Mixing the solar wind proton and electron scales. Theory and 2D-PIC simulations of firehose instability

TL;DR

This paper investigates firehose instabilities in solar wind plasmas through linear theory and 2D PIC simulations, revealing the dominance of oblique aperiodic modes and their effects on particle distributions.

Contribution

It provides new insights into the interplay of electron and proton firehose instabilities, highlighting the prominence of oblique aperiodic modes near thresholds and their impact on plasma anisotropy.

Findings

Oblique aperiodic modes have higher growth rates than parallel modes.

Electron firehose instability saturates rapidly at low fluctuations.

Proton anisotropy enhances both electron and proton firehose instabilities.

Abstract

Firehose-like instabilities (FIs) are cited in multiple astrophysical applications. Of particular interest are the kinetic manifestations in weakly-collisional or even collisionless plasmas, where these instabilities are expected to contribute to the evolution of macroscopic parameters. Relatively recent studies have initiated a realistic description of FIs, as induced by the interplay of both species, electrons and protons, dominant in the solar wind plasma. This work complements the current knowledge with new insights from linear theory and the first disclosures from 2D PIC simulations, identifying the fastest growing modes near the instability thresholds and their long-run consequences on the anisotropic distributions. Thus, unlike previous setups, these conditions are favorable to those aperiodic branches that propagate obliquely to the uniform magnetic field, with (maximum) growth rates higher than periodic, quasi-parallel modes. Theoretical predictions are, in general, confirmed by the simulations. The aperiodic electron FI (a-EFI) remains unaffected by the proton anisotropy, and saturates rapidly at low-level fluctuations. Regarding the firehose instability at proton scales, we see a stronger competition between the periodic and aperiodic branches. For the parameters chosen in our analysis, the a-PFI is excited before than the p-PFI, with the latter reaching a significantly higher fluctuation power. However, both branches are significantly enhanced by the presence of anisotropic electrons. The interplay between EFIs and PFIs also produces a more pronounced proton isotropization.