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A particle-in-cell simulation of symmetric reconnection with zero guide field is carried out to understand the dynamics of ions along the separatrices. Through the investigation of ion velocity distributions at different moments and locations along the separatrices, a typical distribution is found: two counter-streaming populations in the perpendicular direction, with another two populations accelerated into distinct energy levels in the parallel direction. Backward tracing of ions reveals that the counter-streaming cores are mostly composed of ions initially located at the same side of the separatrix, while the other two accelerated populations in the parallel direction are composed of ions crossing through the neutral sheet. Through analysis of energy conversion of these populations, it is found that the ion energization along the separatrix is attributable primarily to the Hall electric field, while that in the region between the two separatrices is caused primarily by the induced reconnection electric field. For the counter-streaming population, the low-energy ions that cross the separatrix twice are affected by both Hall and reconnection electric fields, while the high-energy ions that directly enter the separatrix from the unperturbed plasma are energized mainly by the Hall electric field. For the two energized populations in the parallel direction, the ions with lower-energy are accelerated mainly by the in-plane electric field and the Hall electric field on the opposite side of the separatrix, whereas the ions with higher-energy not only experience the same energization process but also are constantly accelerated by the reconnection electric field.

Magnetotail reconnection plays a crucial role in explosive energy conversion in geospace. Because of the lack of in-situ spacecraft observations, the onset mechanism of magnetotail reconnection, however, has been controversial for decades. The key question is whether magnetotail reconnection is externally driven to occur first on electron scales or spontaneously arising from an unstable configuration on ion scales. Here, we show, using spacecraft observations and particle-in-cell (PIC) simulations, that magnetotail reconnection starts from electron reconnection in the presence of a strong external driver. Our PIC simulations show that this electron reconnection then develops into ion reconnection. These results provide direct evidence for magnetotail reconnection onset caused by electron kinetics with a strong external driver., Magnetotail reconnection plays a crucial role in explosive energy conversion in geospace. Here, the authors show that magnetotail reconnection starts from electron reconnection in the presence of a strong external driver, which then develops into ion reconnection.

Magnetic reconnection is a fundamental plasma process, by which magnetic energy is explosively released in the current sheet to energize charged particles and to create bi-directional Alfvénic plasma jets. Numerical simulations predicted that evolution of the reconnecting current sheet is dominated by formation and interaction of magnetic flux ropes, which finally leads to turbulence. Accordingly, most volume of the reconnecting current sheet is occupied by the ropes, and energy dissipation occurs via multiple relevant mechanisms, e.g., the parallel electric field, the rope coalescence and the rope contraction. As an essential element of the reconnecting current sheet, however, how these ropes evolve has been elusive. Here, we present direct evidence of secondary reconnection in the filamentary currents within the ropes. The observations indicate that secondary reconnection can make a significant contribution to energy conversion in the kinetic scale during turbulent reconnection., Magnetic reconnection is a fundamental plasma process of magnetic energy conversion to kinetic energy. Here, the authors show direct evidence of secondary reconnection in the filamentary currents within the flux ropes indicating a significant contribution to energy conversion in the kinetic scale during turbulent reconnection.

How ion escape from the near-Mars space is one of the biggest puzzles for understanding the atmospheric evolution of Mars. Ions in the plasma wake region continuously escape from the unmagnetized planet. Although the average ion escape rate in the wake region is relatively low, observations also have revealed the presence of events that contribute bursty and enhanced ion escape fluxes. Boundary instabilities and magnetic reconnection are suggested to be the candidate mechanisms. However, there is a lack of evaluation of ion escape caused by reconnection and comparison of the two mechanisms under a similar plasma environment. Here, we show an exciting reconnection event in the Martian wake. Two types of flux ropes are observed during the event. One was generated by reconnection, while others were produced by dayside boundary instability and convected to tail. The escape rate of oxygen ions in the reconnection region was estimated to be about 53–72% of the total tailward escape. Furthermore, the escape flux in the flux rope produced by reconnection was over twice that caused by dayside instabilities.

