Pressure Anisotropy-Driven Instabilities in Solar and Astrophysical Plasmas

Abstract

Pressure anisotropy is a feature found in plasmas that are preferentially heated parallel or perpendicular to the local magnetic field by processes such as expansion or turbulent dissipation. This anisotropy can drive instabilities that affect energy transport and plasma evolution in a variety of solar and astrophysical contexts. In this dissertation, we investigate pressure anisotropy-driven instabilities that are observed in the solar wind and operate in other weakly collisional plasmas. We first provide a broad overview of these instabilities, including a description of current observations and the analytic and numeric methods used to analyze their growth and saturation. We then leverage linear kinetic theory to investigate the impact of distribution function structure on the stability of Alfvén ion-cyclotron modes. Solar wind proton velocity distribution functions extracted from Wind spacecraft observations of the solar wind are used to evaluate the efficacy of existing simplified models in predicting stable or unstable behavior of these modes. While kinetic theory can provide detailed information on particle behavior, including resonant wave-particle interactions, it can pose significant computational challenges to use this prescription to run high-resolution simulations of plasmas. We utilize a 10-moment, multi-fluid model to run nonlinear simulations that incorporate sufficient physics to drive and saturate firehose instabilities. The fidelity of these simulations in modeling real space plasma systems is increased by proceeding from 1D simulations to 2D simulations that can capture both the parallel and oblique firehose modes. As part of this work, we develop the expanding box model within the 10-moment, multi-fluid computational framework. With preliminary test cases validating the implementation of the 10-moment expanding box model, we anticipate that this model will allow self-consistent modeling of plasma expansion. Utilizing solar wind or astrophysical plasma parameters, the 10-moment expanding box can explore a variety of plasma phenomena, including the evolution of turbulence and instabilities in these systems.