earth is to the left. Solid black lines denote magnetic field lines. Green symbols present the selfgenerated quadrupole out-of-plane magnetic field component of B y due to Hall currents which were observed by the Geotail spacecraft during the passage of the magnetic islands and the crossing of the neutral sheet. The observed quadrupole pattern is different from that in the original single X line picture. In single X line picture, the quadrupole pattern is centered at the X line, but in the multiple X lines picture, each X line produces a quadrupole pattern B y structure. Direction reversal of the electron beams was detected in the vicinity of the plasma sheet boundary layer near the central X line. by the changes of plasma density and temperature), it first detected three bursts of tailward directed plasma flow (L 1 to L 3 ) during the period of 15:10 UT to 15:40 UT. It then detected two bursts of earthward directed plasma flow (L 4 and L 5 ) during the period of 15:40 UT to 15:54 UT, and finally detected three bursts of tailward directed plasma flow (L 6 to L 8 ) during the period of 17:50 UT to 18:20 UT. The high-speed accelerated plasma flow is a typical signature of magnetic reconnection [Paschmann et al., 1979]. The reversal of the plasma jets from tailward to Earthward and then to tailward implies that the spacecraft moved from the tailward side to the Earthward side and then finally returned to the tailward side of the active reconnection X line. Simultaneously with these plasma jets there are several bipolar signatures of B z (identified as P 1 to P 8 in Figure 2d) which indicate that magnetic islands passed through Geotail (see Figure 2a). [4] During the period of 15:10 to 15:40 UT there were three bipolar signatures in B z (identified as P 1 to P 3 in Figure 2d) which were observed simultaneously with the three jets of tailward directed plasma flow V x (L 1 to L 3 ). When Geotail approached the neutral sheet at the time of 15:12 UT, it observed the tailward directed jet of L 1. This means that Geotail was located at the tailward side of the X line. Thus, when a magnetic island passed through Geotail, we observed a bipolar signature in B z changing from positive (northward) to negative (southward) around the time averaged zero value of B z. During the period of 15:40 to 15:54 UT, two earthward directed jets (L 4 and L 5 ) were observed, which indicates that Geotail traveled from the tailward side to the Earthward side of the X line. Two bipolar signatures of B z changing from negative to positive (identified as P 4 and P 5 in Figure 2d) were recorded simultaneously. Note that the polarities of the bipolar signatures of B z associated with the Earthward directed jets are changed correspondingly. Even more interesting, during the period of 17:50 to 18:20 UT, three large tailward directed plasma flows were observed (L 6 to L 8 ). Associated with the tailward directed jets, there are three bipolar signatures of B z changing from positive to negative. The polarities of the bipolar signatures of B z associated with the tailward directed jets are again changed correspondingly Dynamics of Electron and Ion Beams [5] The tailward, Earthward and then tailward directed electron and ion beams corresponding to the above mentioned tailward, Earthward and tailward directed plasma jets were observed by the Low-Energy Particle experiment. Taking the L 3 and L 4 events as examples, Figures 3a and 3b and Figures 4a and 4b show the time evolutions of the ion and electron distribution functions in the velocity plane which is defined such that the Y axis is the magnetic field directions (V // ) and the X axis is the direction of the convection velocity (V? ) with reference to the spacecraft frame. The phase space density is scaled by color coding on logarithmic scale. During the period of the tailward plasma jet of L 3, when Geotail entered the plasma sheet as the magnitude of B x decreased (also confirmed by the change of the plasma density and temperature), it detected energetic ion beams (Figure 3a) and electron beams (Figure 3b), which flow tailward (parallel to the magnetic field). During the period of the Earthward plasma jet of L 4, Earthward ion (Figure 4a) and electron beams (Figure 4b) were detected, which flow in the direction antiparallel to the magnetic field. When Geotail was further inside the boundary and moving toward the neutral sheet, the electron distributions appear to be rather hot and become nearly isotropic. The electron distributions show some counter streaming beams. 2of9