In this paper, we analyze one reconnection event observed by the Magnetospheric Multiscale (MMS) mission at the earth’s magnetopause. In this event, the spacecraft crossed the reconnection current sheet from the magnetospheric side to the magnetosheath side, and whistler waves were observed on both the magnetospheric and magnetosheath sides. On the magnetospheric side, the whistler waves propagated quasi-parallel to the magnetic field and toward the X-line, while on the magnetosheath side they propagated almost anti-parallel to the magnetic field and away from the X-line. Associated with the enhancement of the whistler waves, we find that the fluxes of energetic electrons are concentrated around the pitch angle 90° when their energies are higher than the minimum energy that is necessary for the resonant interactions between the energetic electrons and whistler waves. This observation provides in situ observational evidence of resonant interactions between energetic electrons and whistler waves in the magnetic reconnection.

Flux ropes are ubiquitous at Earth’s magnetopause and play important roles in energy transport between the solar wind and Earth’s magnetosphere. In this paper, structure and coalescence of the magnetopause flux ropes formed by multiple X line reconnection in cases with different southward interplanetary magnetic field (IMF) clock angles are investigated by using three-dimensional global hybrid simulations. As the IMF clock angle decreases from 180°, the axial direction of the flux ropes becomes tilted relative to the equatorial plane, the length of the flux ropes gradually increases, and core field within flux ropes is formed by the increase in the guide field. The flux ropes are formed mostly near the subsolar point and then move poleward towards cusps. The flux ropes can eventually enter the cusps, during which their helical structure collapses, their core field weakens gradually, and their axial length decreases. When the IMF clock angle is large (i.e., the IMF is predominantly southward), the flux ropes can coalesce and form new ones with larger diameter. The coalescence between flux ropes can occur both near the subsolar point when they are newly formed and away from the subsolar point (e.g., in the southern hemisphere) when they move towards cusps. However, when the IMF clock angle is small (≤ 135° ), we do not find coalescence between flux ropes.

The microphysics in the separatrix region (SR) plays an important role for the energy conversion in reconnection. Based on the Magnetospheric Multiscale observations in the magnetotail, we present a complete crossing of the current sheet with ongoing magnetic reconnection. The field‐aligned inflowing electrons were observed in both separatrix regions (SRs) and their energy extended up to several times of the thermal energy. Along the SR, a net parallel electrostatic potential was estimated and could be the reason for the inflowing electron streaming. In the northern SR, the electron frozen‐in condition was violated and nonideal electric field was inferred to be caused by the gradient of the electron pressure tensor. The nongyrotropic electron distribution and significant energy dissipation were observed at the same region. The observations indicate that the inner electron diffusion region can extend along the separatrices or some electron‐scale instability can be destabilized in the SR.

A major subset of solar–terrestrial relations, responsible, in particular, for the driver of space weather phenomena, is the interaction between the Earth's magnetosphere and the solar wind. As one of the most important modes of the solar–wind–magnetosphere interaction, magnetic reconnection regulates the energy transport and energy release in the solar–terrestrial relation. In situ measurements in the near-Earth space are crucial for understanding magnetic reconnection. Past and existing spacecraft constellation missions mainly focus on the measurement of reconnection on plasma kinetic-scales. Resolving the macro-scale and cross-scale aspects of magnetic reconnection is necessary for accurate assessment and predictions of its role in the context of space weather. Here, we propose the AME (self-Adaptive Magnetic reconnection Explorer) mission consisting of a cross-scale constellation of 12+ CubeSats and one mother satellite. Each CubeSat is equipped with instruments to measure magnetic fields and thermal plasma particles. With multiple CubeSats, the AME constellation is intended to make simultaneous measurements at multiple scales, capable of exploring cross-scale plasma processes ranging from kinetic scale to macro scale. publishedVersion

Observations of a current sheet as thin as the electron scale are extremely rare in the near-Earth magnetotail. By measurement from the novel Magnetospheric Multiscale mission in the near-Earth mag ...

