Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere

TL;DR

This paper presents a 2D simulation of the solar atmosphere incorporating non-equilibrium hydrogen ionization, revealing significant differences from LTE assumptions in temperature variations and hydrogen level populations.

Contribution

The authors developed and integrated a non-equilibrium hydrogen ionization algorithm into an MHD code for 2D solar atmosphere simulations, providing more accurate modeling of chromospheric dynamics.

Findings

Non-equilibrium ionization reduces hydrogen ionization variations compared to LTE.

Chromospheric temperature variations are larger with non-equilibrium ionization.

Hydrogen level populations influence Halpha opacity and are affected by shocks.

Abstract

The ionization of hydrogen in the solar chromosphere and transition region does not obey LTE or instantaneous statistical equilibrium because the timescale is long compared with important hydrodynamical timescales, especially of magneto-acoustic shocks. We implement an algorithm to compute non-equilibrium hydrogen ionization and its coupling into the MHD equations within an existing radiation MHD code, and perform a two-dimensional simulation of the solar atmosphere from the convection zone to the corona. Analysis of the simulation results and comparison to a companion simulation assuming LTE shows that: a) Non-equilibrium computation delivers much smaller variations of the chromospheric hydrogen ionization than for LTE. The ionization is smaller within shocks but subsequently remains high in the cool intershock phases. As a result, the chromospheric temperature variations are much larger than for LTE because in non-equilibrium, hydrogen ionization is a less effective internal energy buffer. The actual shock temperatures are therefore higher and the intershock temperatures lower. b) The chromospheric populations of the hydrogen n = 2 level, which governs the opacity of Halpha, are coupled to the ion populations. They are set by the high temperature in shocks and subsequently remain high in the cool intershock phases. c) The temperature structure and the hydrogen level populations differ much between the chromosphere above photospheric magnetic elements and above quiet internetwork. d) The hydrogen n = 2 population and column density are persistently high in dynamic fibrils, suggesting that these obtain their visibility from being optically thick in Halpha also at low temperature.