Electron-only Reconnection in an Ion-scale Current Sheet at the Magnetopause

S Y Huang,X H Deng,Z G Yuan,L Yu,J Nan,K Jiang,L F Song,Q Y Xiong

doi:10.3847/1538-4357/ac2668

Abstract

Abstract In the standard model of magnetic reconnection, both ions and electrons couple to the newly reconnected magnetic field lines and are ejected away from the reconnection diffusion region in the form of bidirectional burst ion/electron jets. Recent observations propose a new model: electron-only magnetic reconnection without ion coupling in an electron-scale current sheet. Based on the data from the Magnetospheric Multiscale (MMS) mission, we observe a long-extension inner electron diffusion region (EDR) at least 40 d i away from the X-line at the Earth’s magnetopause, implying that the extension of EDR is much longer than the prediction of the theory and simulations. This inner EDR is embedded in an ion-scale current sheet (the width of ∼4 d i, d i is ion inertial length). However, such ongoing magnetic reconnection was not accompanied with burst ion outflow, implying the presence of electron-only reconnection in an ion-scale current sheet. Our observations present a new challenge for understanding the model of standard magnetic reconnection and the electron-only reconnection model in an electron-scale current sheet.

Highlights

Magnetic reconnection is a widespread important physical process that allows the rapid energy conversion of magnetic field into the plasmas, resulting in the particle’s acceleration/heating and the changing of magnetic field topology (Priest et al, 2000; Yamada et al, 2010)
We report a textbook inner electron diffusion region (EDR) emerging in an ion-scale current sheet at the magnetopause boundary layer, which is characterized by superAlfvén electron jet, electron nongyrotropy, and positive energy dissipation, clear parallel electric field
The outer EDR with electron demagnetization and super-Alfvénic outflow jet is identified in the magnetosphere (Xiao et al, 2007; Chen et al, 2019)

Summary

Introduction

Magnetic reconnection is a widespread important physical process that allows the rapid energy conversion of magnetic field into the plasmas, resulting in the particle’s acceleration/heating and the changing of magnetic field topology (Priest et al, 2000; Yamada et al, 2010). We report a textbook inner EDR emerging in an ion-scale current sheet (the thickness up to 4 di) at the magnetopause boundary layer, which is characterized by superAlfvén electron jet, electron nongyrotropy, and positive energy dissipation, clear parallel electric field. This EDR with a thickness of ~0.53 di is extended about 40 di away from the Xline in the downstream of magnetic reconnection but without burst ion outflow. Our observations demonstrate a new feature of reconnection in space, i.e., electron-only reconnection in the ion-scale current sheet, which is different from the traditional magnetic reconnection model, and challenges the previous observations as well

Event overview

Conclusion