It is generally accepted that collisionless magnetic reconnection is initiated on electron scales, which is mediated by electron kinetics. In this paper, by performing a two-dimensional particle-in-cell simulation, we investigate the transition of collisionless magnetic reconnection from electron scales to ion scales in a Harris current sheet with and without a guide field. The results show that after magnetic reconnection is triggered on electron scales, the electrons are first accelerated by the reconnection electric field around the X line, and then leave away along the outflow direction. In the Harris current sheet without a guide field, the electron outflow is symmetric and directed away from the X line along the center of the current sheet, while the existence of a guide field will distort the symmetry of the electron outflow. In both cases, the high-speed electron outflow is decelerated due to the existence of the magnetic field B z , then leading to the pileup of B z . With the increase of B z , the ions are accelerated by the Lorentz force in the outflow direction, and an ion outflow at about one Alfvén speed is at last formed. In this way, collisionless magnetic reconnection is transferred from the electron scales to the ion scales.

In this paper, by performing a two-dimensional particle-in-cell simulation, we investigate magnetic reconnection in the downstream of a quasi-perpendicular shock. The shock is nonstationary, and experiences a cyclic reformation. At the beginning of reformation process, the shock front is relatively flat, and part of upstream ions are reflected by the shock front. The reflected ions move upward in the action of Lorentz force, which leads to the upward bending of magnetic field lines at the foot of the shock front, and then a current sheet is formed due to the squeezing of the bending magnetic field lines. The formed current sheet is brought toward the shock front by the solar wind, and the shock front becomes irregular after interacting with the current sheet. Both the current sheet brought by the solar wind and the current sheet associated with the shock front are then fragmented into many small filamentary current sheets. Electron-scale magnetic reconnection may occur in several of these filamentary current sheets when they are convected into the downstream, and magnetic islands are generated. A strong reconnection electric field and energy dissipation are also generated around the X line, and high-speed electron outflow is also formed.

A flux rope event observed in the magnetotail exhibits a double-peaked core field feature. The generation of such double-peaked feature within the flux rope is explored with Hall-MHD simulations and theoretical analysis based on multiple X-line reconnection. Simulations with a guide field produce flux ropes bounded by two active X lines in the thin current sheet. The guide field, combined with Hall-generated field, leads to a donut-shaped core field (having a double-peaked profile) near the magnetic separatrix. Subsequently, it rotates into the central region of the flux rope, which tends to be the force-free configuration. The analysis shows that there are three major factors affecting the evolution of the core field, including the guide field, convective, and Hall terms originating from the generalized Ohm's law. The convective term can become stronger near the central region of flux rope, and the Hall term dominates the region next to the separatrix during the early stages of the flux rope evolution. It implies that several different factors contribute to the generation of the double-peaked core field. The results may help explain a variety of core fields available in magnetotail flux ropes.

One-dimensional (1-D) hybrid simulations have demonstrated that a quasi-parallel shock is non-stationary and undergoes a reformation process. Recently, two-dimensional (2-D) hybrid simulations have revealed that ripples along the shock front is an inherent property of a quasi-parallel shock. In this paper, we investigate reformation process of a rippled quasi-parallel shock with a 2-D hybrid simulation model. The simulation results show that at a rippled shock, incident particles behave differently and just can be partially reflected at some specific locations along the rippled shock front, and the reflected particles will form an ion beam that moves back to the upstream along the magnetic field. Then, the beam locally interacts with upstream waves, and the waves are enhanced and finally steepen into a new shock front. As the upstream incident plasma moves to the shock front, the new shock front will approach and merge with the old shock front. Such a process occurs only before these locations along the shock front, and after the merging of the new shock front and old shock front is finished, a relatively plane shock front is formed. Subsequently, a new rippled shock front is again generated due to its interaction with the upstream waves, and it will repeat the previous process. In this pattern, the shock reforms itself quasi-periodically, at the same time, ripples can shift along the shock front. The simulations present a more complete view of reformation for quasi-parallel shocks.

