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Abstract Electron-only magnetic reconnection was first detected by the Magnetospheric Multiscale (MMS) mission in Earth’s turbulent magnetosheath. Its prevalence in kinetic-scale turbulence has attracted great interest in heliophysics, but also revealed a great challenge in identifying it in turbulence, where electron flows are often complex. The magnetic flux transport (MFT) method is an innovative method to identify active reconnection in numerical simulations and in situ observations of turbulent plasmas. Here we extend this method to distinguish between electron-only and ion-coupled reconnection. The coupling of magnetic field motion with plasma flows in the diffusion regions sets distinct scales in the MFT velocity. While both forms of reconnection satisfy the MFT signature for active reconnection as MFT inflows and outflows at an X-line, the specific electron-only MFT signature is only an electron-scale MFT outflow along the current sheet normal direction, whereas the specific ion-coupled signature is a two-scale, outer-ion-and-inner-electron-scale MFT outflow in the electron diffusion region, which evolves into a single ion-scale in the ion diffusion region. These signatures are verified in a simulation of gyrokinetic turbulence. The dependence of the MFT outflow on the distance downstream from the X-lines also agrees well with the framework of magnetic field–plasma flow coupling. The new MFT signatures provide a clear and reliable tool for investigating electron-only reconnection in turbulence, independent of the development of electron outflows. They are directly applicable to kinetic and fluid simulations, and have potential application to observations of diffusion region crossings by spacecraft missions such as MMS. 

Abstract “Electron-only” reconnection, which is both uncoupled from the surrounding ions and much faster than standard reconnection, is arguably ubiquitous in turbulence. One critical step to understanding the rate in this novel regime is to model the outflow speed that limits the transport of the magnetic flux, which is super ion Alfvénic but significantly lower than the electron Alfvén speed based on the asymptotic reconnecting field. Here we develop a simple model to determine this limiting speed by taking into account the multiscale nature of reconnection, the Hall-mediated electron outflow speed, and the pressure buildup within the small system. The predicted scalings of rates and various key quantities compare well with fully kinetic simulations and can be useful for interpreting the observations of NASA’s Magnetospheric-Multiscale (MMS) mission and other ongoing missions. 

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. 

Abstract Magnetic reconnection occurs ubiquitously in the universe and is often invoked to explain fast energy release and particle acceleration in high-energy astrophysics. The study of relativistic magnetic reconnection in the magnetically dominated regime has surged over the past two decades, revealing the physics of fast magnetic reconnection and nonthermal particle acceleration. Here we review these recent progresses, including the magnetohydrodynamic and collisionless reconnection dynamics as well as particle energization. The insights in astrophysical reconnection strongly connect to the development of magnetic reconnection in other areas, and further communication is greatly desired. We also provide a summary and discussion of key physics processes and frontier problems, toward a better understanding of the roles of magnetic reconnection in high-energy astrophysics. 

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. 

Abstract The scaling of the relativistic reconnection outflow speed is studied in the presence of both shear flows parallel to the reconnecting magnetic fields and guide fields pointing out of the reconnection plane. In nonrelativistic reconnection, super-Alfvénic shear flows have been found to suppress reconnection. We extend the analytical model of this phenomenon to the relativistic regime and find similar behavior, which is confirmed by particle-in-cell simulations. Unlike the nonrelativistic limit, the addition of a guide field lowers the in-plane Alfvén velocity, contributing to slower outflow jets and the more efficient suppression of reconnection in strongly magnetized plasmas. 

Abstract The past decade has seen an outstanding development of nonthermal particle acceleration in magnetic reconnection in magnetically dominated systems, with clear signatures of power-law energy distributions as a common outcome of first-principles kinetic simulations. Here we propose a semianalytical model for systematically investigating nonthermal particle acceleration in reconnection. We show particle energy distributions are well determined by particle injection, acceleration, and escape processes. Using a series of kinetic simulations, we accurately evaluate the energy- and time-dependent model coefficients. The resulting spectral characteristics, including the spectral index and lower and upper bounds of the power-law distribution, agree well with the simulation results. Finally, we apply the model to predict the power-law indices and break energies in astrophysical reconnection systems. 

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.