A single frequency approach to nonequilibrium modeling of the chromosphere

TL;DR

This paper introduces a single-frequency approximation method within the MURaM code to model non-LTE radiative transfer of Lyman lines in the solar chromosphere, improving simulation accuracy while reducing computational costs.

Contribution

The paper presents an extension to the MURaM code that incorporates a single-frequency approximation for Lyman line radiative transfer, enabling more accurate and efficient chromospheric modeling.

Findings

Good agreement with Lightweaver reference solutions

Improved hydrogen level population accuracy in the deep chromosphere

Maintained computational efficiency with the approximation

Abstract

The solar chromosphere is a region where radiation plays a critical role in energy transfer and interacts strongly with the plasma. In this layer, strong spectral lines, such as the Lyman lines, contribute significantly to radiative energy exchange. Due to the long ionization/relaxation timescale, departures from LTE become significant in the chromosphere. Accurately modeling this layer therefore requires one to solve the non-LTE radiative transfer for the Lyman transitions. We present an updated version of the MURaM code to enable more accurate simulations of chromospheric hydrogen level populations and temperature evolution. In the previous extension, a non-LTE equation of state, collisional transitions of hydrogen, and radiative transitions of non-Lyman lines were already implemented in the code. Building on this, we have now incorporated radiative transfer for the Lyman lines to compute radiative rate coefficients and the associated radiative losses. These were used to solve the population and temperature evolution equations, rendering the system self-consistent. To reduce computational cost, a single-frequency approximation was applied to each line in the numerical solution of the radiative transfer problem. The extended model shows good agreement with reference solutions from the Lightweaver framework, accurately capturing the radiative processes associated with Lyman lines in the chromosphere. The extension brings the simulated hydrogen level populations in the deep chromosphere closer to detailed radiative balance, while those in the upper chromosphere remain significantly out of balance, consistent with the expected conditions in the real solar atmosphere. The extension enables the MURaM code to accurately capture chromospheric dynamics.