Magnetic reconnection is considered to be one important source to produce energetic electrons. In this paper, two-dimensional (2D) particle-in-cell (PIC) simulations are performed to investigate electron acceleration during the merging process of multiple magnetic islands in a current sheet with a strong guide field. Due to the existence of the strong guide field, we can analyze the contributions of the parallel electric field, Fermi, and betatron mechanisms to the electron acceleration with the guiding-center theory. During the coalescence process of magnetic islands, the islands merge each other continuously until only one large island remains. Energetic electrons are generated during such process, and the electrons with sufficient high energy develop a power-law spectrum. We also investigate the dependence of the index of the power-law spectra on the shape of magnetic islands, electron plasma beta, the number of magnetic islands, and the guide field.

A series of magnetic flux ropes embedded in the ion diffusion region of a magnetotail magnetic reconnection event were investigated in this paper. Waves near the lower hybrid frequency were measured within each of the flux ropes and can be associated with the enhancements of energetic electrons in some of the flux ropes. The waves in the largest flux ropes were further explored in more detail. The electrostatic lower hybrid frequency range waves are detected at the edge, while electromagnetic lower hybrid frequency range waves are observed at the center of the flux rope. The electromagnetic waves are right-hand polarized and propagated nearly perpendicular to magnetic field lines, with a wavelength of ion-electron hybrid scale. The observations are analogous to simulations in which the electrostatic lower hybrid waves are confined to the edge of current sheet but can directly penetrate into the current sheet center in the form of the electromagnetic mode. The observations indicate that the electromagnetic lower hybrid frequency range waves can be excited inside magnetic flux ropes.

Merging magnetic flux ropes, which are believed to play an important role in magnetic reconnection, have now been clearly identified. Observations show that coalescence is indeed closely related to reconnection dynamics and also to turbulence.

Reconnection electric field is a key element of magnetic reconnection. It quantifies the change of magnetic topology and the dissipation of magnetic energy. In this work, two-dimensional (2D) particle-in-cell (PIC) simulations are performed to study the growth of the reconnection electric field in the electron diffusion region (EDR) during magnetic reconnection with a guide field. At first, a seed electric field is produced due to the excitation of the tearing-mode instability. Then, the reconnection electric field in the EDR, which is dominated by the electron pressure tensor term, suffers a spontaneous growth stage and grows exponentially until it saturates. A theoretical model is also proposed to explain such a kind of growth. The reconnection electric field in the EDR is found to be directly proportional to the electron outflow speed. The time derivative of electron outflow speed is proportional to the reconnection electric field in the EDR because the outflow is formed after the inflow electrons are accelerated by the reconnection electric field in the EDR and then directed away along the outflow direction. This kind of reinforcing process at last leads to the exponential growth of the reconnection electric field in the EDR.

A large-scale two-dimensional (2-D) particle-in-cell simulation is performed in this paper to investigate electron acceleration in the dipolarization front (DF) region during magnetic reconnection. It is found that the DF is mainly driven by an ion outflow which also generates a positive potential region behind the DF. The DF propagates with an almost constant speed and gets growing, while the electrons in the DF region can be highly energized in the perpendicular direction due to betatron acceleration. For the first time, we reveal that there exists a velocity threshold; only the electrons below the threshold can be trapped by the parallel electric potential in the DF region and then energized by betatron acceleration.

