Influence of ion-to-electron temperature ratio on tearing instability and resulting subion-scale turbulence in a low-$β_e$ collisionless plasma

Abstract

A two-field gyrofluid model including ion finite Larmor radius (FLR) corrections, magnetic fluctuations along the ambient field and electron inertia is used to study two-dimensional reconnection in a low $β_e$ collisionless plasma, in a plane perpendicular to the ambient field. Both moderate and large values of the ion-to-electron temperature ratio $τ$ are considered. The linear growth rate of the tearing instability is computed for various values of $τ$, confirming the convergence to reduced electron magnetodynamics (REMHD) predictions in the large $τ$ limit. Comparisons with analytical estimates in several limit cases are also presented. The nonlinear dynamics leads to a fully-developed turbulent regime that appears to be sensitive to the value of the parameter $τ$. For $τ= 100$, strong large-scale velocity shears trigger Kelvin-Helmholtz instability, leading to the propagation of the turbulence through the separatrices, together with the formation of eddies of size of the order of the electron skin depth. In the $τ= 1$ regime, the vortices are significantly smaller and their accurate description requires that electron FLR effects be taken into account.