  To study acoustic wave propagation and the corresponding energy deposition in
partially ionized plasmas, we use a two-fluid computational model that treats
neutrals and charged particles (electrons and ions) as two separate fluids.
This two-fluid model takes into account the ion-neutral collisions, ionization,
and recombination, allowing us to investigate both the collisional and reactive
interactions between uncoupled ions and neutrals in the plasmas. In the present
numerical simulations, the initial density is specified to reach hydrostatic
equilibrium, and as a comparison, chemical equilibrium is also taken into
account to provide a density profile that differs from typical hydrostatic
equilibrium profiles. External velocity drivers are then imposed to generate
monochromatic acoustic waves. As is well known, the upward propagating acoustic
waves steepen in gravitationally stratified plasmas due to the exponentially
decreasing density, and they heat the plasmas in the nonlinear regimes where
kinetic energy is dissipated by shock waves and collisional interactions. In
particular, the lower ionization fraction resulting from the present initial
chemical equilibrium significantly enhances the heating efficiency. Moreover,
the ionization process absorbs a significant amount of energy, and the
decoupling between ions and neutrals is also enhanced while considering
ionization and recombination. Therefore, simulations without considering
ionization and recombination may overestimate the overall heating effects but
also underestimate the energy dissipation. The results also suggest that a more
accurate ionization and recombination model could be essential for improving
the modeling of partially ionized plasmas.