Using the Cluster observation in the magnetotail, we investigate the dynamic processes associated with a bursty bulk flow (BBF) event. The BBF is inferred to be caused by magnetic reconnection proceeding to the lobe region in its tail, called "primary reconnection." On the BBF front, another reconnection was directly encountered by one of the four Cluster satellites, and no signatures of this reconnection were simultaneously measured by the satellite at the plasma sheet boundary. It indicates that this reconnection on the BBF front remained within the plasma sheet, called "secondary reconnection." The secondary reconnection moved earthward and was followed by a magnetic island. A few earthward moving pulses of B-z were detected between the island and the primary reconnection site. These B-z pulses, propagating faster than the island ahead of it, would lead to a more compressed B-z magnetic field in the wake of the island. The observational scenario is in accordance to the model proposed to explain the generation of dipolarization front in simulations. Furthermore, both electrons and ions were significantly accelerated in this process. The mechanism is discussed also.

An in situ measurement at the magnetopause shows that the quadrupole pattern of the Hall magnetic field, which is commonly observed in a symmetric reconnection, is still evident in an asymmetric component reconnection, but the two quadrants adjacent to the magnetosphere are strongly compressed into the electron scale and the widths of the remaining two quadrants are still ion scale. The bipolar Hall electric field pattern generally created in a symmetric reconnection is replaced by a unipolar electric field within the electron-scale quadrants. Furthermore, it is concluded that the spacecraft directly passed through the inner electron diffusion region based on the violation of the electron frozen-in condition, the energy dissipation, and the slippage between the electron flow and the magnetic field. Within the inner electron diffusion region, magnetic energy was released and accumulated simultaneously, and it was accumulated in the perpendicular directions while dissipated in the parallel direction. The localized thinning of the current sheet accounts for the energy accumulation in a reconnection.

We present in situ observation of double layer (DL) and associated electron measurement in the subspin time resolution in the separatrix region during reconnection for the first time. The DL is inferred to propagate away from the X line at a velocity of about ion acoustic speed and the parallel electric field carried by the DL can reach −20 mV/m. The electron displays a beam distribution inside the DL and streams toward the X line with a local electron Alfven velocity. A series of electron holes moving toward the X line are observed in the wake of the DL. The identification of multiple similar DLs indicates that they are persistently produced and therefore might play an important role in energy conversion during reconnection. The observation suggests that energy dissipation during reconnection can occur in any region where the DL can reach.

An electron phase-space hole (electron hole) is considered to be unstable to the transverse instability. In this paper, two-dimensional (2D) electrostatic particle-in-cell (PIC) simulations are used to explore the dissipation process of a one-dimensional (1D) electron hole in a weakly magnetized plasma and its consequence on electron heating, which consists of two stages. In the first stage, the electron hole is still kept as a quasi-1D structure, however, with the excitation of the transverse instability and the generation of the perpendicular electric field, the electrons are scattered and then heated along the perpendicular direction in the electron hole. In the second stage, the quasi-1D electron hole is broken into several 2D electron holes. The temperature of the electrons outside of these 2D electron holes also increase, and at last the velocity distribution of the electrons become almost isotropic in the whole simulation domain. Our results provide a new dissipation mechanism of an electron hole.

[1] We investigate a direct south-north crossing of a reconnection ion diffusion region in the magnetotail. During this crossing, multiple electron density dips with a further density decrease within the cavity, called sub-cavities, adjacent to the northern separatrix are observed. The correlation between electron density sub-cavities and strong electric field fluctuations is obvious. Within one of the sub-cavities, a series of very strong oscillating perpendicular electric field and patchy parallel electric field are observed. The parallel electric field is nearly unipolar and directs away from X line. In the same region, inflow electrons with energy up to 100 keV are injected into the X line. Based on the observations, we conclude that the high-energy inflowing electrons are accelerated by the patchy parallel electric field. Namely, electrons have been effectively accelerated while they are flowing into the X line along the separatrix. The observations indicate that the electron acceleration region is widely larger than the predicted electron diffusion region in the classical Hall magnetic reconnection model.