3 Figure 2. Plasma and magnetic field data observed by Geotail as it traveled through the Earth s magnetotail 38 Earth radii behind the Earth during the interval from 1505 to 1820 UT on 11 December (a) Schematic diagrams of the collisionless reconnection configuration and the relative location of Geotail during the observations. (b) Plot of the plasma flow speed component of V x. (c, d) Plots of the magnetic field components B x and B z, respectively. Large tailward, then Earthward and finally tailward plasma jets are observed as the spacecraft traveled from the tailward side to the Earthward side, and then to the tailward side of the X line. neutral sheet near 18:03 UT, it observed the tailward directed jet of L 7. Thus when a magnetic island passed through Geotail, we observed a bipolar signature in B z changing from positive to negative (Figure 5e). Simultaneously with the B z bipolar signature, the bipolar signature of B y was observed, first negative and then positive, around the time-averaged zero value (Figure 5d). This series of the bipolar signatures of B y associated with the magnetic island structures of B z can be well explained by the quadrupole pattern of B y caused by the electron Hall currents schematically drawn in Figure 1. The B y structure associated with the Hall currents was first suggested by Sonnerup [1979]. Such Hall effects and the associated B y structure have been seen in various simulations [Terasawa, 1983; Otto, 2001; Karimabadi et al., 1999; Lin and Swift, 2002]. More convincingly, such quadrupole out-of-plane magnetic field patterns have been revealed by the polarity changing of B y during the two brief crossings of the neutral sheet (indicated by the pink dashed line in Figure 5c). During the period of 17:50 to 18:20 UT, Geotail twice crossed briefly to the northward side of the neutral sheet before returning to the southward side as identified by the sign changing of B x during the events of L 6 and L 8. During the crossings of the neutral sheet, the polarity changes of B y were recorded and are indicated by the blue shaded areas in Figure 5d. Taking Though there are reports of ion beams associated with reconnection [Scholer et al., 1984; Mukai et al., 1994; Nagai et al., 2001; Nishida, 2001], this is the first report of the time evolution of ion and electron beams associated with reconnection on the both sides of the X line Quadrupole Structure of the Out-of-Plane Magnetic Field Component B y [6] We observed out-of-plane B y which had localized bipolar structures during the passage of magnetic islands identified by bipolar signatures in B z and associated with the high-speed plasma jets of V x. Figure 5 shows the observations in an expanded timescale for the interval from 17:55 to 18:20 UT. During this period, three large tailward directed plasma flows were observed (identified as L 6 to L 8 in Figure 5b). The most striking feature of Figure 5d is that there are several bipolar variations in B y around the timeaveraged value of B y (identified as P 6 to P 8 ). They are well correlated with the bipolar signatures of B z observed simultaneously with the three jets of tailward directed plasma flow V x (L 6 to L 8 ). Taking the L 7 event as example, Geotail stayed on the southward side of the neutral sheet (identified by the negative sign of B x ). As Geotail approached the Figure 3a. Observation of tailward ion beams during the periods of the tailward plasma jet L 3. The distribution functions of ions are the slices of the three-dimensional (3-D) distribution functions in a plane which is defined such that the Y axis is the magnetic field directions (V // ), and the X axis is the direction of the convection velocity (V? ) with reference to the spacecraft frame. The phase space density is scaled by color coding on logarithmic scale shown at the right-hand side. 3of9

4 Figure 3b. Observation of tailward electron beams during the periods of the tailward plasma jet L 3. The distribution functions of electrons are the slices of the 3-D distribution functions in a plane which is defined such that the Y axis is the magnetic field directions (V // ) and the X axis is the direction of the convection velocity (V? ) with reference to the spacecraft frame. The phase space density is scaled by color coding on logarithmic scale shown at the right-hand side. the P 8 event as a sample, when Geotail briefly crossed the neutral sheet from the southward side to the northward side around 18:14 UT as is apparent from the changing of the sign of the B x from negative to positive, the B y component first turned negative and then became positive as Geotail returned to the southward side of the neutral sheet (indicated by the blue shaded areas in Figure 5d). The polarity change of B y during the two brief excursions into the northern hemisphere can be well explained by the internally generated quadrupole structure of B y as illustrated in Figure 1. The amplitude of the observed out-of-plane Hall magnetic field is 5 8 nt, which corresponds to about 35 40% of the total magnetic field magnitude (about nt), and close to the theoretical predictions [Pritchett, 2001] Direction Reversal of Electron Beams [7] The new evidence of the Hall current loop is the observations of the directional reversal of the electron beams during the crossing of the plasma sheet boundary layer (PSBL). The Hall current system in the vicinity of the magnetic reconnection site of the near-earth magnetotail have been observed by the observations of electrons having counterstreaming features from the same distribution function [Fujimoto et al., 1997; Nagai et al., 2001; Oieroset et al., 2001]. It is shown that in the outermost region near the plasma sheet/tail lobe boundary, fieldaligned currents flow out of the magnetic reconnection Figure 4a. Observation of Earthward ion beams during the periods of the Earthward plasma jet L 4 with the form as same as Figure 3a. site. In the adjacent region, just inside the outflowing current layer, field-aligned currents flow into the magnetic reconnection site. Figure 6 shows the time-spatial evolutions of the electron distribution functions with the form Figure 4b. Observation of Earthward electron beams during the periods of the Earthward plasma jet L 4 with the form as same as Figure 3b. 4of9

