The aperiodic firehose instability of counter-beaming electrons in space plasmas

TL;DR

This paper investigates the aperiodic firehose instability caused by counter-beaming electrons in space plasmas, revealing how background electrons inhibit its growth and its potential role in space plasma dynamics.

Contribution

It provides a kinetic theory analysis and PIC simulations showing the conditions under which the counter-beaming electron firehose instability occurs and how background electrons influence it.

Findings

BEFI can be excited with beam speeds comparable to thermal speed if beams are dense enough.

Background electrons inhibit the growth rate and unstable wave-number range of BEFI.

BEFI can develop as a secondary instability after electrostatic interactions.

Abstract

Recent studies have revealed new unstable regimes of the counter-beaming electrons specific to hot and dilute plasmas from astrophysical scenarios. The (counter-)beaming electron firehose instability (BEFI) is induced for highly oblique angles of propagation relative to the magnetic field, resembling the fast growing and aperiodic mode triggered by the temperature anisotropy. It is investigated here for space plasma conditions that includes the influence of an embedding background plasma of electrons and protons. Kinetic theory is applied to prescribe the unstable regimes, and differentiate from the regimes of interplay with other instabilities. Linear theory predicts a systematic inhibition of the BEFI, by reducing the growth rates and the range of unstable wave-number with increasing the relative density of the background electrons. To obtain finite growth rates, the beam speed does not need to be high (just comparable to thermal speed), but beams must be dense enough, with a relative density at least 15-20\% of the total density. The plasma conditions favorable to this instability are reduced under the influence of background electrons. PIC simulations confirm not only that BEFI can be excited in the presence of background electrons, but also the inhibiting effect of this population. In the regimes of transition to electrostatic (ES) instabilities, BEFI is still robust enough to develop as a secondary instability, after the relaxation of beams under a quick interaction with ES fluctuations. BEFI resembles the properties of firehose heat-flux instability triggered by the electron strahl. However, BEFI is driven by a double (counter-beaming) strahl, and develops at oblique angles, which makes it effective in the regularization of the electron counter-beams observed in closed magnetic field topologies and interplanetary shocks.