Ca ii 854.2 nm in an enhanced network region simulated with MURaM-ChE

TL;DR

This study evaluates how well the MURaM-ChE simulation reproduces the average Ca ii 854.2 nm line profile of the Sun's chromosphere, emphasizing the roles of isotopic splitting and atmospheric dynamics.

Contribution

It demonstrates that the MURaM-ChE simulation can match observed line profiles when including isotopic splitting and sufficient atmospheric dynamics.

Findings

Simulated spectra agree well with observations.

Atmospheric dynamics are necessary to match line width.

Isotopic splitting explains the observed line asymmetry.

Abstract

The Ca ii 854.2 nm line is widely used to study the chromosphere of the Sun. In the quiet Sun, the spatially averaged line profile shows a red asymmetry and a redshift of the line center. It is known that the effect of isotopic splitting must be taken into account in the forward modeling to reproduce the observed asymmetry. So far, no numerical model could match an average observed line profile in terms of the line width and asymmetry. Our goal is to investigate how well a simulation computed with the chromospheric extension of the MURaM code (MURaM-ChE) reproduces the spatially averaged Ca ii 854.2 nm line profile. We aim to determine the contributions from the isotopic splitting versus the dynamics in the atmosphere to the resulting line width and asymmetry. We solve the radiative transfer problem three times, once considering only the most abundant isotope of calcium in the atmosphere, once taking six calcium isotopes into account, and finally using a single composite atom model. We find the forward modeled spatially and temporally averaged spectra to be in good agreement with an average observation of the quiet Sun. In order to match the observed line width, the simulated atmosphere must be sufficiently dynamic. The typical red asymmetry can only be reproduced by taking the isotopic splitting effect into account, as suggested in the literature.