Shaping the solar wind temperature anisotropy by the interplay of electron and proton instabilities

TL;DR

This study investigates how electron and proton instabilities interact to shape the temperature anisotropy in the solar wind, using linear kinetic theory to compare theoretical thresholds with observations at 1 AU.

Contribution

It introduces a dynamical model that captures the interplay between electron and proton instabilities affecting temperature anisotropy in the solar wind.

Findings

Identifies the principal instabilities driven by proton and electron anisotropies.

Compares instability thresholds with observed anisotropy bounds at 1 AU.

Highlights the combined effect of electrons and protons on plasma relaxation.

Abstract

A variety of nonthermal characteristics like kinetic, e.g., temperature, anisotropies and suprathermal populations (enhancing the high energy tails of the velocity distributions) are revealed by the in-situ observations in the solar wind indicating quasistationary states of plasma particles out of thermal equilibrium. Large deviations from isotropy generate kinetic instabilities and growing fluctuating fields which should be more efficient than collisions in limiting the anisotropy (below the instability threshold) and explain the anisotropy limits reported by the observations. The present paper aims to decode the principal instabilities driven by the temperature anisotropy of electrons and protons in the solar wind, and contrast the instability thresholds with the bounds observed at 1~AU for the temperature anisotropy. The instabilities are characterized using linear kinetic theory to identify the appropriate (fastest) instability in the relaxation of temperature anisotropies $A_{e,p} = T_{e,p,\perp}/ T_{e,p,\parallel} \ne 1$. The analysis focuses on the electromagnetic instabilities driven by the anisotropic protons ($A_p \lessgtr 1$) and invokes for the first time a dynamical model to capture the interplay with the anisotropic electrons by correlating the effects of these two species of plasma particles, dominant in the solar wind.