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To study acoustic wave propagation and the corresponding energy deposition inpartially ionized plasmas, we use a two-fluid computational model that treatsneutrals 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 reactiveinteractions between uncoupled ions and neutrals in the plasmas. In the presentnumerical simulations, the initial density is specified to reach hydrostaticequilibrium, and as a comparison, chemical equilibrium is also taken intoaccount to provide a density profile that differs from typical hydrostaticequilibrium profiles. External velocity drivers are then imposed to generatemonochromatic acoustic waves. As is well known, the upward propagating acousticwaves steepen in gravitationally stratified plasmas due to the exponentiallydecreasing density, and they heat the plasmas in the nonlinear regimes wherekinetic energy is dissipated by shock waves and collisional interactions. Inparticular, the lower ionization fraction resulting from the present initialchemical equilibrium significantly enhances the heating efficiency. Moreover,the ionization process absorbs a significant amount of energy, and thedecoupling between ions and neutrals is also enhanced while consideringionization and recombination. Therefore, simulations without consideringionization and recombination may overestimate the overall heating effects butalso underestimate the energy dissipation. The results also suggest that a moreaccurate ionization and recombination model could be essential for improvingthe modeling of partially ionized plasmas.

Two-fluid Modeling of Acoustic Wave Propagation in Gravitationally Stratified Isothermal Media