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Magnetic reconnection is a fundamental plasma process by which magnetic field lines on two sides of the current sheet flow inward to yield an X-line topology. It is responsible for producing energetic electrons in explosive phenomena in space, astrophysical, and laboratorial plasmas. The X-line region is supposed to be the important place for generating energetic electrons. However, how these energetic electrons are generated in such a limited region is still poorly understood. Here, using Magnetospheric multiscale mission data acquired in Earth's magnetotail, we present direct evidence of super-thermal electrons up to 300 keV inside an X-line region, and the electrons display a power-law spectrum with an index of about 8.0. Concurrently, three-dimensional network of dynamic filamentary currents in electron scale is observed and leads to electromagnetic turbulence therein. The observations indicate that the electrons are effectively accelerated while the X-line region evolves into turbulence with a complex filamentary current network. Magnetotail reconnection plays an important role in explosive energy conversion. Here, the authors show direct evidence of super-thermal electrons up to 300 keV within X-line region in Earth's magnetotail, indicating effective electron acceleration due to turbulence. [ABSTRACT FROM AUTHOR]

In this paper, by performing two‐dimensional (2‐D) particle‐in‐cell simulations of collisionless magnetic reconnection in a Harris current sheet, we analyze the formation of electron inflow along the separatrices toward the X‐line from the perspective of fluid. The results show that both the parallel electric field and mirror force can drive the electron inflow to stream toward the X‐line. The speed of the electron inflow can reach about 4VA(where VA is the Alfvén speed based on the upstream asymptotic magnetic field magnitude and peak density in the current sheet). Although the contribution of the parallel electric field is much larger than that of the mirror force, the mirror force cannot be ignored. When the electron inflow streams toward the X‐line, it is heated in the parallel direction. The resulted gradient of the parallel electron pressure leads to the attenuation of the electron inflow. We also investigate the effects of the density and temperature of the background plasma on the formation of the electron inflow. With the decrease of the background plasma density, the contribution of the parallel electric field becomes larger while that of the mirror force almost remains unchanged. With the decrease of the background plasma temperature, the contribution of the parallel electric field becomes larger while that of the mirror force becomes smaller. Plain Language Summary: Magnetic reconnection is a universal process in which the topology of the magnetic field changes, accompanied by plasma heating and particle acceleration. During reconnection, electrons moving along the separatrices toward the X‐line form the electron inflow. The electron inflow has been confirmed in simulations and observations, but its generation mechanism remains unclear. This paper focuses on the dynamics of inflow electrons along the separatrices during magnetic reconnection, and quantitatively analyzes the formation of electron inflow. We found that the formation of electron inflow is contributed by three effects: accelerated by the parallel electric field, accelerated by the magnetic mirror force, and decelerated by the gradient of parallel electron pressure. The acceleration contribution of the parallel electric field is dominant, but that of mirror force cannot be ignored, especially when the background plasma density and temperature are sufficiently high. Key Points: Electron inflow is accelerated by parallel electric field and mirror force and decelerated by the gradient of parallel electron pressureParallel electric field plays a major role in the formation of electron inflow, while the contribution of mirror force cannot be ignoredThe contributions of parallel electric field and mirror force change with the background plasma density and temperature [ABSTRACT FROM AUTHOR]

In this study, we report observations from the Magnetospheric Multiscale (MMS) mission of large‐scale parallel electric fields in a magnetosheath reconnecting current sheet. The four MMS satellites successively crossed an electron jet embedded in a broad current sheet and detected a unipolar E|| being colocated with the jet. The strong electron pressure anisotropy inside the broad current sheet suggests that the jet could be generated by the firehose instability. The observations show that the large‐amplitude E|| can fill the entire electron diffusion region (EDR) in the magnetosheath reconnection and thus dominates electron acceleration therein, dramatically different from EDRs in the magnetotail and magnetopause where large‐amplitude unipolar E|| was rarely detected. The appearance of this E|| could be due to the strong guide field which is common in the magnetosheath. Plain Language Summary: Magnetic reconnection is a fundamental physical process in space and astrophysical plasmas in which magnetic energy is converted into plasma kinetic energy and heat. Magnetic reconnection is believed to be initiated in a small‐scale electron diffusion region (EDR) where electrons are decoupled from the magnetic field lines. Recent spacecraft observations in Earth's magnetosheath have revealed a new form of magnetic reconnection without ion coupling in electron‐scale current sheets. The detailed properties of the EDR in these electron‐only reconnection events have been scarce. In this letter, we report Magnetospheric Multiscale mission observations of large‐amplitude and unipolar parallel electric fields being colocated with the EDR of a magnetosheath reconnecting current sheet. The parallel electric field has a large spatial extent that fills the entire EDR. This parallel electric field generates a potential drop of 120 V, which can accelerate electrons passing through the EDR. Altogether, our results suggest that large‐scale parallel electric fields can dominate the electron dynamics in the EDR of magnetosheath electron‐only reconnection events that usually have large guide fields. Key Points: A strong electron jet was detected in a broad current sheet with a significant pressure anisotropyThe electron jet was identified as an extended electron diffusion region and large‐scale parallel electric field was observed inside itParallel electric field generates a potential of 120 V and dominates electron acceleration therein [ABSTRACT FROM AUTHOR]

