Acoustic/shock wave heating in the gravitationally stratified partially ionized plasmas: the two-fluid effects

 The chromospheric heating problem is a long-standing intriguing topic of solar physics, and the acoustic wave/shock wave heating in the chromospheric plasma has been investigated in the last several decades. It has been confirmed that acoustic waves, and the shock waves induced by the steepening acoustic waves in the gravitationally stratified chromospheric plasma, are able to transport energy to the chromosphere, but the energy supplied in this way is not necessarily sufficient for heating the chromosphere. Here, we further investigate the acoustic/shock wave heating process while taking into account the two-fluid effects.

 As the plasma in the chromosphere is weakly or partially ionized,  neutrals play an important role in wave propagation in this region. Therefore,  a two-fluid computational model treating neutrals and charged particles (electrons and ions) as two separate fluids is used for modelling the acoustic/shock wave propagation in idealised partially ionized plasmas, while taking into account the ion-neutral collisions, ionization and recombination. We have thus investigated  the collisional and reactive interactions between separated ions and neutrals, as well as the resulting effects in the acoustic/shock wave propagation and damping. In the numerical simulations, both the initial hydrostatic equilibrium and chemical equilibrium are taken into account to provide different density profiles for comparison.

We have found that the shock heating in the partially ionized plasmas strongly depends on the ionization fraction. In particular, the relatively smaller ionization fraction resulting from the initial chemical equilibrium significantly enhances the shock wave heating, which dominates the overall heating effect according to an approximated quantitative comparison. Moreover, the decoupling between ions and neutrals is also enhanced while considering ionization and recombination, resulting in stronger collisional heating.