Electromagnetic compatibility test in reverberation chamber has unique superiority. However, the problem of uncertainty on reverberation chamber testing is focused all the time. In some immunity test, the maximum of electric field is concerned. The standard deviation relative to mean value, the width of the confidence interval and the ratio of maximum and minimum value are used to describe the uncertainty of maximum of electric field. The effects of the number of independent samples on the uncertainty are analyzed. The results show that the uncertainty decreases with the increase of the number of independent samples, but the velocity is slow when the number is big. The ratio of maximum and minimum value can also be used to describe the stirrer efficiency.

In this chapter, we review the progress in understanding the processes near the separatrices during magnetic reconnection. The results are obtained from numerical simulation and spacecraft measurement. The reconnection separatrices represent the surface (cross curve in two-dimensional regime) separating the reconnected magnetic field lines from the reconnecting lines, and thereby connect to the reconnection X-line. The average properties of the particle distribution and physical processes in the separatrix region are summarized. Recent studies confirm that various instabilities occur in the separatrix region and lead to a complex interplay and affecting the plasma in this region. The microphysics in the separatrix region should play an important role in reconnection dynamics. Furthermore, electrons are accelerated up to 100 keV before they enter into the electron diffusion region: a significant part of energy conversion takes place in the separatrix region during reconnection.

Using the high resolution field and plasma data obtained from the Magnetospheric Multiscale mission (MMS) at the magnetopause, a series of three flux transfer events was observed one after another inside southward ion flows, without time gap between any two successive flux ropes. Using the plasma measurements, the current densities within the flux ropes were studied in detail. The currents within the first two flux ropes, dubbed Fr1 and Fr2, was composed of a series of well-separated filamentary currents. The thickness of the filamentary currents and the gap between them were sub-ion scale, occasionally dropped down to electron-scale. In the third flux rope Fr3 which was closest to the expected reconnection X-line, the current displayed a singular compact current layer, was ion scale in width and concentrated on its center. Considering the location of the flux ropes relative to the reconnection X-line, we suggested the current density could be a singular structure when the flux rope was just created and then fragmented into a series of filamentary currents as time. By examining the inter-regions between Fr1 and Fr2, and between Fr2 and Fr3, reconnection was only confirmed to occur between Fr2 and Fr3 and no reconnection signature was found between Fr1 and Fr2. It seems that magnetic field compression resulted from collision of two neighboring flux ropes is one necessary condition for the occurrence of the coalescence.

Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy. It is closely associated with many explosive phenomena in space plasma, usually collisionless in character. For this reason, researchers have become more interested in collisionless magnetic reconnection. In this paper, the various roles of electron dynamics in collisionless magnetic reconnection are reviewed. First, at the ion inertial length scale, ions and electrons are decoupled. The resulting Hall effect determines the reconnection electric field. Moreover, electron motions determine the current system inside the reconnection plane and the electron density cavity along the separatrices. The current system in this plane produces an out-of-plane magnetic field. Second, at the electron inertial length scale, the anisotropy of electron pressure determines the magnitude of the reconnection electric field in this region. The production of energetic electrons, which is an important characteristic during magnetic reconnection, is accelerated by the reconnection electric field. In addition, the different topologies, temporal evolution and spatial distribution of the magnetic field affect the accelerating process of electrons and determine the final energy of the accelerated electrons. Third, we discuss results from simulations and spacecraft observations on the secondary magnetic islands produced due to secondary instabilities around the X point, and the associated energetic electrons. Furthermore, progress in laboratory plasma studies is also discussed in regard to electron dynamics during magnetic reconnection. Finally, some unresolved problems are presented.