When the interplanetary magnetic field (IMF) is northward, magnetic reconnection between the IMF and the geomagnetic field can occur at the high latitude magnetopause, which allows energy transfer from the solar wind into Earth's magnetosphere. In this letter, by using three‐dimensional global hybrid simulations, we find that during the northward IMF, high latitude magnetic reconnection both poleward and equatorward of the cusp can occur almost simultaneously. High latitude magnetopause flux ropes (HLMFRs) with four topologies of magnetic field lines can be formed by multiple X‐line reconnection. At the center of the HLMFRs, there is a decrease in the surrounding magnetic field magnitude (weak core field) and an increase in parallel temperature. When the IMF is purely northward, the HLMFRs are located at the noon‐midnight meridian plane. When the IMF has a positive By component, the HLMFRs in the northern hemisphere shift duskward. Plain Language Summary: When the interplanetary magnetic field (IMF) is southward, magnetic reconnection between the IMF and the Earth's magnetic field can occur at the low latitude magnetopause, and flux ropes are formed. However, when the IMF is northward, the reconnection site is at high latitude magnetopause. In many previous studies, the high latitude magnetopause reconnection with the northward IMF was considered as no flux rope is formed. In this letter, we use computer simulations to investigate the high latitude magnetopause reconnection during the northward IMF in different directions. When the IMF is northward, the high latitude magnetopause flux ropes (HLMFRs) with four types of magnetic field lines are formed. At the center of the HLMFRs, there is a decrease in the surrounding magnetic field magnitude, an increase in parallel temperature. Compared with the flux ropes formed during the southward IMF, the HLMFRs are fewer and smaller. When the IMF is purely northward, the HLMFRs are located at the noon‐midnight meridian plane; when the IMF rotates to the duskward (but still northward), the HLMFRs in the northern hemisphere shift duskward. Key Points: High latitude magnetopause reconnection with different northward IMF clock angles is studied by using 3‐D global hybrid simulationsHigh latitude magnetopause flux ropes (HLMFRs) with four topologies of magnetic field lines are formed by multiple X‐line reconnectionIMF clock angle effects where the HLMFRs occurs and the direction of the HLMFRs' axis [ABSTRACT FROM AUTHOR]

Collisionless magnetic reconnection occurs ubiquitously in space plasma environments and plays an important role in energy conversion therein. In collisionless magnetic reconnection, the reconnection site is usually unsteady and ejects reconnection fronts away from it. Using two‐dimensional particle‐in‐cell simulations, we study the energy budgets from the collisionless magnetic reconnection site to reconnection fronts. It is concluded that the reconnection rate cannot well reflect energy conversion of nonsteady state magnetic reconnection because energy conversion occurs predominantly at the reconnection fronts, whereas the reconnection rate can only represent the energy conversion at the reconnection site. We clarify the connection between the reconnection site and the reconnection fronts in terms of energy conversion. The reconnection site functions as a trigger and energy source that generates the outflow of Poynting flux, bulk kinetic energy flux, and enthalpy flux forming the reconnection fronts that move downstream. The well‐developed reconnection fronts are no longer related to the reconnection site. The energy income at the reconnection fronts is mainly the Poynting flux from their top and bottom boundaries, most of which is transformed to P¯⋅V flux flowing downstream out of the moving front through the work by the electric field. The work done by the electric force is compensated with the work done by the thermal pressure gradient, which guarantees that the released magnetic energy is mostly converted to thermal energy. Key Points: Energy budgets from collisionless magnetic reconnection site to reconnection fronts are studied through two‐dimensional particle‐in‐cell simulationsThe reconnection fronts that dominate the energy conversion are generated from the reconnection siteEnergy conversion at the well‐developed reconnection front is unrelated to the reconnection site. The Poynting flux inflow is mainly converted to enthalpy flux [ABSTRACT FROM AUTHOR]

