A new low-beta regime for unstable proton firehose modes in bi-Kappa distributed plasmas

TL;DR

This study investigates how suprathermal protons in solar wind plasmas lower the thresholds and amplify the growth of proton firehose instabilities, revealing a new low-beta regime with enhanced electromagnetic fluctuations.

Contribution

It introduces a comprehensive analysis of proton firehose modes in bi-Kappa distributed plasmas, highlighting the impact of suprathermal populations on instability thresholds and growth rates.

Findings

Suprathermals amplify growth rates of firehose modes.

Instability thresholds are lowered to $eta_{p \\parallel}<1$ with suprathermals.

Electromagnetic fluctuations are significantly enhanced, leading to faster anisotropy relaxation.

Abstract

In the solar wind plasma an excess of kinetic temperature along the background magnetic field stimulates proton firehose modes to grow if the parallel plasma beta parameter is sufficiently high, i.e., $\beta_{p \parallel}\gtrsim 1$. This instability can prevent the expansion-driven anisotropy from increasing indefinitely, and explain the observations. Moreover, such kinetic instabilities are expected to be even more effective in the presence of suprathermal Kappa-distributed populations, which are ubiquitous in the solar wind, are less affected by collisions than the core population, but contribute with an additional free energy. In this work we use both linear and extended quasi-linear (QL) frameworks to characterize the unstable periodic proton firehose modes (propagating parallel to the magnetic field) under the influence of suprathermal protons. Linear theory predicts a systematic stimulation of the instability, suprathermals amplifying the growth rates and decreasing the instability thresholds to lower anisotropies and lower plasma betas ($\beta_{p \parallel}<1$). In perfect agreement with these results, the QL approach reveals a significant enhancement of the resulting electromagnetic fluctuations up to the saturation with a stronger back reaction on protons, leading also to a faster and more efficient relaxation of the temperature anisotropy.