Density cavity in magnetic reconnection diffusion region in the presence of guide field

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010ja016324, 2011 Density cavity in magnetic reconnection diffusion region in the presence of guide field M. Zhou, 1,2 Y. Pang, 1,2 X. H. Deng, 1,3 Z. G. Yuan, 3 and S. Y. Huang 3 Received 28 November 2010; revised 11 March 2011; accepted 7 April 2011; published 30 June [1] Understanding the structure of the diffusion region of magnetic reconnection is crucial to pinpoint the mechanism of energy conversion from magnetic field to plasma. Characteristics of a diffusion region with guide field (i.e., component reconnection) may be significantly different from those of a diffusion region without guide field (i.e., antiparallel reconnection). In this study, we attempt to understand the structure of a diffusion region with guide field by studying the density cavity along separatrix. We present an event in which a density cavity was detected by the Cluster spacecraft in a diffusion region in the presence of guide field. The cavity was located around the separatrix region on the southern hemisphere of the neutral sheet and earthward of the X line and was coincident with strong magnetic field compression. The width of the cavity was on the ion inertial scale. This cavity contained a relatively strong antiparallel current, which was mainly contributed by parallel streaming electrons with energy of 1 10 kev. Enhancements of lower hybrid wave and electromagnetic whistler wave were observed inside the cavity. These waves are probably excited by parallel streaming electrons along separatrix via electron beam instability. Two dimensional electromagnetic particle in cell simulation was employed to study the structure of the density cavity. The location and scale of the cavity and the signature of electric current and electron velocity are consistent with our observations. It is found that there was displacement between the position of electron density minimum and out of plane magnetic field maximum in reconnection with guide field. However, this displacement is much less than that in reconnection without guide field. There was no significant acceleration for electrons to reach energy larger than 30 kev at the cavity. Citation: Zhou, M., Y. Pang, X. H. Deng, Z. G. Yuan, and S. Y. Huang (2011), Density cavity in magnetic reconnection diffusion region in the presence of guide field, J. Geophys. Res., 116,, doi: /2010ja Introduction [2] Magnetic reconnection is a fundamental plasma process that enables topology reconfiguration of magnetic field lines, during which magnetic field energy could release to plasma energy in an explosive way [Vasyliunas, 1975]. It is believed that the key processes that control fast reconnection occur in a small region around X line, that is, the diffusion region, where plasma is no longer frozen in magnetic fields. [3] The structure of reconnection diffusion region has been extensively studied both theoretically and experimentally for many years. The diffusion region includes a characteristic two scale structure, that is, the ion diffusion region and electron diffusion region, depending on the mass of different species. In the ion diffusion region, ions and electrons no longer move together. The decoupling of ions 1 Institute of Space Science and Technology, Nanchang University, Nanchang, China. 2 Institute of Astronomy, Nanchang University, Nanchang, China. 3 School of Electronic Information, Wuhan University, Wuhan, China. Copyright 2011 by the American Geophysical Union /11/2010JA and electrons motion leads to the Hall effect, which is believed to be essential for fast reconnection [Birn et al., 2001; Deng and Matsumoto, 2001]. Typical manifestation of the Hall effect are quadrupolar structure of out of plane magnetic fields, and bipolar electric fields pointing toward central current sheet at the edge of diffusion region, as well as density depletion layers along magnetic separatrices [Shay et al., 2001; Runov et al., 2003; Eastwood et al., 2007]. A moderate guide field could dramatically change the structure of diffusion region, even though the reconnection rate remains fast. The familiar quadrupolar B y pattern is replaced by an enhancement of B y between the separatrices. Quadrupolar density structure appears along the separatrices, with one pair of separatrix arms negative charged while the other pair of separatrix arms positive charged [Pritchett and Coroniti, 2004; Ricci et al., 2004]. [4] Density cavity around diffusion region was first noticed by Shay et al. [2001], who showed that there was density depletion layer lying just downstream from the magnetic separatrix. The width of the layer is on the ion inertial scale. The essential role of Hall terms in the formation of depletion layer near separatrix region has been 1of10

