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  1. Abstract

    Magnetic reconnection is a ubiquitous plasma process that transforms magnetic energy into particle energy during eruptive events throughout the universe. Reconnection not only converts energy during solar flares and geomagnetic substorms that drive space weather near Earth, but it may also play critical roles in the high energy emissions from the magnetospheres of neutron stars and black holes. In this review article, we focus on collisionless plasmas that are most relevant to reconnection in many space and astrophysical plasmas. Guided by first-principles kinetic simulations and spaceborne in-situ observations, we highlight the most recent progress in understanding this fundamental plasma process. We start by discussing the non-ideal electric field in the generalized Ohm’s law that breaks the frozen-in flux condition in ideal magnetohydrodynamics and allows magnetic reconnection to occur. We point out that this same reconnection electric field also plays an important role in sustaining the current and pressure in the current sheet and then discuss the determination of its magnitude (i.e., the reconnection rate), based on force balance and energy conservation. This approach to determining the reconnection rate is applied to kinetic current sheets with a wide variety of magnetic geometries, parameters, and background conditions. We also briefly review the key diagnostics and modeling of energy conversion around the reconnection diffusion region, seeking insights from recently developed theories. Finally, future prospects and open questions are discussed.

     
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  2. Abstract

    Energy dissipation in collisionless plasmas is one of the most outstanding open questions in plasma physics. Magnetic reconnection and turbulence are two phenomena that can produce the conditions for energy dissipation. These two phenomena are closely related to each other in a wide range of plasmas. Turbulent fluctuations can emerge in critical regions of reconnection events, and magnetic reconnection can occur as a product of the turbulent cascade. In this study, we perform 2D particle-in-cell simulations of a reconnecting Harris current sheet in the presence of turbulent fluctuations to explore the effect of turbulence on the reconnection process in collisionless nonrelativistic pair plasmas. We find that the presence of a turbulent field can affect the onset and evolution of magnetic reconnection. Moreover, we observe the existence of a scale-dependent amplitude of magnetic field fluctuations above which these fluctuations are able to disrupt the growing of magnetic islands. These fluctuations provide thermal energy to the particles within the current sheet and preferential perpendicular thermal energy to the background population.

     
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  3. Abstract

    The rate of magnetic reconnection is of the utmost importance in a variety of processes because it controls, for example, the rate energy is released in solar flares, the speed of the Dungey convection cycle in Earth’s magnetosphere, and the energy release rate in harmful geomagnetic substorms. It is known from numerical simulations and satellite observations that the rate is approximately 0.1 in normalized units, but despite years of effort, a full theoretical prediction has not been obtained. Here, we present a first-principles theory for the reconnection rate in non-relativistic electron-ion collisionless plasmas, and show that the same prediction explains why Sweet-Parker reconnection is considerably slower. The key consideration of this analysis is the pressure at the reconnection site (i.e., the x-line). We show that the Hall electromagnetic fields in antiparallel reconnection cause an energy void, equivalently a pressure depletion, at the x-line, so the reconnection exhaust opens out, enabling the fast rate of 0.1. If the energy can reach the x-line to replenish the pressure, the exhaust does not open out. In addition to heliospheric applications, these results are expected to impact reconnection studies in planetary magnetospheres, magnetically confined fusion devices, and astrophysical plasmas.

     
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  4. Abstract

    Using combined MHD/test particle simulations, we explore characteristics of ion (proton) acceleration tailward of a near‐Earth reconnection site. We present spatial distributions and explore acceleration mechanisms and sources of accelerated ions. Acceleration is due primarily due simple crossings of the enhanced electric field near the x‐line or in the departing plasmoid. The energetic particle distributions show the expected energy dispersed tailward streaming at the plasma sheet boundary, while equatorial distributions are more complicated, resulting from different acceleration sites within the moving plasmoid. Sources are mostly inside the central plasma sheet dawnward of the plasmoid.

