Onset of magnetic reconnection in a collisionless, high-beta plasma

TL;DR

This paper investigates how pressure anisotropy and mirror instability influence magnetic reconnection in collisionless, high-beta plasmas, revealing that these effects can significantly alter the reconnection process and current sheet stability.

Contribution

It introduces an analytical model showing how mirror instability affects current sheet formation and tearing modes in high-beta collisionless plasmas, highlighting deviations from standard reconnection profiles.

Findings

Mirror instability triggers strong field line distortions at high beta.

Reconnection geometry can differ radically from Harris-sheet profiles.

Tearing modes may be disrupted before standard growth due to mirror effects.

Abstract

In a magnetized, collisionless plasma, the magnetic moment of the constituent particles is an adiabatic invariant. An increase in the magnetic-field strength in such a plasma thus leads to an increase in the thermal pressure perpendicular to the field lines. Above a $\beta$-dependent threshold (where $\beta$ is the ratio of thermal to magnetic pressure), this pressure anisotropy drives the mirror instability, producing strong distortions in the field lines on ion-Larmor scales. The impact of this instability on magnetic reconnection is investigated using a simple analytical model for the formation of a current sheet (CS) and the associated production of pressure anisotropy. The difficulty in maintaining an isotropic, Maxwellian particle distribution during the formation and subsequent thinning of a CS in a collisionless plasma, coupled with the low threshold for the mirror instability in a high-$\beta$ plasma, imply that the geometry of reconnecting magnetic fields can differ radically from the standard Harris-sheet profile often used in simulations of collisionless reconnection. As a result, depending on the rate of CS formation and the initial CS thickness, tearing modes whose growth rates and wavenumbers are boosted by this difference may disrupt the mirror-infested CS before standard tearing modes can develop. A quantitative theory is developed to illustrate this process, which may find application in the tearing-mediated disruption of kinetic magnetorotational "channel" modes.