On the interplay of solar wind proton and electron instabilities: Linear and quasi-linear approaches

TL;DR

This paper investigates how electron and proton plasma instabilities interact in the solar wind using linear and quasi-linear models, revealing complex competition and relaxation effects that influence plasma anisotropy.

Contribution

It introduces a combined linear and quasi-linear framework to analyze the interplay of electron and proton instabilities in space plasmas, highlighting their mutual influence on growth rates and saturation levels.

Findings

Linear theory shows strong competition between electron and proton instabilities.

Quasi-linear analysis reveals different saturation levels and timescales for fluctuations.

Whistler fluctuations inhibit proton firehose instability effects.

Abstract

Important efforts are currently made for understanding the so-called kinetic instabilities, driven by the anisotropy of different species of plasma particles present in the solar wind and terrestrial magnetosphere. These instabilities are fast enough to efficiently convert the free energy of plasma particles into enhanced (small-scale) fluctuations with multiple implications, regulating the anisotropy of plasma particles. In this paper we use both linear and quasilinear (QL) frameworks to describe complex unstable regimes, which realistically combine different temperature anisotropies of electrons and ions (protons). Thus parameterized are various instabilities, e.g., proton and electron firehose, electromagnetic ion cyclotron, and whistler instability, showing that their main linear properties are markedly altered by the interplay of anisotropic electrons and protons. Linear theory may predict a strong competition of two instabilities of different nature when their growth rates are comparable. In the QL phase wave fluctuations grow and saturate at different levels and temporal scales, by comparison to the individual excitation of the proton or electron instabilities. In addition, cumulative effects of the combined proton and electron induced fluctuations can markedly stimulate the relaxations of their temperature anisotropies. Only whistler fluctuations inhibit the efficiency of proton firehose fluctuations in the relaxation of anisotropic protons. These results offer valuable premises for further investigations in numerical simulations, to decode the full spectrum of kinetic instabilities resulting from the interplay of anisotropic electrons and protons in space plasmas.