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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]

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]

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]

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. [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]

It is well known that the parallel cuts of the parallel and perpendicular electric field in electron phase-space holes (electron holes) have bipolar and unipolar structures, respectively. Recently, electron holes in the Earth's plasma sheet have been observed by THEMIS satellites to have detectable fluctuating magnetic field with regular structures. Du et al. () investigated the evolution of a one-dimensional (1D) electron hole with two-dimensional (2D) electromagnetic particle-in-cell (PIC) simulations in weakly magnetized plasma (Ω< ω, where Ω and ω are the electron gyrofrequency and electron plasma frequency, respectively), which initially exists in the simulation domain. The electron hole is unstable to the transverse instability and broken into several 2D electron holes. They successfully explained the observations by THEMIS satellites based on the generated magnetic structures associated with these 2D electron holes. In this paper, 2D electromagnetic particle-in-cell (PIC) simulations are performed in the x- y plane to investigate the nonlinear evolution of the electron two-stream instability in weakly magnetized plasma, where the background magnetic field $(\mathbf{B}_{0} =B_{0}\vec{\mathbf{e}} _{x})$ is along the x direction. Several 2D electron holes are formed during the nonlinear evolution, where the parallel cuts of E and E have bipolar and unipolar structures, respectively. Consistent with the results of Du et al. (), we found that the current along the z direction is generated by the electric field drift motion of the trapped electrons in the electron holes due to the existence of E, which produces the fluctuating magnetic field δB and δB in the electron holes. The parallel cuts of δB and δB in the electron holes have unipolar and bipolar structures, respectively. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the use of floating metal ring (FMR) edge termination (ET) structure to improve breakdown voltage (V BD) 4H-SiC Schottky barrier diodes (SBDs) is reported. Influence of the FMR’s parameters, such as the number and width of metal rings, the space between two neighboring metal rings, etc., on surface electric filed distribution and breakdown voltage were studied. Two-dimensional simulation results reveal that the induced potential voltage on FMR increases with decreasing the distance to the main electrode, which is beneficial to reduce surface electric field intensity. As compared to the diode without ET design, about 142% improvement in V BD of SBDs has been realized in SBDs with a 2-ring FMR ET structure. In experiments, the same Schottky metal including Al, Ti, Ni, and Au has been used for both the main electrode and FMR structure. 4H-SiC SBDs with 1–3 FMRs ET design have been successfully fabricated and V BD in the range of 476–1080V has been achieved. Comparisons between the calculated and experimental results were made and a good agreement has been obtained. [Copyright &y& Elsevier]

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. [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]