2 Figure 1. Magnetic field and plasma bulk flow of SC4 between 2325 and 2335 UT: (a c) three components of magnetic field, (d) plasma bulk velocity, and (e) ion density. X, Y, and Z components of bulk velocity in Figure 1d are represented by black, red, and green lines, respectively. Red dashed line marks the time when the density cavity was detected. confirmed by Hall MHD simulation [Yang et al., 2006]. In addition, the cavity was proposed as a region for the production of energetic electrons in the presence of guide field [Drake et al., 2005]. [5] Retinò et al. [2006] studied a density cavity structure at the magnetopause separatrix region, which was about 50 ion inertial lengths away from X line. The density cavity was associated with strong electric fields, electron beams, as well as intense turbulence between lower hybrid frequency ( f lh ) and electron plasma frequency ( f pe ). Khotyaintsev et al. [2006] presented an observation of density cavity which was formed owing to the escape of magnetospheric electrons along the newly opened field lines on the magnetopause. Inside the cavity a strong perpendicular electric field was observed, which constituted a potential jump of several kv. They suggested that the potential jump and filed aligned currents could be responsible for strong auroral. [6] Density cavities have also been detected around reconnection region [Øieroset et al., 2001; Vaivads et al., 2004; Cattell et al., 2005]. Electrostatic solitary waves (ESWs) were detected at the edge of density cavity, and were closely associated with narrow electron beams [Cattell et al., 2005]. In their simulation, satellite observation can be reproduced only when a small guide field was added in reconnection region. [7] In this paper, we report an observation of density cavity in a reconnection diffusion region in the presence of finite guide field, and focus on the microphysics. Some features observed inside the cavity are different from those of Cattell et al. [2005]. The rest of the paper is organized as follows: in section 2 we briefly review the Cluster observations, and in section 3 we present the features of waves and particles associated with density cavity. The results are discussed in section 4 and briefly summarized in section Event Overview [8] The magnetic field, wave and particle instrument onboard of Cluster were used to study the detailed structure of cavity. The FGM instrument provides 22 Hz highresolution magnetic field data [Balogh et al., 2001]. Twentyfive Hz high resolution electric field data are provided by the EFW instrument [Gustafsson et al., 1997]. The STAFF instrument provides 1 s resolution magnetic and electric field power spectrums in the frequency between 8 and 4000 Hz [Cornilleau Wehrlin et al., 2003]. Thermal (<25 kev) electron distribution data are provided by the PEACE instrument [Johnstone et al., 1997], and RAPID instrument gives high energy (>30 kev) electron fluxes data [Wilken et al., 2001]. All the variables are presented in the Geocentric Solar Magnetospheric (GSM) coordinate except the electric field, which is shown in the Geocentric Solar Ecliptic (GSE) coordinate. [9] On 19 September 2003, Cluster encountered an X line in the Earth s magnetotail. During the tail season of 2003, the distance between four spacecraft was as small as 200 km, which provides a good opportunity to study smallscale structures. Figure 1 shows the magnetic field, plasma flow, and density measured by spacecraft SC4 between 2325 and 2335 UT. Cluster was located at around ( 17.5, 3.7, 0.4) R E. A plasma bulk flow reversal from tailward to earthward was detected by Cluster between 2325 UT and 2334 UT. Magnetic field B z component changed from negative to positive accompanying with this flow reverse, 2of10

