Non-equilibrium ionisation plasmas in the interstellar medium

Abstract

Obtaining astrophysical information from diffuse cool, warm and hot plasmas in interstellar and intergalactic media by electromagnetic radiation is based on highly non-linear heating and cooling processes, which are largely determined by atomic physical time scales and reaction rates. To calculate spectra is further complicated by gas dynamical interactions and processes, such as e.g. shock waves, fast adiabatic expansion and catastrophic cooling. In essence this leads to a non-linear coupling between atomic physics and hydro- or magnetohydrodynamics, which renders radiative cooling to become time- and space-dependent, contrary to the often conveniently used assumption of collisional ionisation equilibrium for optically thin plasmas. Computing power and new algorithms for high performance computing have made it possible to trace the dynamical and thermal evolution of a sufficiently large section of interstellar space over an appreciable time scale to derive characteristic quantities like temperature and density distribution as well as spectra, which can be compared to X-ray, UV and optical observations. In this review we describe diffuse interstellar plasma simulations, the physical processes which drive the temporal and spatial evolution, and present high resolution numerical simulations, including time-dependent cooling, which further our understanding of the state and evolution of interstellar (magnetised) plasmas. We also discuss briefly the rôle of cosmic rays and their interaction with the plasma.