The Effective Mean-Free-Path of the Solar Wind

Abstract

The high temperature and rarefied ionised gas (plasma) that constitutes the corona of the sun escapes the gravitational bound and flows out into interplanetary space. This plasma is called the solar wind. It is characterised by a long collision mean-free-path (i.e., weakly collisional); it is not in thermodynamic equilibrium. While the plasma is ultimately governed by a kinetic equation, it does appear that the solar wind is described by fluid equations, where it is assumed to be at equilibrium. This is in stark contradiction to the long collision mean-free-path. The suggestion is that collisionless relaxation processes are playing a strong role in dictating the dynamics of the solar wind. These processes are wave-particle interactions that cause the plasma to relax towards equilibrium, i.e., they are effective collision processes. This thesis takes a novel route to measure the effective mean-free-path of the solar wind, by modelling compressive fluctuations of arbitrary effective mean-free-path, and making a robust comparison to solar wind observations. The effective mean-free-path is measured to be approximately 10$^3$ times shorter than the collisional mean-free-path. It is shown to be consistent with and justify decades of past solar wind research that use fluid equations. The theory for the numerical model is derived from first principles and is shown to coincide with previous results, and draw together many concepts about compressive plasma waves. The solar wind dataset used in this thesis was not previously used for scientific analysis, so verification of the data quality is demonstrated. In addition, data analysis tools are constructed to measure some of the potential effective collision mechanisms. The analysis is tested on simulation data, to verify the accuracy, by measuring key quantities in identifying the relevant role of various effective collision mechanisms. The analysis of the numerical simulation data is shown to be satisfactory and can be employed on spacecraft data. This measurement resolves a long-standing debate on the utility and accuracy of fluid equations in studying the solar wind. The direct measurement of the effective mean-free-path is important for the field of plasma physics because it dictates the transport and thermodynamics of weakly collisional plasmas.