3 Figure 2. Data from around the density cavity during 2331: :00 UT: (a c) magnetic field, (d) spacecraft potential, (e) electric current density, and (f, g) electric fields. The black line in Figure 2e shows the parallel component, while the red line shows the perpendicular component. which implies that a tailward moving X line passed the Cluster spacecraft. A diffusion region was identified during the interval of 2325: :00 UT, manifesting itself by quadrupolar structure of out of plane magnetic fields, as well as bipolar Hall electric fields pointing toward current sheet in the normal direction [Borg et al., 2005]. Plasma flow V z reversed from negative to positive when spacecraft crossed current sheet from northern to southern hemisphere. The flow pattern is consistent with inflow of conventional reconnection model. The average ion density was about 0.12/cm 3 in the diffusion region. Electromagnetic lower hybrid drift waves [Zhou et al., 2009] and kinetic Alfvén waves [Chaston et al., 2009; Huang et al., 2010] were also detected in this reconnection diffusion region. During the tailward flow, Cluster stayed at the northern hemisphere with B x larger than 10 nt. During the earthward flow, Cluster crossed the dynamic current sheet a few times and then stayed at the southern hemisphere for several minutes. Figure 3. Electron phase space density (PSD) as a function of energy obtained from the Pitch Angle Distribution (PAD) data of the Plasma Electron and Current Experiment (PEACE) instrument on board SC4 around (a) 2331:48 UT and (b) 2331:52 UT. Black, red, and green lines denote the phase space density at 0, 90, and 180 pitch angle, respectively. 3of10

4 the observation of kinetic Alfvén waves implies the diffusion region includes a nonnegligible guide field. Figure 4. Power spectrum and polarization analysis of wave activities around the density cavity. (a) Power spectrum of both magnetic field (blue line) and electric field (red line) between 2331:47 and 2331:53 UT. The solid lines represent power spectrum from the FluxGate Mgnetometer (FGM) and Electric Field and Wave (EFW) instruments, while the dashed lines represent power spectrum from the Spatio Temporal Analysis of Field Fluctuations (STAFF) instrument. The three vertical dashed lines indicate f ci, f lh, and f ce, respectively. (b) Hodogram of electric field in the x y plane. The red line indicates the direction of ambient magnetic field. [10] There are a few evidences indicating the existence of nonnegligible guide field in the diffusion region. First, the average value of B y component in the diffusion region is about 5 nt, which is approximately 20% of the asymptotic magnetic field 25 nt. Second, Kinetic Alfvén waves were detected in this diffusion region and believed to drive significant transport [Chaston et al., 2009]. It has been suggested that dispersive whistler waves and kinetic Alfvén waves play important roles in facilitating fast reconnection [Rogers et al., 2001]. In the presence of guide field, the dominated wave mode facilitating fast reconnection is kinetic Alfvén waves, in contrast to the antiparallel reconnection which whistler waves dominate [Deng and Matsumoto, 2001; Drake and Shay, 2007; Eastwood et al., 2009]. Therefore, 3. Features of Waves and Particles Associated With Density Cavity [11] A density cavity was detected at around 2331:50 UT, when Cluster was located at the southern hemisphere of neutral sheet (B x < 0), and earthward of X line (V x >0, B z > 0). Figure 2 shows three components of magnetic field, spacecraft potential, electric current density, and electric fields from 2331:42 to 2332:00 UT. The spacecraft potential can reflect the fast change of plasma density, the lower the spacecraft potential, the lower the plasma density and vice versa [Pedersen et al., 2008]. It was found that the density cavity corresponded to strong compression of magnetic field. The compression is primarily on the out of plane component B y (Figure 2b). On the basis of the timing of the maximum B y, we obtained the velocity of current sheet motion, in the direction of n = [0.39, 0.20, 0.90] with speed of about 240 km/s. The estimated current sheet normal direction agrees well with the result given by Borg et al. [2005]. The scale size of the cavity was estimated as 1400 km, approximately 2 ion inertial lengths. Electric current density was calculated by multispacecraft analysis [Dunlop et al., 2002]. There was an increase of antiparallel current (up to 38 na/m 2 ) at the leading edge of density cavity, after that the parallel current dropped down to about 20 na/m 2. The perpendicular current density was relatively small and stable (around 10 na/m 2 ) all through the layer. [12] Figure 3 shows the electron distribution of SC4 around the time when the cavity was detected. The electron distribution was measured by the PEACE instrument in the SPINPAD mode [Johnstone et al., 1997]. The parallel velocity distribution in Figure 3a shows a bump between 2 kev and 5 kev, and the phase space density (PSD) of parallel component above 1 kev is larger than the antiparallel and perpendicular component. Electron distribution at 4s later shows similar parallel dominated anisotropy distribution. There is a parallel beam component between 2 kev and 4 kev, and