Electromagnetic instabilities of low-beta alpha/proton beams in space plasmas

TL;DR

This study analyzes electromagnetic instabilities caused by alpha/proton drifts in low-beta space plasmas, revealing conditions under which alpha cyclotron instabilities grow, potentially affecting solar wind dynamics.

Contribution

It provides a comprehensive dispersion relation analysis for low-beta alpha/proton drifts, highlighting the dominance of alpha cyclotron instabilities without thermal anisotropies.

Findings

Alpha cyclotron mode instability is prominent in low-beta solar wind conditions.

Growth rates increase with plasma beta and alpha-proton temperature ratio.

Instability can regulate alpha particle drift in space plasmas.

Abstract

Relative drifts between different species or particle populations are characteristic to solar plasma outflows, e.g., in the fast streams of the solar winds, coronal mass ejections and interplanetary shocks. This paper characterizes the dispersion and stability of the low-beta alpha/proton drifts in the absence of any intrinsic thermal anisotropies, which are usually invoked in order to stimulate various instabilities. The dispersion relations derived here describe the full spectrum of instabilities and their variations with the angle of propagation and plasma parameters. The results unveil a potential competition between instabilities of the electromagnetic proton cyclotron and alpha cyclotron modes. For conditions specific to a low-beta solar wind, e.g., at low heliocentric distances in the outer corona, the instability operates on the alpha cyclotron branch. The growth rates of the alpha cyclotron mode are systematically stimulated by the (parallel) plasma beta and/or the alpha-proton temperature ratio. One can therefore expect that this instability develops even in the absence of temperature anisotropies, with potential to contribute to a self-consistent regulation of the observed drift of alpha particles.