Instabilities of collisionless current sheets revisited: the role of anisotropic heating

TL;DR

This study examines how anisotropic electron heating influences the stability and evolution of collisionless current sheets, highlighting the importance of numerical methods and anisotropy in triggering or stabilizing instabilities.

Contribution

It demonstrates the impact of anisotropic heating on current sheet instabilities and emphasizes the significance of macroparticle shape functions and numerical effects in PIC simulations.

Findings

Higher order shape functions improve anisotropic heating estimates.

Temperature anisotropies can stabilize or trigger instabilities.

Numerical effects are significant at higher mass ratios.

Abstract

In this work, we investigate the influence of the anisotropic heating on the spontaneous instability and evolution of thin Harris-type collisionless current sheets, embedded in antiparallel magnetic fields. In particular, we explore the influence of the macroparticle shape-function using a 2D version of the PIC code ACRONYM. We also investigate the role of the numerical collisionality due to the finite number of macroparticles in PIC codes. It is shown that it is appropriate to choose higher order shape functions of the macroparticles compared to a larger number of macroparticles per cell. This allows to estimate better the anisotropic electron heating due to the collisions of macroparticles in a PIC code. Temperature anisotropies can stabilize the tearing mode instability and trigger additional current sheet instabilities. We found a good agreement between the analytically derived threshold for the stabilization of the anisotropic tearing mode and other instabilities, either spontaneously developing or initially triggered ones. Numerical effects causing anisotropic heating at electron time scales, become especially important for higher mass ratios (above $m_i/m_e = 180$). If numerical effects are carefully taken into account, one can recover the theoretical estimated linear growth rates of the tearing instability of thin isotropic collisionless current sheets, also for higher mass ratios.