Collisional and Reactive Multi-fluid Modeling of Acoustic Wave Propagation and Heating in Gravitationally Stratified Chromospheric plasma

Abstract:

To study acoustic wave propagation and the corresponding energy deposition in the partially ionized solar chromosphere, we use a multi-fluid computational model which 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 chromospheric plasma. In the present numerical simulations, the initial density is first specified to reach hydrostatic equilibrium, and chemical equilibrium is also taken into account to provide a consistent density profile that differs from the hydrostatic equilibrium density profiles. While the equilibrium is reached, external photospheric velocity drivers are imposed to introduce monochromatic acoustic waves. As is well known, the acoustic waves steepen in the lower solar atmospheric plasma due to the exponentially decreasing density, and at the same time they heat the plasma via the nonlinear collisional interaction leading to more dissipation. In particular, the present numerical results suggest that the initial chemical equilibrium is an important factor which eventually changes the heating rate. Moreover, introducing the ionization and recombination slows down the heating rate, since ionization process itself absorbs a significant amount of energy. In addition, the heating rate relates to the frequency of the acoustic waves, of which the steepening wave fronts introduce significant decoupling between ions and neutrals. More specifically, low frequency waves tend to heat the higher layers of the atmosphere at higher heating rates, because their kinetic energy is essentially not reduced in the lower regions and thus more energy may be deposited in higher altitudes where the density is much lower.