Propagation of thermal and hydromagnetic waves in an ionizing-recombining hydrogen plasma

Abstract

The propagation of thermal and magnetohydrodynamic (MHD) waves in a heat-conducting, hydrogen plasma, threaded by an external uniform magnetic field (B) and in which photoionization and photorecombination [H++e−⇌H+hν(χ)] processes are progressing, is investigated here using linear analysis. The resulting dispersion equation is solved analytically for varied strength (β≪1 and ∼1) and orientation of the magnetic field, where β denotes the ratio of plasma to magnetic pressures. Application of this model to the interstellar medium shows that heat conduction governs the propagation of thermal waves only at relatively high frequencies regardless of the plasma temperature, strength, and orientation of the magnetic field. When the direction of wave propagation is held perpendicular to B (i.e., k ⊥B), the magnetosonic phase velocity is closely Alfvénic for β≪1, while for β∼1 both the hydrostatic and magnetic pressures determine the wave velocity. As long as k ∥B, the fast (transverse) magnetosonic wave becomes an Alfvén wave for all frequencies independent of the plasma temperature and field strength, while the slow (longitudinal) magnetosonic wave becomes a pure sound wave. Amplification of thermal and MHD waves always occur at low frequencies and preferentially at temperatures for which the plasma is either weakly or partially ionized. Compared to previous analysis for the same hydrogen plasma model with B=0, the presence of the magnetic field makes the functional dependence of the physical quantities span a longer range of frequencies, which becomes progressively longer as the field strength is increased.