A variety of plasma waves have been observed in the separatrix region during magnetic reconnection, and their roles on reconnection and exciting mechanisms are still controversial. In this paper, we report the whistler wave with a very narrow frequency band just above half of the electron cyclotron frequency in the separatrix region. This whistler wave is accompanied with the electron cyclotron harmonic (ECH) waves. The ECH waves have frequencies between the first and fifth harmonic of electron cyclotron frequency fce. The wave normal angle of the whistler waves is ∼20°–∼40°, meaning the quasi‐parallel propagation, while the ECH waves propagate perpendicular to the background magnetic field with θ∼87°±2°. In the separatrix region, the electrons have a loss‐cone distribution while these two kinds of waves are observed. Based on the linear theory, such an electron distribution is unstable for both whistler waves and ECH waves. It indicates that these two kinds of waves are generated by the electron loss‐cone instability. Key Points: A very narrow band whistler wave and electron cyclotron harmonic waves are simultaneously observed in the magnetosphere separatrix regionThe electron loss‐cone distributions are continuously detected while the two kinds of waves are observedThe two kinds of waves are probably excited by the electron loss‐cone instability [ABSTRACT FROM AUTHOR]

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 cyclic reformation. At the beginning of the reformation process, the shock front is relatively flat, and part of the upstream ions are reflected by the shock front. The reflected ions move upward in the action of the Lorentz force, which leads to the upward bending of the 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 carried 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 a high-speed electron outflow is also formed. [ABSTRACT FROM AUTHOR]

During magnetic reconnection, such as in the magnetopause of magnetized planets, the upstream plasma conditions between the two inflow regions are usually different. In this paper, we demonstrate that such a kind of asymmetric reconnection can be studied in the laboratory using the recently proposed experimental scheme where reconnection is driven by laser-powered capacitor coils. Two-dimensional particle-in-cell simulations on the plane in a cylindrical coordinate are conducted to study magnetic reconnection with the inflow along the direction. Magnetic reconnection is found to be asymmetric with a stronger magnetic field in the inner (small) inflow region and a weaker magnetic field in the outer (large) inflow region due to the cylindrical symmetric geometry. Electron crescent velocity distributions are observed near the flow stagnation point while ion crescent velocity distributions are observed in the region with Larmor electric field. The out-of-plane Hall magnetic field is asymmetric between the two inflow regions with a larger spatial scale in the outer inflow region. This asymmetric Hall magnetic field configuration is different from that in previous studies. The typical reconnection rate increases with a stronger driver and the highest rate is around 0.2 , where is the typical value of the magnetic field and is the Alfven speed. This study provides a new method to experimentally study asymmetric reconnection in the laboratory and has potential applications regarding magnetic reconnection in the magnetopause of magnetized planets. [ABSTRACT FROM AUTHOR]

