Ohm's Law, the Reconnection Rate, and Energy Conversion in Collisionless Magnetic Reconnection

TL;DR

This review synthesizes recent advances in understanding collisionless magnetic reconnection, focusing on the reconnection electric field, rate, and energy conversion mechanisms based on kinetic simulations and observations.

Contribution

It provides a comprehensive overview of the current understanding of reconnection electric fields, rates, and energy conversion in collisionless plasmas, integrating recent theoretical and observational insights.

Findings

Reconnection electric field breaks the frozen-in flux condition.

Reconnection rate determined by force balance and energy conservation.

Energy conversion processes are analyzed around the diffusion region.

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 of 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.