Clarifying the solar wind heat-flux instabilities

TL;DR

This paper provides a detailed numerical analysis of heat-flux instabilities in the solar wind, focusing on whistler and firehose modes, and examines how plasma parameters and suprathermal electrons influence their dominance.

Contribution

It offers the first comprehensive linear Vlasov-Maxwell dispersion analysis of both instabilities considering suprathermal electrons in space plasmas.

Findings

Whistler and firehose instabilities are influenced by beam-core velocity and plasma beta.

Suprathermal electrons inhibit firehose but stimulate whistler instability.

Instability thresholds inversely or directly correlate with plasma beta depending on the mode.

Abstract

In the solar wind electron velocity distributions reveal two counter-moving populations which may induce electromagnetic (EM) beaming instabilities known as heat flux instabilities. Depending on plasma parameters two distinct branches of whistler and firehose instabilities can be excited. These instabilities are invoked in many scenarios, but their interplay is still poorly understood. An exact numerical analysis is performed to resolve the linear Vlasov-Maxwell dispersion and characterize these two instabilities, e.g., growth rates, wave frequencies and thresholds, enabling to identify their dominance for conditions typically experienced in space plasmas. Of particular interest are the effects of suprathermal Kappa-distributed electrons which are ubiquitous in these environments. The dominance of whistler or firehose instability is highly conditioned by the beam-core relative velocity, core plasma beta and the abundance of suprathermal electrons. Derived in terms of relative drift velocity the instability thresholds show an inverse correlation with the core plasma beta for the whistler modes, and a direct correlation with the core plasma beta for the firehose instability. Suprathermal electrons reduce the effective (beaming) anisotropy inhibiting the firehose modes while the whistler instability is stimulated.