By comparing an electron‐only reconnection event with a traditional reconnection event observed in the magnetotail, we illustrate the differences between and similarities of the two events. The electron behaviors are very similar in both events, but intensities of the electron flows and temperature in the traditional reconnection are much stronger than those in the electron‐only reconnection. The Hall electric field in the traditional reconnection occurs on the ion‐scale and is deflected from the normal direction by the significant magnetic field reconnected, while this field varies on the electron‐scale and points to the middle plane in the electron‐only reconnection. The comparison indicates that the electrons are undergoing the same process in both events, and the electron‐only reconnection was prior to the traditional reconnection. The Hall electric field could control the form of reconnection: producing either electron‐only reconnection or traditional reconnection. Key Points: We illustrate the differences between and similarities of the electron‐only reconnection and the normal reconnectionThe electron reconnection was prior to the normal reconnectionThe Hall electric field and the reconnected magnetic field intensity could control the form of reconnection [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Using measurements by the Magnetospheric Multiscale spacecraft in the magnetotail, we studied electron distribution functions across an electron diffusion region. The dependence of the nongyrotropic distribution on the energy and vertical distance from the electron diffusion region midplane was revealed for the first time. The nongyrotropic distribution was observed everywhere except for an extremely narrow layer right at the electron diffusion region midplane. The energy of the nongyrotropic distribution increased with growth of the vertical distance from the midplane. For the electrons within certain energy range, they exhibited the nongyrotropic distribution at the distance further away from the midplane than that expected from the meandering motion. The correlation between the crescent‐shaped distribution with multiple stripes and the large Hall electric field was established. It appears that the measured nongyrotropic distribution and the crescent‐shaped distribution were caused by the meandering motion and the Hall electric field together. Plain Language Summary: The Magnetospheric Multiscale mission is designed to study electron physics of magnetic reconnection, a key process for many explosive phenomena in solar flares and magnetosphere. Understanding electron motion is highly important in the study of magnetic reconnection, and the electron velocity distribution is an intuitive and effective way to study the electron dynamics. In this paper, we present a complete electron diffusion region crossing by Magnetospheric Multiscale during a nearly symmetric magnetic reconnection in the magnetotail and show the evolution of the electron velocity distributions across the electron diffusion region. In addition, the relationship between Hall electric field and crescent‐shaped electron distribution is also shown in this paper. Key Points: The electron nongyrotropic distribution depended on the energy and the normal distance from the midplane of the electron diffusion regionThe electron crescent distributions were caused by the meandering motion and the Hall electric field togetherThe fine substructures were observed in the narrow electron diffusion region [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The microphysics in the separatrix region (SR) plays an important role for the energy conversion in reconnection. In this letter, we present nonideal electric field in the SR and the electron acceleration therein. These observations indicate that a significant part of energy conversion takes place in the SR during reconnection. Key Points: Nonideal electric field was observed in the separatrix region and inferred to be caused by gradient of the electron pressure tensorThe nongyrotropic electron distribution and substantial energy dissipation were observed in the separatrix regionAsymmetry between two sides of the current sheet was observed in the symmetric reconnection [ABSTRACT FROM AUTHOR]

With the measurements of the Magnetospheric Multiscale mission at the magnetopause, we investigated the electron distribution and the whistler waves associated with a series of six ion‐scale flux transfer events (FTEs). Based on the magnetic field signature, each FTE can be divided into the core region and the draping region. In the draping regions of the most FTEs, the low‐energy electrons displayed a bidirectional field‐aligned distribution. The medium‐energy electrons showed a field‐aligned or beam distribution in the leading part, while a pancake distribution was presented for the electrons in the trailing part of the draping region, which has never been reported previously. The close correlation between the pancake distribution and the compression of the localized magnetic field suggests that the pancake distribution may be due to the betatron acceleration. The whistler waves associated with the FTEs were observed and categorized into the lower and upper bands according to the frequency range. The lower band whistler waves propagated in variable directions and therefore could be generated locally. The trailing part of the draping region with the electron pancake distribution was considered to be one possible source region. On the contrary, the upper band whistler waves were all found in the core region and propagated antiparallel to the magnetic field and therefore originated from the same source region. The observations confirmed that the FTEs are important channels for the mass and wave transport between the magnetosheath and the inner magnetosphere, and the electron dynamics can be modified during the FTE evolution. Key Points: The electron distribution and whistler wave characteristics are significantly different in the core and draping regions of the FTEsThe close correlation between the pancake distribution and compression of the localized magnetic field is observed in the trailing partThe whistler waves associated with the FTEs are categorized into the lower and upper bands according to the frequency range [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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 magnetotail, we identified such an electron‐scale current sheet and determined its detailed properties. The electron current sheet was bifurcated, with a half‐thickness of nine electron inertial lengths, and was sandwiched between the Hall field. Because of the strong Hall electric field, the super‐Alfvénic electron bulk flows were created mainly by the electric field drift, leading to the generation of the strong electron current. Inevitably, a bifurcated current sheet was formed since the Hall electric field was close to zero at the center of the current sheet. Inside the electron current sheet, the electrons were significantly heated while the ion temperature showed no change. The ions kept moving at a low speed, which was not affected by this electron current sheet. The energy dissipation was negligible inside the current sheet. The observations indicate that a thin current sheet, even as thin as electron scale, is not the sufficient condition for triggering bursty reconnection. [ABSTRACT FROM AUTHOR]