the PSD in parallel direction is larger than the other directions between 2 and 10 kev. The parallel distribution is fitted by two Maxwellian distributions: core plasma and a beam. The beam component obtained from the fitting are: n b = 0.002/cm 3, T b = 300 ev, v b = km/s 3 kev. Parameters for core plasma are: n 0 = 0.06/cm 3, T 0 = 3000 ev. This indicates that the distribution consists of a cold electron beam and hot ambient electrons. Parallel current contributed by ions, estimated by J ik = N i * V ik, where N i is ion density and V ik is ion parallel bulk velocity, was about 1 na/m 2, 1 order of magnitude lower than the total parallel current. However, parallel electron beams could provide electric current density approx 10 na/m 2. This suggests that electrons are the main carriers for the antiparallel current inside the cavity. [13] We found that there were strong electric field fluctuations with amplitudes up to 40 mv/m in the cavity (Figures 2f and 2g). Beyond this region the electric field was significantly small, which implies that the electric field was localized in the layer. We obtained power spectrum of magnetic field by combing both FGM and STAFF 4of10

5 Figure 5. Particle in cell (PIC) simulation results at w ci t = 34 with guide field B g = 0.5 B 0 : (a) out ofplane magnetic field component B y, normalized by asymptotic magnetic field B 0,inthex z plane, (b) electron density N e, normalized by initial electron density in the current sheet n 0,inthex z plane, and (c) B y (black line) and N e (red line) along the vertical line at x =27d i. instrument, and electric field power spectrum is obtained by combing EFW and STAFF instrument. The combined power spectrums cover the frequency range from 0.02 Hz up to 4000 Hz. Power spectrum of magnetic (electric) field from FGM (EFW) instrument was calculated by continuous wavelet analysis. Figure 4a shows the power spectrums of both electric and magnetic field. We can see that the power spectrums of magnetic field and electric field diverge around the ion cyclotron frequency ( f ci ), and the magnetic field power spectrum drops rapidly above f ci, whereas the power spectrum of electric field decreases slower than magnetic field and has a peak near f lh. The extremely large value of E / B suggests the observed fluctuation between f ci and f lh is electrostatic. The humps in power spectrum of both magnetic field and electric field around 0.25 Hz are due to spin tone contamination, which are not real plasma wave activities. The polarization property of these electric field fluctuations was further examined. Figure 4b shows the hodogram of the electric field vectors on the X Y plane, which are filtered above 1 Hz. The fluctuated electric fields were primarily linearly and quasi perpendicularly polarized on the X Y plane. Since the ambient magnetic field almost lay on the X Y plane with elevation angle smaller than 10 degrees, the third component of electric field (E z ) would not change the highly oblique polarization, no matter how large it is. Thus we conclude that the wave was linearly and highly obliquely polarized to the magnetic field. The frequency and polarization analysis suggest the electrostatic wave is lower hybrid wave (LHW). In addition, both magnetic field and electric field power spectrums increase between 200 and 600 Hz, which is slightly below electron cyclotron frequency ( f ce ). Polarization analysis show the wave is right hand circularly polarized (not shown), which confirms it is electromagnetic whistler wave. 4. Discussion 4.1. Comparison With Particle in Cell Simulation [14] Two dimensional particle in cell (PIC) simulation was employed to study the structures of density cavity. We require a somewhat larger initial guide field (0.5 times the asymptotic field) than that be inferred from the observations ( 0.2 times the asymptotic field) to fully reproduce the observed features. The discrepancy might be due to the fact that the average B y in observation may not be the real value of guide field because the guide field may be not uniform all through the diffusion region and may varies with time. [15] Our two dimensional fully electromagnetic relativistic PIC code was modified from the TRISTAN code to study magnetic reconnection and current sheet instabilities [Buneman, 1993]. The initial magnetic field is given by two Harris current sheets: B x = B 0 tanh((z L z /4)/L 0 ) B 0 tanh ((z 3L z /4)/L 0 ) B 0, where B 0 is the asymptotic magnetic field, L z is the simulation box length in z direction, L 0 is the initial half width of current sheet, which is set to 0.5 d i, and d i =c/w pi is the ion inertial length in the central current sheet. An initial flux perturbation is introduced in order to make the system enter the nonlinear stage quickly. Periodic boundary condition are employed at both x and z direction. Other parameters for this simulation are: w ci Dt = , 5of10

