Waves in Partially Ionised Solar Atmospheric Plasmas in Ionisation Non-equilibrium

Abstract

Energy transport in the solar atmosphere, particularly the heating of the chromosphere and corona, remains a significant challenge in solar physics. Magnetohydrodynamic waves are considered a primary mechanism for this transport; however, in the weakly ionised lower atmosphere, their damping is governed by interactions between charged particles and the predominant neutral population. Ideal single-fluid models do not capture these interactions. The principal contribution of this study is the analysis of chemical processes at the perturbation level. In particular, the perturbations to ionisation and recombination rates are incorporated into the linearised equations, enabling ionisation non-equilibrium to be included directly in the dispersion relation rather than neglected. This methodology has not previously been applied to the linear magnetoacoustic spectrum. This thesis analyses the influence of ionisation and recombination on magnetoacoustic wave propagation and damping in ionised solar plasmas. Using a two-fluid model with separate charged and neutral components coupled through collisions, thermal exchange, and ionisation and recombination, we investigate collisional coupling, propagation direction, and plasma-$\beta$. The resulting dispersion relation is analysed using Friedrichs diagrams. Ionisation non-equilibrium introduces an additional relaxation mechanism that modifies wave damping and anisotropy. In weakly collisional regimes, chemical coupling enables neutral slow modes to inherit magnetic anisotropy despite weak friction, whereas in strongly collisional regimes these modes become damped and can transition to overdamped behaviour. These effects depend critically on plasma-$\beta$. In strongly ionised plasmas, a minority neutral component can influence wave propagation and damping. Chemical relaxation is particularly important for slow modes because their compressibility produces density and temperature perturbations that alter the ionisation balance. Finally, a three-fluid model of non-magnetised photospheric plasma examines charge exchange between protons and neutral hydrogen. Charge exchange strengthens ion-neutral coupling, enhances damping of charged-fluid perturbations, and promotes a common acoustic response, with greater influence in weakly ionised regimes where neutrals dominate.