Using the high-resolution field and plasma data obtained from the Magnetospheric Multiscale mission 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, were 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 that 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 interregions 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. [ABSTRACT FROM AUTHOR]

Recently, magnetic reconnection has been realized in high-energy-density laser-produced plasmas. Plasma bubbles with self-generated magnetic fields are created by focusing laser beams to small-scale spots on a foil. The bubbles expand into each other, which may then drive magnetic reconnection. The reconnection experiment in laser-produced plasmas has also been conducted at Shenguang-II (SG-II) laser facility, and the existence of a plasmoid was identified in the experiment [Dong et al., Phys. Rev. Lett. 108, 215001 (2012)]. In this paper, by performing two-dimensional (2-D) particle-in-cell simulations, we investigate such a process of magnetic reconnection based on the experiment on SG-II facility, and a possible explanation for the formation of the plasmoid is proposed. The results show that before magnetic reconnection occurs, the bubbles squeeze strongly each other and a very thin current sheet is formed. The current sheet is unstable to the tearing mode instability, and we can then observe the formation of plasmoid(s) in such a multiple X-lines reconnection. [ABSTRACT FROM AUTHOR]

By performing two-dimensional particle-in-cell simulations, we investigate the transfer between electron bulk kinetic and electron thermal energy in collisionless magnetic reconnection. In the vicinity of the X line, the electron bulk kinetic energy density is much larger than the electron thermal energy density. The evolution of the electron bulk kinetic energy is mainly determined by the work done by the electric field force and electron pressure gradient force. The work done by the electron gradient pressure force in the vicinity of the X line is changed to the electron enthalpy flux. In the magnetic island, the electron enthalpy flux is transferred to the electron thermal energy due to the compressibility of the plasma in the magnetic island. The compression of the plasma in the magnetic island is the consequence of the electromagnetic force acting on the plasma as the magnetic field lines release their tension after being reconnected. Therefore, we can observe that in the magnetic island the electron thermal energy density is much larger than the electron bulk kinetic energy density. [ABSTRACT FROM AUTHOR]

Recent experiments have observed magnetic reconnection in laser-produced high-energy-density (HED) plasma bubbles. We perform two-dimensional (2-D) particle-in-cell (PIC) simulations to investigate magnetic reconnection between two approaching HED plasma bubbles. It is found that the expanding velocity of the bubbles has a great inñuence on the process of magnetic reconnection. When the expanding velocity is small, a single X line reconnection is formed. However, when the expanding velocity is sufficiently large, we can observe a plasmoid in the vicinity of the X line. At the same time, the structures of the electromagnetic field in HED plasma reconnection are similar to that in Harris current sheet reconnection. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) particle-in-cell simulations are performed to investigate the structures of the out-of-plane magnetic field in magnetic island, which is produced during anti-parallel collisionless magnetic reconnection. Regular structures with alternate positive and negative values of the out-of-plane magnetic field along the x direction are formed in magnetic island. The generation mechanism of such structures is also proposed in this paper, which is due to the Weibel instability excited by the temperature anisotropy in magnetic island. [ABSTRACT FROM AUTHOR]

As observation have shown, magnetic islands often appear before and/or after the onset of magnetic reconnections in the current sheets, and they also appear in the current sheets in the solar corona, Earth's magnetotail, and Earth's magnetopause. Thus, the existence of magnetic islands can affect the initial conditions in magnetic reconnection. In this paper, we propose a model of quasi-static magnetic island eigenmodes in the current sheet. This model analytically describes the magnetic field structures in the quasi-static case, which will provide a possible approach to reconstructing the magnetic structures in the current sheet via observation data. This model is self-consistent in the kinetic theory. Also, the distribution function of charged particles in the magnetic island can be calculated. [ABSTRACT FROM AUTHOR]