     
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  5. Abstract

    We investigate waves close to the lower‐hybrid frequency in 12 magnetotail reconnection electron diffusion region (EDR) events with guide field levels of near‐zero to 30%. In about half of the events, the wave vector has a small component along the current sheet normal, consistent with known lower‐hybrid drift wave properties, but the perpendicular magnetic field fluctuations can be comparable or greater than the parallel component, a feature unique to the waves inside and adjacent to EDRs. Another new wave property is that the wave vector has a significant component along the current sheet normal in some events and completely along the normal for one event. In 1/4 of the events, theterm has a significant contribution to the wave electric field, possibly a feature of lower‐hybrid waves more likely to exist in the diffusion region than further away from the X‐line. Electron temperature variations are correlated with the wave potential, due to wave electric field acceleration and crossings at the corrugated separatrix region with different amounts of mixing between reconnection inflowing and outflowing populations. The latter also leads to the anti‐correlation between parallel and perpendicular temperature components. Using four‐spacecraft measurements, the magnetic field line twisting is demonstrated by the correlated fluctuations inand. The lower‐hybrid wave in the EDR of weak guide field reconnection may be generated near separatrices and penetrate to the mid‐plane or locally generated, and the latter possibility is beyond the prediction of previous reconnection simulations.

     
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  6. Abstract

    Three‐dimensional X‐line spreading is important for the coupling between global dynamics and local kinetic physics of magnetic reconnection. Using large‐scale 3‐D particle‐in‐cell simulations, we investigate the spreading of the X‐line out of the reconnection plane under a strong guide field in asymmetric reconnection. The X‐line spreading speed depends strongly on the equilibrium current sheet thickness. In a simulation with a thick, ion‐scale equilibrium current sheet (CS), the X‐line spreads at the ambient species drift speeds, which are significantly lower than the Alfvén speed based on the guide field(sub‐Alfvénic spreading). In simulations with a sub‐ion‐scale CS, the X‐line spreads atinstead (Alfvénic spreading). An Alfvénic signal consistent with kinetic Alfvén waves develops and propagates, leading to CS thinning and extending, which ultimately causes reconnection onset. The continuous onset of reconnection along the propagation direction of the signal manifests as Alfvénic X‐line spreading. The strong dependence on the CS thickness of the spreading speeds and the orientation of the X‐line are consistent with the collisionless tearing instability. Our simulations indicate that when the collisionless tearing growth is sufficiently strong in a thinner CS such that, Alfvénic X‐line spreading can effectively take place. Our results compare favorably with a number of numerical simulations and recent magnetopause observations. An important implication of this work is that the magnetopause CS is typically too thick for the X‐line to spread at the Alfvén speed.

     
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  7. Abstract

    The current sheet structure and ion behaviors in a magnetotail reconnection diffusion region are investigated. The multispacecraft analysis suggests a corrugated current sheet structure, interpreted as due to a flapping motion that propagates along geocentric solar magnetospheric along the +ydirection in the Geocentric Solar Magnetospheric (GSM) coordinate. The electric field (E) and ion distributions have similarities with those in a planar current sheet. Energetic ions move along the current direction, suggesting the acceleration by the observed reconnectionEduring the meandering motion. Counterstreaming ions along the current sheet normal suggest the acceleration by the HallEthat is observed to be the dominant component. However, at certain locations,Eand counterstreaming ions significantly deviate from the local normal direction, and more than one pair of counterstreaming populations exist, possibly because the corrugated current sheet enables ions entering the current sheet at different locations with different velocities to mix together.

     
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  8. Abstract

    We report evidence of magnetic reconnection in the transition region of the Earth's bow shock when the angle between the shock normal and the immediate upstream magnetic field is 65°. An ion‐skin‐depth‐scale current sheet exhibits the Hall current and field pattern, electron outflow jet, and enhanced energy conversion rate through the nonideal electric field, all consistent with a reconnection diffusion region close to the X‐line. In the diffusion region, electrons are modulated by electromagnetic waves. An ion exhaust with energized field‐aligned ions and electron parallel heating are observed in the same shock transition region. The energized ions are more separated from the inflowing ions in velocity above the current sheet than below, possibly due to the shear flow between the two inflow regions. The observation suggests that magnetic reconnection may contribute to shock energy dissipation.

     
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