Non-equilibrium helium ionization in an MHD simulation of the solar atmosphere

TL;DR

This study presents a 2D MHD simulation of the solar atmosphere that incorporates non-equilibrium helium ionization, revealing significant impacts on temperature distribution and radiating material, crucial for understanding chromospheric dynamics.

Contribution

The paper introduces the first 2D radiation-MHD simulation including non-equilibrium helium ionization, improving modeling accuracy of the solar chromosphere's thermal and dynamic properties.

Findings

Non-equilibrium helium ionization causes higher temperatures in wave fronts.

It results in more radiating material in the 11-18 kK temperature range.

LTE assumption leads to a thermostat-like behavior in temperature distribution.

Abstract

The ionization state of the gas in the dynamic solar chromosphere can depart strongly from the instantaneous statistical equilibrium commonly assumed in numerical modeling. We improve on earlier simulations of the solar atmosphere that only included non-equilbrium hydrogen ionization by performing a 2D radiation-magneto-hydrodynamics simulation featuring non-equilibrium ionization of both hydrogen and helium. The simulation includes the effect of hydrogen Lyman-$\alpha$ and the EUV radiation from the corona on the ionization and heating of the atmosphere. Details on code implementation are given. We obtain helium ion fractions that are far from their equilibrium values. Comparison with models with LTE ionization shows that non-equilibrium helium ionization leads to higher temperatures in wave fronts and lower temperatures in the gas between shocks. Assuming LTE ionization results in a thermostat-like behaviour with matter accumulating around the temperatures where the LTE ionization fractions change rapidly. Comparison of DEM curves computed from our models shows that non-equilibrium ionization leads to more radiating material in the temperature range 11-18 kK compared to models with LTE helium ionization. We conclude that non-equilbrium helium ionization is important for the dynamics and thermal structure of the upper chromosphere and transition region. It might also help resolve the problem that intensities of chromospheric lines computed from current models are smaller than those observed.