Satellite observations with high‐resolution measurements have demonstrated the existence of intermittent current sheets and occurrence of magnetic reconnection in a quasi‐parallel magnetosheath behind the terrestrial bow shock. In this letter, by performing a three‐dimensional global hybrid simulation, we investigated the characteristics of the quasi‐parallel magnetosheath of the bow shock, which is formed due to the interaction of the solar wind with the Earth's magnetosphere. Current sheets with widths of several ion inertial lengths are found to be produced in the magnetosheath after the upstream large‐amplitude electromagnetic waves penetrate through the shock and are then compressed in the downstream. Magnetic reconnection consequently occurs in these current sheets, where high‐speed ion flow jets are identified in the outflow region. Simultaneously, flux ropes with the extension (along the y direction) of about several Earth's radii are also observed. Our simulation shed new insight on the mechanism for the occurrence of magnetic reconnection in the quasi‐parallel shocked magnetosheath. Plain Language Summary: Benefited from the high‐resolution measurements provided by Cluster and Magnetospheric Multiscale missions, there are abundant evidences demonstrating the existence of intermittent current sheets in the terrestrial magnetosheath downstream of a quasi‐parallel shock, where magnetic reconnection can occur. However, the process for the formation of these current sheets and occurrence of magnetic reconnection is still unknown. In this letter, the interaction between the high‐speed solar wind and the terrestrial magnetosphere is studied with the help of a three‐dimensional global hybrid simulation. The bow shock is found to be formed in front of the magnetosphere, and in the subsolar and north parts of the bow shock, the shock is quasi‐parallel. In the upstream of the quasi‐parallel shock, there exists plenty of large‐amplitude low‐frequency electromagnetic waves that are convected toward the shock by the solar wind. These waves then penetrate through the shock, and at last current sheets with widths of about several ion inertial lengths are formed in the downstream after these waves are highly compressed by the shock. There are strong evidences supporting the occurrence of magnetic reconnection in these current sheets: the generation of flux ropes, the existence of the reconnection electric field and energy dissipation around the X line, and high‐speed plasma flow in the outflow region. The flux ropes have a helical structure of magnetic field lines, and their extension along the y direction is about several Earth's radii. Key Points: We describe the process for the formation of current sheets and occurrence of reconnection in the quasi‐parallel shocked magnetosheathThe downstream current sheets are formed after the upstream waves penetrate through the shock and are then compressedThe evidences of reconnection in the downstream include the formation of flux ropes and high‐speed flows in the outflow region [ABSTRACT FROM AUTHOR]

We first perform a two-dimensional particle-in-cell simulation of anti-parallel magnetic reconnection to verify that in the electron diffusion region the reconnection electric field is mainly balanced by the gradient of the electron pressure. Then, by following typical electron trajectories in the fixed electromagnetic field of anti-parallel reconnection, we calculate the gradient of the electron pressure. We find that the resulted gradient of the electron pressure is equal to the reconnection electric field. This indicates that in the electron diffusion region the reconnection electric field is balanced by the gradient of the electron pressure, which results from the electron nongyrotropic motions. Our result gives a microphysical explanation of the balance between the reconnection electric field and the gradient of the electron pressure. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is one of the most important processes in astrophysical, space, and laboratory plasmas, and magnetic island is an important feature in reconnection. Therefore, identifying the structures of magnetic island is crucial to improving our understanding of magnetic reconnection. Using two-dimensional (2-D) particle-in-cell (PIC) simulations, we demonstrate that the out-of-plane magnetic field has a dip in the center of magnetic island, which is formed during multiple X line guide field reconnection. Such structures are considered to be produced by the current system in the magnetic island. At the edge of the magnetic island, there exists a current anti-parallel to the in-plane magnetic field, while the current is parallel to the in-plane magnetic field inside the magnetic island. Such a dual-ring current system, which is attributed to the electron dynamics in the magnetic island, leads to the dip of the out-of-plane magnetic field in the center of the island. The relevance between our simulations and crater flux transfer events (C-FTEs) is also discussed. [ABSTRACT FROM AUTHOR]

We use a 2-D hybrid numerical simulation to study velocity-driven magnetic reconnection in low-beta plasmas. Our results show that Alfven waves can be produced in the process. Under their action newborn ions undergo pitch-angle scattering and assume a spherical shell distribution. A part of the ions are accelerated to a maximum energy of about 4(miV2A0/2). The acceleration lasts over a time scale of 100/Ωi, and is extremely rapid. The energy spectrum of the accelerated ions is a double power-law spectrum. [Copyright &y& Elsevier]