6 Figure 6. PIC simulation results at w ci t = 34 with guide field B g = 0.5 B 0 : (a) parallel current density J k, normalized by n 0 * V A, in the x z plane, (b) parallel electron velocity, normalized by Alfvén velocity V A, in the x z plane, and (c) J k (black line) and V ek (red line) along the vertical line at x =27d i. w pe /w ce = 2, c/v A = 10, n b /n 0 = 0.2, where w ci is ion cyclotron frequency, w ce is electron cyclotron frequency while w pe is electron plasma frequency, c is the light speed, V A is the Alfvén speed based on asymptotic magnetic field and plasma density in the central current sheet n 0, and n b is the background plasma density. We used an artificial mass ratio m i /m e = 25 because of the limitation of computational power. In this simulation, we employed 1280 * 1280 grids, equivalent to physical size of 51.2 * 51.2 c/w pi. The initial number of electron ion pairs is 100 million, so the peak number density exceeds 600 particles per grid. [16] Figure 5 shows the out of plane component of magnetic field B y and electron density n e in x z plane. A primary X line was located at around (19.6, 12.8) d i. A secondary magnetic island with 10 d i length was formed on the lefthand side (tailward) of X line. Magnetic field and density were both significantly enhanced in the center of magnetic island. This secondary island is likely formed because the extended current sheet is unstable to tearing instability [Daughton et al., 2006]. Similar as previous simulations, the quadrupolar B y component is replaced by enhancement of B y between separatrices. Electron density shows asymmetric structure along separatrices. Separatrix arms on the upper right and lower left quadrant has higher density than the other two arms. Figure 5c shows B y and n e along the vertical dashed line at x =27d i. We can see that there is a density cavity at around (x, z) = (27, 10.6) d i, which is on the earthward of X line and southern hemisphere of neutral sheet, in the same quadrant as observation. The width of density cavity is about 2 d i in simulation, consistent with observation. [17] Figure 6 shows the parallel current and electron velocity. It is evident that large parallel electron velocities are concentrated near X line and along separatrices. There is an enhancement of antiparallel current at the location of density cavity, corresponding to the peak of parallel electron velocity. Parallel current density on the northern hemisphere, which is stronger than that on the southern hemisphere, also corresponds to large parallel electron velocity. The formation of density cavity in the presence of guide field, similar as the antiparallel reconnection, is probably caused by the motion of electrons moving toward X line [Lu et al., 2010]. Electrons move toward X line along the upper left and lower right separatrices. These electrons are accelerated near X line and then expelled out along other two separatrices. Therefore, electrons are removed from the upper left and lower right separatrices and density cavity structures are formed. [18] Recently, Lu et al. [2010] concluded that density cavity along separatrix was outside the peaks of out ofplane magnetic field in antiparallel reconnection. They showed that density cavity was formed owing to the depletion of electrons moving toward X line along separatrix; nevertheless, maximum of B y strength is located between two in plane currents. In their simulation, the displacement between the location of density minimum in cavity and B y maximum is about 0.6 d i. From Figure 5c, we can see that there is a slight displacement between the location of density minimum and B y maximum, which is 0.16 d i. In our observation, there is also a shift of detection time, around 0.5 s, between the density minimum and B y maximum. Considering the flapping speed of current sheet 6of10