In this paper, with a two-dimensional particle-in-cell simulation model, we study the formation of power-law spectra of energetic electrons in multiple X line magnetic reconnection with a strong guide field. The processes of both magnetic reconnection and electron acceleration can be separated into two stages. In the first stage, two X lines appear at the border and center of the simulation domain, and then, two magnetic islands are formed. In this stage, electrons are accelerated mainly by parallel electric fields, and a power-law spectrum of energetic electrons is generated with the appearance of the second X line. In the second stage, the two magnetic islands are merged into one big island. Besides parallel electric fields, the Fermi mechanism also plays an important role in the production of energetic electrons, and its contribution is comparable to that of parallel electric fields when the electron energy is sufficiently large. In this stage, the generated power-law spectrum of energetic electrons becomes hard. In general, the acceleration efficiencies by both the parallel electric fields and Fermi mechanism become higher with the increase in electron energy, and the tendency is more obvious for the Fermi mechanism. Therefore, both the parallel electric fields and Fermi mechanism are important in the formation of power-law spectra of energetic electrons during multiple X line reconnection. We also investigate the influences of the ion-electron temperature ratio, guide field, and initial flux perturbation on the formed power-law spectra of energetic electrons.In this paper, with a two-dimensional particle-in-cell simulation model, we study the formation of power-law spectra of energetic electrons in multiple X line magnetic reconnection with a strong guide field. The processes of both magnetic reconnection and electron acceleration can be separated into two stages. In the first stage, two X lines appear at the border and center of the simulation domain, and then, two magnetic islands are formed. In this stage, electrons are accelerated mainly by parallel electric fields, and a power-law spectrum of energetic electrons is generated with the appearance of the second X line. In the second stage, the two magnetic islands are merged into one big island. Besides parallel electric fields, the Fermi mechanism also plays an important role in the production of energetic electrons, and its contribution is comparable to that of parallel electric fields when the electron energy is sufficiently large. In this stage, the generated power-law spectrum of energetic electrons be...

The nature of magnetic reconnection in planetary magnetospheres may differ between various planets. We report the first observations of a rapidly evolving magnetic reconnection process in Mercury's magnetotail by the MESSENGER spacecraft. The reconnection process was initialized in the plasma sheet and then evolved into the lobe region during a ~35 s period. The tailward reconnection fronts of primary and secondary flux ropes with clear Hall signatures and energetic electron bursts were observed. The reconnection timescale of a few seconds is substantially shorter than that of terrestrial magnetospheric plasmas. The normalized reconnection rate during a brief quasi-steady period is estimated to be ~0.2 on average. The observations show the rapid and impulsive nature of the exceedingly driven reconnection in Mercury's magnetospheric plasma that may be responsible for the much more dynamic magnetosphere of Mercury.

In this paper, by taking advantage of a two-dimensional particle-in-cell simulation model, we study the structure of the out-of-plane magnetic field (Hall magnetic field) during asymmetric magnetic reconnection without a guide field, and the associated in-plane current system is also analyzed. The evolution of asymmetric reconnection has two stages. At the first stage, the electrons move toward the X line along the separatrix in the magnetosheath side, and depart from the X line along the separatrix in the magnetosphere side. Another electron flow toward the X line exists above the separatrix in the magnetosphere side. The resulted in-plane current system, which is mainly determined by electron dynamics, generates the quadrupolar structure of the Hall magnetic field, where the two quadrants in the magnetosheath side are much stronger than those in the magnetosphere side. At the second stage, besides these electron flows, an additional electron flow away from the X line is formed along the magnetic field below the separatrix in the magnetosheath side. A hexapolar structure of the Hall magnetic field is then generated by such a current system.In this paper, by taking advantage of a two-dimensional particle-in-cell simulation model, we study the structure of the out-of-plane magnetic field (Hall magnetic field) during asymmetric magnetic reconnection without a guide field, and the associated in-plane current system is also analyzed. The evolution of asymmetric reconnection has two stages. At the first stage, the electrons move toward the X line along the separatrix in the magnetosheath side, and depart from the X line along the separatrix in the magnetosphere side. Another electron flow toward the X line exists above the separatrix in the magnetosphere side. The resulted in-plane current system, which is mainly determined by electron dynamics, generates the quadrupolar structure of the Hall magnetic field, where the two quadrants in the magnetosheath side are much stronger than those in the magnetosphere side. At the second stage, besides these electron flows, an additional electron flow away from the X line is formed along the magnetic field b...

