Ion-neutral interactions and non-equilibrium ionization in the solar chromosphere

TL;DR

This study investigates how non-equilibrium ionization and ion-neutral interactions influence the thermal structure of the solar chromosphere using advanced 2.5D radiative MHD simulations, revealing significant differences in heating processes.

Contribution

It introduces a comprehensive simulation including NEI and ion-neutral effects, highlighting their impact on chromospheric thermal dynamics beyond previous LTE-based models.

Findings

Heating in shock wakes is less efficient with NEI.

Heating in shock fronts is more efficient under NEI.

Significant thermal differences observed in various chromospheric features.

Abstract

The thermal structure of the chromosphere is regulated through a complex interaction of various heating processes, radiative cooling, and the ionization degree of the plasma. Here we study the impact on the thermal properties of the chromosphere when including the combined action of non-equilibrium ionization (NEI) of hydrogen and helium and ion-neutral interaction effects. We have performed a 2.5D radiative magnetohydrodynamic simulation including ion-neutral interaction effects by solving the generalized Ohm's law (GOL) as well as NEI for hydrogen and helium using the Bifrost code. The GOL equation includes ambipolar diffusion and the Hall term. We compare this simulation with another simulation that computes the ionization in local thermodynamic equilibrium (LTE) including ion-neutral interaction effects. Our numerical models reveal substantial thermal differences in magneto-acoustic shocks, the wake behind the shocks, spicules, low-lying magnetic loops, and the transition region. In particular, we find that heating through ambipolar diffusion in shock wakes is substantially less efficient, while in the shock fronts themselves it is more efficient, under NEI conditions than when assuming LTE.