Comparing the counter-beaming and temperature anisotropy driven aperiodic electron firehose instabilities in collisionless plasma environments

TL;DR

This paper compares the properties of electron firehose instabilities driven by temperature anisotropy and counter-beaming populations in collisionless plasmas, revealing differences in growth rates and the influence of additional instabilities at higher beam speeds.

Contribution

It provides a detailed comparative analysis of counter-beaming and temperature anisotropy driven electron firehose instabilities, highlighting their similarities and differences in growth rates and conditions.

Findings

Counter-beaming FI has higher growth rates than TAFI at small drifts.

At higher beam speeds, electrostatic two-stream instability may dominate.

Both instabilities occur in similar wave-number and oblique angle ranges.

Abstract

The electron firehose instabilities are among the most studied kinetic instabilities, especially in the context of space plasmas, whose dynamics is mainly controlled by collisionless wave-particle interactions. This paper undertakes a comparative analysis of the aperiodic electron firehose instabilities excited either by the anisotropic temperature or by the electron counter-beaming populations. Two symmetric counter-beams provide an effective kinetic anisotropy similar to the temperature anisotropy of a single (non-drifting) population, with temperature along the magnetic field direction larger than that in perpendicular direction. Therefore, the counter-beaming plasma is susceptible to firehose-like instabilities (FIs), parallel and oblique branches. Here we focus on the oblique beaming FI, which is also aperiodic when the free energy is provided by symmetric counter-beams. Our results show that, for relative small drifts or beaming speeds ($U$), not exceeding the thermal speed ($\alpha$), the aperiodic FIs exist in the same interval of wave-numbers and the same range of oblique angles (with respect to the magnetic field direction), but the growth rates of counter-beaming FI (CBFI) are always higher than those of temperature anisotropy FI (TAFI). For $U/\alpha > 1$, however, another electrostatic two-stream instability (ETSI) is also predicted, which may have growth rates higher than those of CBFI, and may dominate in that case the dynamics.