7 Figure 7. PIC simulation results at w ci t = 30 without guide field: (a) electron density N e, normalized by initial electron density in the current sheet n 0, in the x z plane, (b) B y (black line) and N e (red line) along the vertical line at x =24d i, and (c) J k along the vertical line at x =24d i. was 240 km/s, the displacement is about 120 km 0.17 d i, which is close to our simulation result but much less than simulation result without guide field. [19] We preformed another run, which the same parameters were used except that the guide field was set to zero. Figure 7a shows the electron density at x z plane, and Figure 7b shows the B y and electron density along the vertical line at x =24d i. There is an evident displacement between the position of density minimum and B y maximum. The displacement is about 0.56 d i, which agrees with the result of Lu et al. [2010]. We predict that, around the separatrix, the location between the dip of electron density and peak of out of plane magnetic field become closer as guide field strength increases. [20] We provide further evidence that the observed current system cannot be generated in antiparallel reconnection. Figure 7c shows the parallel current density along the vertical line at x =24d i. In contrast to guide field case, parallel current reverses sign crossed the separatrix. At the inner region of separatrix the current flows parallel to the magnetic field while on the outer region the current flows antiparallel to the magnetic field. Similar Hall current system can be found in other literatures [Nagai et al., 2003; Lu et al., 2010]. If a spacecraft cross the current sheet from northern to southern hemisphere, which is the case of our observation, then the spacecraft would first detect a parallel current and then an antiparallel current. Our observation shows that only the antiparallel current was observed, which is inconsistent with Hall current system in antiparallel reconnection Possible Generation Mechanism of LHW and Whistler Waves [21] There are several ways to generate waves near lower hybrid frequency. One possibility is transverse current driven instability, such as lower hybrid drift instability, which could be excited by diamagnetic drift in the presence of magnetic field or density gradient [Davidson et al., 1977]. From Figure 2 we found that the strongest electric field fluctuations did not correlate well with the greatest density gradient. It implies that lower hybrid drift instability is unlikely the mechanism to excite the LHW. Another possibility is that the LHW was generated by nonlinear evolution of electron beam or Buneman instability [Miyake et al., 2000; Drake et al., 2003]. The Buneman instability has a positive growth rate when the electron ion drift velocity V d is larger than the electron thermal velocity V te [Drake et al., 2003]. In the cavity, the average electric current density 24 na/m 2, plasma density 0.07/cm 3 and electron temperature 2.2 kev, so the drift velocity is estimated as 2200 km/s, much less than V te km/s. Therefore, Buneman instability, suggested to be responsible for the 7of10

8 Figure 8. Relation between density cavity and energetic electron fluxes: (a) magnetic field, (b) spacecraft potential, and (c) energetic electron differential fluxes, with energy range between 37 and 244 kev. The unit of electron differential fluxes is 1/(cm 2 sr kev s). generation of ESWs around density cavity [Cattell et al., 2005], is unlikely be excited inside the cavity. On the other hand, electron beams drifting along magnetic field can excite two stream instability, which results in the formation of ESWs. Owing to ion dynamics, the LHW can be strongly excited owing to coupling with parallel drifting potentials [Miyake et al., 2000]. The possible free energy source of whistler waves is also probably the parallel streaming electron beams since there is no apparent electron temperature anisotropy (T? /T k > 1) associated with whistler waves. Electron beams might be unstable to the electron cyclotron instability to trigger cyclotron waves [Teste and Parks, 2009]. [22] We have to point out that our two dimensional simulation could not reproduce the LHW and whistler wave observed inside the density cavity. Electron beam instability generally propagates parallel to the ambient magnetic field, which has the largest component in the out of plane direction inside the density cavity. Therefore, to fully understand the generation mechanism and feedback of these waves to reconnection process, three dimensional kinetic simulation is required Relationship Between Density Cavity and Energetic Electron Production [23] Drake et al. [2005] suggested that in the presence of guide field, there is parallel electric field inside density cavity along separatrix, and electrons could reach relativistic energies via multiple acceleration by this parallel electric field. Pritchett [2006, 2008] argued that acceleration at X line by inductive electric field is necessary for electrons to become energetic. He proposed that electrons were first preaccelerated by parallel electric field in the cavity to form cold electron