The onset of electron-only reconnection

TL;DR

This paper investigates the formation and onset of electron-only magnetic reconnection in low plasma beta conditions, revealing how electron-scale current sheets can rapidly reconnect without ion involvement, unifying different reconnection regimes.

Contribution

It introduces a theoretical framework for electron-only reconnection, deriving conditions for sheet formation and reconnection onset based on plasma parameters and aspect ratios.

Findings

Ideal sheets collapse to sub-ion scales

Reconnection occurs once sheets reach a critical aspect ratio

Electron-only reconnection can happen at a few electron scales

Abstract

Motivated by recent observations of "electron-only" magnetic reconnection, without an ion-scale sheet or ion outflows, in both the Earth's magnetosheath and in numerical simulations, we study the formation and reconnection of electron-scale current sheets at low plasma beta. We first show that ideal sheets collapse to thicknesses much smaller than the ion scales, by deriving an appropriate analogue of the Chapman-Kendall collapse solution. Second, we show that, in practice, reconnection onset happens in these collapsing sheets once they reach a critical aspect ratio, because the tearing instability then becomes faster than their collapse timescale. We show that this can happen for sheet thicknesses larger than the ion scale or at only a few times the electron scale, depending on plasma parameters and the aspect ratio of the collapsing structure, thereby unifying the usual picture of ion-coupled reconnection and the new regime of electron-only reconnection. %and thus the usual picture of ion-coupled reconnection and the new regime of electron-only reconnection can be unified. We derive relationships between plasma beta, ion-to-electron temperature ratio, the aspect ratio, electron outflow velocity, and the final thickness of the sheets, and thus determine under what circumstances electron-scale sheets form and reconnect.