Magnetoacoustic waves in two-fluid weakly ionised solar atmospheric plasmas in ionisation non-equilibrium

In the lower solar atmosphere, the plasma is only weakly ionised, so wave dynamics are governed not only by magnetic forces but also by ion–neutral interactions and chemical processes. This paper investigates how ionisation and recombination influence the propagation and damping of magnetoacoustic waves in weakly ionised solar plasmas, and how these effects depend on the strength of collisional coupling, the direction of propagation, and the plasma-β parameter. We used a two-fluid model in which charges and neutrals are treated as separate but interacting fluids coupled through collisions, thermal exchange, and ionisation and recombination processes. We analysed linear wave properties by solving the resulting dispersion relation and investigating the angular dependence of the complex wave frequency using Friedrichs diagrams. Our results confirm that ionisation non-equilibrium introduces an additional relaxation mechanism that can substantially modify wave damping and anisotropy. In weakly collisional regimes, chemical coupling allows neutral slow modes to inherit magnetic anisotropy even when friction is weak. In strongly collisional regimes, neutral slow modes become strongly damped and can transition into non-oscillatory, over-damped behaviour, despite the plasma being weakly ionised. The influence of non-equilibrium effects depends critically on plasma-β. Slow modes are most affected at low β, while at high β the fast mode becomes increasingly compressive and more susceptible to chemical damping. Our results demonstrate that ionisation non-equilibrium plays a key role in determining the nature of wave propagation and dissipation in weakly ionised solar plasmas.