Collisionless relaxation as the origin of the anisotropic, non-thermal, and multi-temperature momentum distributions observed in space plasmas

TL;DR

This paper explains how collisionless space plasmas naturally develop anisotropic, non-thermal, and multi-temperature particle distributions due to anisotropic compression or expansion, without requiring collisions, aligning with observations.

Contribution

It provides a theoretical framework based on Liouville's theorem showing that collisionless plasmas cannot return to thermal equilibrium after anisotropic processes, explaining observed distributions.

Findings

Collisionless plasmas exhibit anisotropic, non-thermal distributions.

Anisotropic compression/expansion leads to non-equilibrium states.

Solar wind measurements support the ubiquity of these distributions.

Abstract

Anisotropic, non-thermal, and multi-temperature distributed particle momenta are commonly observed in collisionless space plasmas, such as the solar wind. Using Liouville's theorem, we argue that anisotropic compression or expansion of the plasma, followed by a relaxation of the resulting anisotropic stress must lead to non-equilibrium states that are either anisotropic, non-thermal distribution functions, different electron and ion temperatures, or a combination of these effects. We present arguments showing that a plasma in thermal equilibrium undergoing anisotropic compression or expansion cannot return to thermal equilibrium in the absence of particle collisions. Since most astrophysical plasmas are practically collisionless and experience significant anisotropic compression or expansion, we expect anisotropic, non-thermal, and multi-temperature particle distributions to be ubiquitous, in agreement with solar wind measurements.