A new fast reconnection model in a collisionless regime

TL;DR

This paper introduces a simple, general model for collisionless magnetic reconnection that predicts much faster reconnection rates than classical models, aligning well with experimental data and explaining rapid phenomena like solar flares.

Contribution

It presents a novel, first-principles-based collisionless reconnection model that is not dependent on initial configurations and predicts micro-physics-limited, rapid reconnection rates.

Findings

Reconnection rate is significantly faster than Sweet-Parker model.

Diffusion region width is micro-physics-dependent and scale-independent.

Model aligns well with experimental data from MRX device.

Abstract

Based on the first principles (i.e. (i) by balancing the magnetic field advection with the term containing electron pressure tensor non-gyrotropic components in the generalised Ohm's law; (ii) using the conservation of mass; and (iii) assuming that the weak magnetic field region width, where electron meandering motion supports electron pressure tensor off-diagonal (non-gyrotropic) components, is of the order of electron Larmor radius) a simple model of magnetic reconnection in a collisionless regime is formulated. The model is general, resembling its collisional Sweet-Parker analogue in that it is not specific to any initial configuration e.g. Harris type tearing unstable current sheet, X-point collapse or otherwise. In addition to its importance from the fundamental point of view, the collisionless reconnection model offers a much faster reconnection rate (M_c'less=(c/omega_pe)^2 /(r_L,e L)) than Sweet-Parker's classical one (M_sp=S^-1/2). The width of the diffusion region (current sheet) in the collisionless regime is found to be delta_c'less=(c/omega_pe)^2/r_L,e, which is independent of global reconnection scale L and is only prescribed by micro-physics (electron inertial length, c/omega_pe, and electron Larmor radius, r_L,e). Amongst other issues, the fastness of the reconnection rate alleviates e.g. the problem of interpretation of solar flares by means of reconnection, as for the typical solar coronal parameters the obtained collisionless reconnection time can be a few minutes, as opposed to Sweet-Parker's equivalent value of < a day. The new theoretical reconnection rate is compared to the MRX device experimental data by [Yamada et al., Phys. Plasmas, 13, 052119 (2006), Ji et al. GRL, 35, 13106 (2008)] and a good agreement is obtained.