Onset of a substorm is closely correlated with the current disruption in the near-Earth magnetotail, about 6~8 RE and magnetic reconnection in the middle magnetotail, around 20~30 RE. It is vital to understand the onset of a substorm by studying the time series among polar expansion of aurora, current disruption and magnetic reconnection. In this paper, with measurements from the Cluster spacecraft in the middle magnetotail, LANL-01 and LANL-97 at the synchronous orbit, POLAR in the near-earth magnetotail and IMAGE at polar region, a single substorm event is presented. The analysis indicates that magnetic reconnection is 3 minutes earlier than the current disruption in the near-earth magnetotail. Aurora brightening is observed by IMAGE 4 minutes after the current disruption. Simultaneously, the sharp increment of the AE index implies the substorm onset. The observational result is consistent with the Near Earth Neutral Line model (NENL).

In collisionless reconnection, the magnetic field near the separatrix is stronger than that around the X-line, so an electron-beam can be formed and flows toward the X-line, which leads to a decrease of the electron density near the separatrix. Having been accelerated around the X-line, the electrons flow out along the magnetic field lines in the inner side of the separatrix. A quadruple structure of the Hall magnetic field By is formed by such a current system. A 2D particle-in-cell (PIC) simulation code is used in this paper to study the collisionless magnetic reconnection without an initial guide field. The current system described above is proved by the simulations. Furthermore, the position of the peak of the Hall magnetic field By is found to be between the separatrix and the center of the current sheet, which is verified by Cluster observations.

We report a tailward high speed flow event observed by Cluster during 0203:00UT-0205:30UT on September 20, 2003. Within the flows, a series of three bipolar Bz signatures were observed. The first and third bipolar Bz signatures are identified as magnetic flux ropes while the middle one is found to result from the collision of the two flux ropes. A vertical thin current layer was embedded in the center of the middle bipolar Bz signature. Combining the plasma, electric field and wave data around the thin current layer, we conclude that the two magnetic flux ropes were coalescing. The observations indicate that coalescence of magnetic flux ropes can happen in the regions away from reconnection site, and can produce energetic electrons and waves. A basic criterion for identify the coalescence in the magnetotail is proposed also.

In this paper, with two-dimensional particle-in-cell simulations, we report that the electron Kelvin–Helmholtz instability is unstable in the current layer associated with a large-scale magnetic island, which is formed in multiple X-line guide field reconnections. The current sheet is fragmented into many small current sheets with widths down to the order of the electron inertial length. Secondary magnetic reconnection then occurs in these fragmented current sheets, which leads to a turbulent state. The electrons are highly energized in such a process.

A magnetic reconnection event with a moderate guide field encountered by Cluster in the near-Earth tail on 28 August 2002 is reported. The guide field points dawnward during this event. The quadrupolar structure of the Hall magnetic field within the ion diffusion region is distorted toward the northern hemisphere in the earthward part while toward the southern hemisphere tailward part of X-line. Observations of current density and electron pitch angle distribution indicate that the distorted quadrupolar structure is formed due to a deformed Hall electron current system. Cluster crossed the ion diffusion region from south to north earthward of the X-line. An electron density cavity is confirmed in the northern separatrix layer while a thin current layer (TCL) is measured in the southern separatrix layer. The TCL is formed due to electrons injected into the X-line along the magnetic field. These observations are different from simulation results where the cavity is produced associated with inflow electrons along the southern separatrix while the strong current sheet appears with the outflow electron beam along the northern separatrix. The energy of the inflowing electron in the separatrix layer could extend up to 10 keV. Energetic electron fluxes up to 50 keV have a clear peak in the TCL. The length of the separatrix layer is estimated to be at least 65 c/omega(pi). These observations suggest that electrons could be pre-accelerated before they are ejected into the X-line region along the separatrix. Multiple secondary flux ropes moving earthward are observed within the diffusion region. These secondary flux ropes are all identified earthward of the observed TCL. These observations further suggest there are numerous small scale structures within the ion diffusion region.