5 beams were observed, which were aligned with the magnetic field and directed away from the x point. The relative observation locations are indicated by the blue shaded boxes in Figure 1. During this period, no direction reversal of the ion beams was observed. To make the electron beams seen more clearly, we have used one-dimensional (1-D) cut of the distribution function along the direction of magnetic field (indicated by the red dashed lines in Figure 6). Figure 7 show the corresponding distribution functions in parallel directions to the magnetic field. A smooth green curve shows the one-count level. The electron beams are indicated by the red circles. The temporal-spatial evolutions of the electron distribution functions during the Figure 5. Plasma and magnetic field data and dynamic frequency spectrogram of electric and magnetic fields observed by Geotail in an expanded timescale for the interval from 1755 to 1820 UT on 11 December (a) Frequency-time spectrograms of the electric and magnetic fields. (b, c, d, e) Plasma flow speed component V x and the magnetic field components B x, B y, and B z, respectively. Bipolar B y field variations consistent with Hall magnetic field pattern were detected during the passage of the magnetic islands identified by the bipolar signatures of B z associated with the plasma jets of V x and during the two brief crossings of the neutral sheet. The enhancement of wave activities are well correlated with the high-speed plasma jets of V x. The observation of the electron beams are confirmed by the exciting of the electron plasma waves (langmuir waves) indicated by the solid arrows in Figure 5a. as same as Figure 3 during the crossing of the PSBL. During the period just after the tailward jet event of L 6 and before the tailward jet event of L 7, Geotail was located at the tailward side of the X line. Around the time of 18:01 UT, Earthward directed electron beams were observed immediately adjacent to the outer of the plasma sheet boundary layer, which flowed in the direction opposite that of the magnetic field and directed toward the x point. The B x component was large and negative and the spacecraft was just outside the plasma sheet boundary layer (PSBL) in the southern hemisphere. At the time of 18:03:11 UT, when Geotail moved inside the boundary region (indicated by the change of B x and confirmed by the changes of plasma density, temperature and the characteristics of the ion beams, tailward electron Figure 6. Detection of the direction reversal of the electron beams during the crossing of the plasma sheet boundary layer. The distribution functions of electrons are shown as the same as Figure 3. Around the time of 1801 UT, Earthward directed electron beams were observed in the region immediately adjacent to the outer of the plasma sheet boundary layer, which were antialigned with the magnetic field and directed toward the x point. At the time of 1803:11 UT, tailward electron beams aligned with the magnetic field and directed away from the x point were detected just inside the plasma sheet boundary layer region. The electron beams can be seen only clearly near the boundary, while closer to the neutral sheet, electrons are quickly thermolized and become nearly isotropic, and the electron distributions show some counter streaming beams. 5of9

6 Figure 7. The corresponding cut of the distribution function along the direction of magnetic field (indicated by the red dashed lines in Figure 6). A smooth green curve shows the one count level. The electron beams are indicated by the red circles. The time intervals for each distribution functions are indicated by the blue boxes at the bottom of Figure 5. crossing of the PSBL region are consistent with the prediction of the simulations [Hoshino et al., 2001]. The existence of the electron beams is also confirmed by the excitation of the bursts of electron plasma waves (Langmuir waves) recorded by the MultiChannel Analyzer of PWI [Kojima et al., 1997] and indicated by the black arrows in Figure 5a. The observation of the electron beams moving toward and away from the x point during the crossing of the plasma sheet boundary layer associated with reconnection provides new evidence of the existence of Hall current loop carried by electrons. [8] By checking the wave data shown in Figure 5a, which were observed by