We study electron distribution functions in a diffusion region of antiparallel asymmetric reconnection by means of particle-in-cell simulations and analytical theory. At the electron stagnation ...point, the electron distribution comprises a crescent-shaped population and a core component. The crescent-shaped distribution is due to electrons coming from the magnetosheath toward the stagnation point and accelerated mainly by electric field normal to the current sheet. Only a part of magnetosheath electrons can reach the stagnation point and form the crescent-shaped distribution that has a boundary of a parabolic curve. The penetration length of magnetosheath electrons into the magnetosphere is derived. We expect that satellite observations can detect crescent-shaped electron distributions during magnetopause reconnection.
Abstract Numerous structures conducive to magnetic reconnection are frequently observed in the turbulent regions at quasi-parallel shocks. In this work, we use a particle-in-cell simulation to study ...3D magnetic reconnection in shock turbulence. We identify and characterize magnetic null points, and focus on reconnection along the separator between them. We identify a reconnection region with strong parallel current, a finite parallel potential, and counterrotating electron flows. Electrons are shown to be accelerated by the parallel electric field before being scattered at the null.
Magnetic reconnection in a quasi‐parallel bow shock is investigated with two‐dimensional local particle‐in‐cell simulations. In the shock transition and downstream regions, large amplitude magnetic ...fluctuations exist, and abundant current sheets form. In some current sheets, reconnection occurs, and ion‐scale and electron‐scale magnetic islands are generated. In electron‐scale island regions, only electron outflow jets are observed, producing a quadrupolar out‐of‐plane magnetic field pattern, while in ion‐scale islands, both ions and electrons are involved and energized in reconnection. Normalized reconnection rates are obtained to be between around 0.1 to 0.2, and particle acceleration signatures are seen in distribution functions.
Abstract
We perform a 2.5-dimensional particle-in-cell simulation of a quasi-parallel shock, using parameters for the Earth’s bow shock, to examine electron acceleration and heating due to magnetic ...reconnection. The shock transition region evolves from the ion-coupled reconnection dominant stage to the electron-only reconnection dominant stage, as time elapses. The electron temperature enhances locally in each reconnection site, and ion-scale magnetic islands generated by ion-coupled reconnection show the most significant enhancement of the electron temperature. The electron energy spectrum shows a power law, with a power-law index around 6. We perform electron trajectory tracing to understand how they are energized. Some electrons interact with multiple electron-only reconnection sties, and Fermi acceleration occurs during multiple reflections. Electrons trapped in ion-scale magnetic islands can be accelerated in another mechanism. Islands move in the shock transition region, and electrons can obtain larger energy from the in-plane electric field than the electric potential in those islands. These newly found energization mechanisms in magnetic islands in the shock can accelerate electrons to energies larger than the achievable energies by the conventional energization due to the parallel electric field and shock drift acceleration. This study based on the selected particle analysis indicates that the maximum energy in the nonthermal electrons is achieved through acceleration in ion-scale islands, and electron-only reconnection accounts for no more than half of the maximum energy, as the lifetime of sub-ion-scale islands produced by electron-only reconnection is several times shorter than that of ion-scale islands.
Highly structured electron distribution functions in the electron diffusion region (EDR) during magnetic reconnection are studied by means of fully kinetic simulations. Four types of structures ...(striations, arcs, swirls, and rings) in momentum space are analyzed to understand their formation mechanisms. Discrete striations are formed by particles undergoing different numbers of meandering bounces in the EDR and are a result of oscillations in the out‐of‐plane force on meandering electrons. Predictions for the separation between striations and the triangular shape of the distribution are obtained analytically. Arcs and swirls are due to partial remagnetization of accelerated electrons. Near the end of the outflow jet, electron remagnetization gives rise to the ring structure. Understanding the distribution structures is critical to unraveling the kinetic processes occurring in the EDR and will guide the identification of EDRs based on satellite measurements.
Key PointsOscillating Lorentz force in bounce motion causes striations in distributionsA triangular distribution is formed due to particle motion in the neutral planeArcs, swirls, and rings are formed as electrons are gradually magnetized
Based on particle‐in‐cell simulations of collisionless magnetic reconnection, the spatiotemporal evolution of electron velocity distributions in the electron diffusion region (EDR) is reported to ...illustrate how electrons are accelerated and heated. Approximately when the reconnection rate maximizes, electron distributions in the vicinity of the X line exhibit triangular structures with discrete striations and a temperature (Te) twice that of the inflow region. Te increases as the meandering EDR populations mix with inflowing electrons. As the distance from the X line increases within the electron outflow jet, the discrete populations swirl into arcs and gyrotropize by the end of the jet with Te about 3 times that of the X line. Two dominant processes increase Te and produce the spatially and temporally evolving EDR distributions: (1) electric field acceleration preferential to electrons which meander in the EDR for longer times and (2) cyclotron turning by the magnetic field normal to the reconnection layer.
Key Points
Reconnection electric and normal magnetic fields heat meandering electrons
Distributions evolve spatially and temporally near and after peak reconnection
Triangular, striated distributions can identify the electron diffusion region
Electron distribution functions in the electron diffusion region during symmetric magnetic reconnection are investigated by means of theory and fully kinetic simulations. Crescent‐like striations are ...formed in distribution functions in the velocity plane perpendicular to the magnetic field. Using an analytical current sheet, we solve the equation of motion for electrons, and derive the shape of a crescent distribution, as a function of the distance from the neutral line, field gradients, and the reconnection electric field. Each crescent is tilted in the velocity plane because of the acceleration by the reconnection electric field, and multiple stripes appear due to multiple meandering bounces. Applying the theory to distribution functions observed in Earth's magnetotail, we deduce the amplitude of the reconnection electric field.
Magnetic reconnection has been observed in the transition region of quasi‐parallel shocks. In this work, the particle‐in‐cell method is used to simulate three‐dimensional reconnection in a ...quasi‐parallel shock. The shock transition region is turbulent, leading to the formation of reconnecting current sheets with various orientations. Two reconnection sites with weak and strong guide fields are studied, and it is shown that reconnection is fast and transient. Reconnection sites are characterized using diagnostics including electron flows and magnetic flux transport. In contrast to two‐dimensional simulations, weak guide field reconnection is realized. Furthermore, the current sheets in these events form in a direction almost perpendicular to those found in two‐dimensional simulations, where the reconnection geometry is constrained.
Plain Language Summary
Quasi‐parallel shocks are regions where there is a large angle between the shock surface and the upstream magnetic field. Particles reflected from the shock move upstream and excite waves, creating a turbulent environment. This is favorable for the generation of current sheets and a process called magnetic reconnection, in which magnetic energy is converted to kinetic energy with a change in field topology. We use simulations to study reconnection, characterizing reconnection sites and providing comparisons to other simulations and observations.
Key Points
Turbulent reconnection events in a 3D quasi‐parallel shock simulation are characterized
Both strong and weak guide field events are found, consistent with the range of values seen in observations
Reconnection sites have different 3D orientations not captured by 2D simulations
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