The Electron Firehose and Ordinary-Mode Instabilities in Space Plasmas

TL;DR

This study investigates the competition between the electron firehose and ordinary-mode instabilities in space plasmas, revealing that the oblique firehose dominates at lower anisotropies and large plasma beta, with the ordinary mode active only at high beta.

Contribution

The paper provides a comprehensive 3D wave-vector analysis of the firehose and ordinary-mode instabilities, clarifying their hierarchy and conditions for growth in nonstreaming plasmas.

Findings

Oblique firehose instability has the lowest threshold among the instabilities.

Ordinary-mode instability grows faster at higher anisotropies and beta.

Only large beta regimes are susceptible to the ordinary-mode instability.

Abstract

The selfgenerated wave fluctuations are particularly interesting in the solar wind and magnetospheric plasmas, where Coulomb collisions are rare and cannot explain the observed states of quasi-equilibrium. Linear theory predicts that the firehose and the ordinary-mode instabilities can develop under the same conditions, confusing the role of these instabilities in conditioning the space-plasma properties. The hierarchy of these two instabilities is reconsidered here for nonstreaming plasmas with an electron temperature anisotropy $T_\parallel > T_\perp$, where $\parallel$ and $\perp$ denote directions with respect to the local mean magnetic field. In addition to the previous comparative analysis, here the entire 3D wave-vector spectrum of the competing instabilities is investigated, paying particular attention to the oblique firehose instability and the relatively poorly known ordinary-mode instability. Results show a dominance of the oblique firehose instability with a threshold lower than the parallel firehose instability and lower than the ordinary-mode instability. For larger anisotropies, the ordinary mode can grow faster, with maximum growth rates exceeding the ones of the oblique firehose instability. In contrast to previous studies that claimed a possible activity of the ordinary-mode in the small $\beta [< 1]$ regimes, here it is rigorously shown that only the large $\beta [> 1]$ regimes are susceptible to these instabilities.