Plasma Wave Instrument (PWI) [Matsumoto et al., 1994], we found a good correlation between the reconnection plasma jets and the enhancement of wave activities. Comparing Figure 5a and Figure 5b, we see that the onsets and cutoffs of the waves happen almost simultaneously with the plasma jets. We have observed the burst of electromagnetic waves with frequencies in the range of whistler waves. Figure 8 shows the burst of the electromagnetic waves detected by the Waveform Capture (WFC) of the PWI at 18:15 UT. The panel a shows the electric field component of En measured by the two sets of orthogonal dipolar antennas, and the panel b displays the magnetic field component of B a observed by the triaxial research coils. The corresponding frequency spectrum is shown Figure 8 (bottom left). The enhanced broadband electromagnetic fluctuations drops in the range of oblique whistler waves, i.e., W lh w W ce, where W lh and W ce are the lower hybrid frequency and the electron cyclotron frequency, respectively. The wave vector k is near parallel to the ambient magnetic field by minimum variance analysis. The polarity of the waves is right-handed in the plane perpendicular to the ambient magnetic field, which is shown in Figure 8 (bottom right). The right-hand polarity is a typical character of electron whistler waves Observation of Electrostatic Solitary Waves (ESW) Associated With Reconnection [9] Though the Waveform Capture (WFC) receivers operate only 8.7 seconds every 5 minutes during some periods of observation, we do have data from the WFC at critical periods when the magnetic reconnection took place near the spacecraft in the magnetotail region. We observed a variety of waveforms in this reconnection event. Figures 9 and 10 show the typical waveforms of the electric field components of the Broadband Electrostatic Noise (BEN). The red arrows at the bottom of Figure 2 show the time when these samples were acquired. The E // and E? are the electric field components parallel and perpendicular to the ambient magnetic field B projected on to the antenna plane. The Electrostatic Solitary Waves (ESWs) shown in Figure 9 are observed both near neutral sheet and around the separatix boundary. Another waveform is the Narrowband Electrostatic Noise (NEN) with its waveform to be more monochromatic shown in panel a of Figure 10. The important characteristic of this emission is the frequency range between electron and ion plasma frequencies. There exist no standard parallel propagation electrostatic modes in this frequency range [Coroniti and Ashour-Abdalla, 1989]. The electrostatic waves with large amplitude modulations near neutral sheet have been observed associated with reconnection, which are shown in Figures 10b and 10c. The waveforms are highly modulated in amplitude. The observed modulated waves have the frequency near to the local electron plasma frequency. By examining the polarity of the modulated electron plasma waves, we found that they include two different polarizations relative to the ambient magnetic field B 0. The polarity of the electric fields shown in panel b is parallel to the ambient magnetic field B 0. However, there is significant difference in their polarizations in Figure 10c. We plot the time variation of the observed parallel and perpendicular electric field components relative to the ambient magnetic field B 0. As is clearly seen, the orientations of the polarizations change very quickly from the direction of almost parallel into that of perpendicular to B 0, respectively. As the frequencies of the modulated electron plasma waves are almost equal to the local electron plasma frequencies, the parallel polarized electron plasma waves are the Langmuir waves, while the perpendicular polarized electron plasma waves are like to be Electron Cyclotron Harmonic waves (ECH). Such modulated 6of9

7 Figure 8. Burst of whistler waves observed by Waveform Capture (WFC) on 11 December (a, b) Electric field component of En measured by the two sets of orthogonal dipolar antennas, and the magnetic field component of B a observed by the triaxial search coils, respectively. Fourier frequency spectrum of En and B a. (bottom left). Hodogram of the two orthogonal magnetic field components B?1 and B?2 in the plane perpendicular to the ambient magnetic field B 0. (bottom right). The wave vector k is near parallel to the ambient magnetic field by minimum variance analysis. The polarity of the waves is right-handed, which is a typical character of electron whistler waves. electron plasma waves with polarization parallel and perpendicular to the ambient magnetic field B 0 are observed in the tail lobe region by Kojima et al. [1997]. 