Onset of magnetic reconnection in a collisionless, high- plasma

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 β-dependent threshold (where β 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-β 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. †

Triggering tearing in a forming current sheet with the mirror instability

“…Our work lends credence to the conclusion made by Alt & Kunz (2019) that ‘numerical simulations of collisionless reconnection in high- plasmas should not initialise with a Maxwellian plasma embedded in an equilibrium CS. Instead, the CS should be allowed to evolve, and the particle distribution function self-consistently with it’.…”

“…First, by appreciably wrinkling the reconnecting field on kinetic scales, nonlinear mirrors cause to depart from that corresponding to an undisturbed Harris sheet . Proposing a simple model for the magnetic profile of a mirror-infested CS, Alt & Kunz (2019) calculated the resulting and showed that it becomes positive (and, thus, the CS becomes tearing-unstable) at wavenumbers up to that corresponding to the inverse location of the innermost magnetic mirror (which effectively replaces as the characteristic thickness of the CS). Those authors then argued that the location of the innermost magnetic mirror is set by the perpendicular wavenumber of that mirror, using with given by and and taking on their local values.…”

“…The purpose of this paper is then to investigate the early evolution and subsequent disruption of a thinning CS in a high-, weakly collisional plasma, while taking into consideration plasma-kinetic effects such as pressure anisotropy and the mirror instability that are self-consistently driven during CS formation. We do so by performing a series of hybrid-kinetic particle-in-cell (PIC) simulations of a thinning CS using the code, with the further aim of testing certain predictions of the analytic model of Alt & Kunz (2019) for the mirror-instigated tearing of a forming CS.…”

“…What high beta and low collisionality bring to the mix are the generation of kinetically unstable pressure anisotropy and the consequent destabilisation of the CS on ion-Larmor scales. The argument runs as follows (Alt & Kunz 2019).…”

The Effect of Thermal Pressure on Collisionless Magnetic Reconnection Rate

“…Third, the model only includes ion heating in the diffusion region, while additional electron heating can be as strong in some regimes (Haggerty et al 2015;Shuster et al 2015;Wang et al 2016;Dahlin et al 2014;Wang et al 2016;Li et al 2015), and the upstream electric potential may be important (Le et al 2009;Egedal et al 2013Egedal et al , 2015Shuster et al 2015). Fourth, pressure anisotropy instabilities (e.g., mirror instability) may arise in high-β plasmas and was suggested to distort the field geometry and the current sheet (Alt & Kunz 2019). Finally, we do not take 3D physics into account, for example, self-generated turbulence (Daughton et al 2011;Liu et al 2013; or localized reconnection layer along the third dimension (Huang et al 2020;Liu et al 2019).…”