Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities

TL;DR

This paper investigates how compressive fluctuations and plasma microinstabilities in the solar wind lead to proton isotropization, explaining why the proton temperature ratio remains near unity in a collisionless environment.

Contribution

It introduces a model of the fluctuating-anisotropy effect using linear Vlasov-Maxwell theory to explain proton isotropization driven by compressive fluctuations and microinstabilities.

Findings

The fluctuating-anisotropy effect can keep the proton temperature ratio near unity.

Microinstabilities scatter protons, reducing pressure anisotropy.

The model explains the observed proton temperature isotropy in the solar wind.

Abstract

Compressive fluctuations are a minor yet significant component of astrophysical plasma turbulence. In the solar wind, long-wavelength compressive slow-mode fluctuations lead to changes in $\beta_{\parallel \mathrm p}\equiv 8\pi n_{\mathrm p}k_{\mathrm B}T_{\parallel \mathrm p}/B^2$ and in $R_{\mathrm p}\equiv T_{\perp \mathrm p}/T_{\parallel \mathrm p}$, where $T_{\perp \mathrm p}$ and $T_{\parallel \mathrm p}$ are the perpendicular and parallel temperatures of the protons, $B$ is the magnetic field strength, and $n_{\mathrm p}$ is the proton density. If the amplitude of the compressive fluctuations is large enough, $R_{\mathrm p}$ crosses one or more instability thresholds for anisotropy-driven microinstabilities. The enhanced field fluctuations from these microinstabilities scatter the protons so as to reduce the anisotropy of the pressure tensor. We propose that this scattering drives the average value of $R_{\mathrm p}$ away from the marginal stability boundary until the fluctuating value of $R_{\mathrm p}$ stops crossing the boundary. We model this "fluctuating-anisotropy effect" using linear Vlasov--Maxwell theory to describe the large-scale compressive fluctuations. We argue that this effect can explain why, in the nearly collisionless solar wind, the average value of $R_{\mathrm p}$ is close to unity.