On the collisionless asymmetric magnetic reconnection rate

TL;DR

This paper predicts the steady-state reconnection electric field in asymmetric collisionless magnetic reconnection by maximizing the reconnection rate, resulting in a parameter-free model that aligns well with previous models and observations.

Contribution

The paper introduces a new model for the reconnection rate that does not rely on free parameters and is based on geometric maximization within MHD constraints.

Findings

Prediction within a factor of two of previous models

Model has no free parameters

Supports the universal normalized reconnection rate of ~0.1

Abstract

A prediction of the steady-state reconnection electric field in asymmetric reconnection is obtained by maximizing the reconnection rate as a function of the opening angle made by the upstream magnetic field on the weak magnetic field (magnetosheath) side. The prediction is within a factor of two of the widely examined asymmetric reconnection model [Cassak and Shay, Phys. Plasmas 14, 102114, 2007] in the collisionless limit, and they scale the same over a wide parameter regime. The previous model had the effective aspect ratio of the diffusion region as a free parameter, which simulations and observations suggest is on the order of 0.1, but the present model has no free parameters. In conjunction with the symmetric case [Liu et al., Phys. Rev. Lett. 118, 085101, 2017], this work further suggests that this nearly universal number 0.1, essentially the normalized fast reconnection rate, is a geometrical factor arising from maximizing the reconnection rate within magnetohydrodynamic (MHD)-scale constraints.