2D Radiative MHD Simulations of the Importance of Partial Ionization in the Chromosphere

TL;DR

This study uses advanced 2.5D radiative MHD simulations to explore how partial ionization affects the solar chromosphere's thermodynamics, revealing significant impacts of ambipolar diffusion and magnetic diffusivity variations.

Contribution

It introduces a comprehensive numerical approach incorporating partial ionization effects via generalized Ohm's law into chromospheric simulations, highlighting the importance of ambipolar diffusion.

Findings

Ambipolar diffusion can be as large as numerical diffusion in the chromosphere.

Magnetic diffusivity varies strongly with ion-neutral collision estimates.

Self-consistent simulations can accurately model ambipolar diffusion effects.

Abstract

The solar chromosphere is weakly ionized and interactions between ionized particles and neutral particles likely have significant consequences for the thermodynamics of the plasma. We investigate the importance of introducing neutral particles using numerical 2.5D radiative MHD simulations obtained with the Bifrost code. The models span from the upper layers of the convection zone to the low corona, and solve the full MHD equations with non-grey and NLTE radiative transfer, and thermal conduction. The effects of partial ionization are implemented using the generalized Ohm's law. The approximations required in going from three fluids to the generalized Ohm's law are tested in our simulations. The Ohmic diffusion, the Hall term, and ambipolar diffusion show strong variations in the chromosphere. These strong variations of the various magnetic diffusivities are absent or significantly underestimated when, as has been common for these types of studies, using the VAL-C model as a basis for estimates. In addition, we find that differences in estimating the magnitude of ambipolar diffusion arise depending on which method is used to calculate the ion-neutral collision frequency. These differences cause uncertainties in the different magnetic diffusivity terms. In the chromosphere, we find that the ambipolar diffusion is of the same order of magnitude or even larger than the numerical diffusion used to stabilize our code. As a consequence, ambipolar diffusion produces a strong impact on the modeled atmosphere. Perhaps more importantly, it suggests that at least in the chromospheric domain, self-consistent simulations of the solar atmosphere driven by magneto-convection can accurately describe the impact of the dominant form of resistivity (ambipolar diffusion). This suggests that such simulations may be more realistic in their approach to the lower solar atmosphere than previously assumed.