3. Discussion and Conclusions [10] By combining the observations of plasma, magnetic field, particles and waves, we report the evidence of multiple X lines collisionless reconnection in the magnetotail region by the observations of the electron and ion beams and the quadrupole structure of the out-of-plane magnetic field component B y during a fortuitous encounter with an active reconnection diffusion region around the X line. Three interesting features were found: One is the detection of the quadrupole pattern of the out-of-plane B y magnetic field component during the passage of magnetic islands and the crossing of the neutral sheet. The second is the observation of the direction reversal of the electron beams in the vicinity of the separatrix of the magnetic topology of reconnection. The observation of the electron beams moving toward and away from the x point during the crossing of the plasma sheet boundary layer provides new evidence of the existence of Hall current loop carried by electrons. The third is a clear plasma flow reversal with local bipolar signatures of B z. [11] The observed quadrupole pattern is different from that in the original single X line picture. In single X line picture, the quadrupole pattern is centered at the X line, but in the multiple X lines picture, each X line produces a quadrupole pattern B y structure, and the pattern that we talks about are the B y structure between two X lines that are present close to each other. Multiple X lines reconnection (MXR) has been extensively investigated following the observation of magnetic flux ropes at the dayside magnetopause, where MXR reconnection has been hypothesized to be responsible for the generation of flux transfer events (see the review by Lee [1995]). Recently, Slavin et al. [2003] showed that flux ropes are a common occurrence in the near-earth plasma sheet and that they are closely associated with fast Earthward and tailward flow. It has been shown by a two fluid code simulation that reconnection develops as multiple X line segments with characteristic scale length of 1 4R e, and the X line segments merge together to a state in which large-scale magnetic energy takes place [Shay et al., 2003]. [12] We found a good correlation between the reconnection plasma jets and the enhancement of wave activities. The burst of electromagnetic waves with frequencies in the range of whistler waves was observed by checking the spectrum, polarization and propagation. We observe Electrostatic Solitary Waves (ESWs), Narrowband Electrostatic Noise (NEN), and electrostatic waves with large amplitude modulations (AMEW) associated with reconnection. The ESWs associated with reconnection at the dayside magnetopause have been observed by Matsumoto et al. [2003] and Cattell et al. [2002]. Our observations clearly show that the 7of9

8 Figure 9. Typical electrostatic waveforms of the Broadband Electrostatic Noise (BEN) associated with reconnection. The Electrostatic Solitary Waves (ESWs) are observed both near neutral sheet and around the separatix boundary. The E // and E? are the electric field components parallel and perpendicular to the ambient magnetic field B projected on to the antenna plane. Figure 10. Typical electrostatic waveforms of the Broadband Electrostatic Noise (BEN) associated with reconnection near the neutral sheet. (a) Narrowband Electrostatic Noise NEN and (b, c) highly amplitude modulated electrostatic waves (AMEW). Modulated electron plasma waves with polarization parallel and perpendicular to the ambient magnetic field B 0 are observed. 8of9

9 enhancement of BEN waves associated with reconnection is not caused by random noise (such as white noise ). Instead, the waves are composed of coherent structure. Recently Drake et al. [2003] have revealed the development of turbulence driven by intense electron beams which form near the magnetic X line and separatrices. The resulting turbulence evolved into distinct nonlinear structures consisting of localized regions of bipolar parallel electric field. [13] Though there is great progress in both observations and theory, many key issues, such as the microscopic structure of the reconnection layer, especially the electron diffusion region, the source of anomalous resistivity, and how the macroscopic structure coupled with microscopic process have not been firmly solved. The changes in the flow directions may suggest that the X line moves tailward or Earthward. However, it cannot rule out the possibility, due to the limitation of a single spacecraft, that the flow reversals were due to the sequential switch-on of different independent X lines. It should be stressed that the above mentioned events are 3-D and multiple X lines transient reconnection. The magnetic island with the high-speed flow has the O-type neutral structure, and the Hall current dynamics of the O-type region is complex and maybe quite different from that of the steady single X type neutral structure. Multipoint observations and 3-D measurements with high time resolution, combined with particle simulations, are crucial for the study of 3-D structure of collisionless reconnection in more detail. [14] Acknowledgments. We thank J. F. Drake, P. L. Pritchett, M. Hoshino, M. Shay, and Y. Omura for valuable discussions. We thank all members of the Geotail team for the high-quality data and for the successful spacecraft operation. X. H. 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