beam, then funneled into the X line, where electrons could be further energized by parallel inductive field, and finally ejected out along the other two pairs of separatrices. We are unable to identify the parallel electric field from the Cluster observation, since Cluster EFW instrument only measures two components of electric field on the spin plane. [24] It is evident, from Figure 8, that energetic electron flux decreased in the cavity, while flux increased before and after the spacecraft crossed the cavity. Interestingly, the former flux peak was associated with a magnetic island, which was identified by negative positive bipolar B z component and enhancement of B y component. The structure of this magnetic island and related electron acceleration is not the main concern of this paper and will be further evaluated. The flux decrease inside the cavity might be partially due to total density decrease, thus the power spectrum of electrons inside cavity was compared with that of two other times when Cluster observed similar level of flux as inside cavity. The power law indices of electron spectrums vary little between three different times as shown in Figure 9, which means that electron spectrum inside the cavity is not harder than the other two times. The result implies that there was no significant acceleration occurred at the cavity. Nevertheless, cold electron beam with drift energy of 3 kev were accelerated in parallel direction, suggesting that density cavity could provide a preacceleration for electrons before they reach X line, but unable to produce energetic electrons. [25] Our results are important supplements to the study of Retinò et al. [2006], who described the details of wave and particle dynamics associated with density cavity along separatrix. They identified counterstreaming electron beams in density cavity, as well as electric field turbulence between f lh 8of10

9 [29] Acknowledgments. We thank the Cluster teams and Cluster Active Archive for providing high quality data and successful operation. We are grateful to M. Dunlop for the discussion about the interpretation of FGM and EFW data. The work was supported by the National Natural Science Foundation of China (NSFC) under grants , , , , and and by the Fundamental Research Funds for the Central Universities ( ). [30] Philippa Browning thanks the reviewers for their assistance in evaluating this paper. Figure 9. PSD for high energy electrons at three different times marked by dashed lines in Figure 8c: 2331:34 UT (black line), 2331:50 UT (green line), and 2332:06 UT (red line). Dashed lines are best fit to power laws E g. and f pe. The hot outflowing and cold inflowing electron beams along separatrix are typical signatures of antiparallel reconnection. We showed that electron and wave dynamics inside density cavity along separatrix of reconnection with guide field were different from that in antiparallel reconnection. 5. Summary [26] In this paper, we present an observation of density cavity inside reconnection diffusion region with finite guide field. The cavity was located around separatrix region and corresponded to strong compression of magnetic field. The width of the layer is about 2 ion inertial lengths. The layer contained relative strong antiparallel current, which was mainly contributed by parallel streaming electrons. These features were reproduced by two dimensional electromagnetic PIC simulation with moderate guide field. There are displacements between the positions of electron density minima and out of plane magnetic field maxima both in reconnection with and without guide field. However, the displacement in reconnection with guide field is much less than that in reconnection without guide field, which is a new feature, to our best knowledge, never reported before. [27] There were strong electric field fluctuations located inside the cavity. Polarization analysis shows the wave below lower hybrid frequency is primarily the lower hybrid wave, which is a linear and highly oblique polarized electrostatic wave. Above the lower hybrid frequency, electromagnetic whistler wave was detected. We suggest that these waves were probably excited by parallel streaming electron breams via electron beam instability. [28] There was no significant acceleration for electrons to reach energy larger than 30 kev in the cavity. Cold electron beam was observed in the cavity and typical energy of the beam is several kev. The production of energetic electrons during reconnection will be further studied by combining satellite observation and kinetic simulation. 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