[Paper Review] Electron-only Reconnection in Ion-scale Current Sheet at the Magnetopause

S. Y. Huang, Q. Y. Xiong+6 more|arXiv (Cornell University)|Sep 27, 2021

Ionosphere and magnetosphere dynamics36 references46 citations

TL;DR

This study presents the first in-situ observation of an electron-only magnetic reconnection event in an ion-scale current sheet at Earth's magnetopause, using high-resolution MMS data. Despite a current sheet thickness of ~4 ion inertial lengths (di), the inner electron diffusion region (EDR) extended at least 40 di downstream from the X-line without burst ion outflow, indicating energy dissipation and electron acceleration without ion participation, challenging standard reconnection models.

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 and electron jets. Recent observations propose a new model: electron only magnetic reconnection without ion coupling in electron scale current sheet. Based on the data from Magnetospheric Multiscale (MMS) Mission, we observe a long extension inner electron diffusion region (EDR) at least 40 di away from the X line at the terrestrial 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 di, di is ion inertial length). However, such ongoing magnetic reconnection was not accompanied with burst ion outflow, implying the presence of electron only reconnection in ion scale current sheet. Our observations present new challenge for understanding the model of standard magnetic reconnection and electron only reconnection model in electron scale current sheet.

Motivation & Objective

• To investigate the nature of magnetic reconnection in ion-scale current sheets at the magnetopause, particularly whether electron-only reconnection can occur in such conditions.
• To determine whether the observed electron diffusion region (EDR) is consistent with standard reconnection or represents a new regime.
• To examine the absence of burst ion outflow in the presence of a long-extending EDR in an ion-scale current sheet.
• To challenge existing models of magnetic reconnection by identifying electron-only reconnection in a regime previously thought to require ion coupling.

Proposed method

• Utilized high-resolution in-situ measurements from the Magnetospheric Multiscale (MMS) mission, including FGM, EDP, and FPI instruments.
• Performed minimum variance analysis (MVA) to define the LMN coordinate system for the current sheet crossing.
• Identified the inner EDR using signatures such as electron nongyrotropy, super-Alfvénic electron flow, parallel electric field, and positive energy dissipation (J•E' > 0).
• Applied timing analysis across four MMS spacecraft to estimate the thickness and extension of the EDR and current sheet.
• Calculated reconnection rate using the formula from Liu et al. (2017) and Nakamura et al. (2018), consistent with prior observations.
• Used cone angle estimation from magnetic field vectors to infer the EDR extension from the X-line based on separatrix crossings.

Experimental results

Research questions

• RQ1 Can electron-only reconnection occur in an ion-scale current sheet, rather than only in electron-scale current sheets?
• RQ2 What is the spatial extent of the inner electron diffusion region (EDR) in an ion-scale current sheet, and does it exceed theoretical predictions?
• RQ3 Why is there no burst ion outflow despite the presence of a well-defined EDR and Hall electromagnetic signatures in an ion-scale current sheet?
• RQ4 How does the observed EDR extension (~40 di) compare to theoretical and simulation expectations for inner EDRs?
• RQ5 What mechanisms might enable electron-only reconnection in an ion-scale current sheet, and how do they differ from standard reconnection?

Key findings

• The inner electron diffusion region (EDR) extended at least 40 ion inertial lengths (di) downstream from the X-line, far exceeding typical theoretical predictions.
• The EDR was embedded in an ion-scale current sheet with a thickness of ~4 di (~165.5 km), indicating reconnection in a regime where ions are expected to participate.
• Despite clear Hall signatures (bipolar Hall electric field, quadrupolar out-of-plane magnetic field, and Hall current), no burst ion outflow was observed, indicating electron-only reconnection.
• The EDR exhibited super-Alfvénic electron flow (up to -300 km/s and 400 km/s in L and M directions), electron nongyrotropy, and positive energy dissipation (J•E' > 0), confirming its identity as an inner EDR.
• The estimated EDR thickness was ~0.54 di (~22 km or ~22 de), consistent with inner EDR characteristics.
• The reconnection rate was calculated to be between 0.021 and 0.034, consistent with previous observations and theoretical expectations.