# Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes

## TL;DR

This study develops semi-empirical models of sunspot chromospheres that accurately reproduce Mg II h&k line profiles, enhancing understanding of sunspot dynamics and chromospheric heating.

## Contribution

The paper introduces four new semi-empirical sunspot chromosphere models based on spectropolarimetric data and radiative transfer simulations, filling a gap in dynamic chromosphere modeling.

## Key findings

- Models reproduce Mg II h&k profiles well in sunspot penumbra.
- Supersonic downflows in models match observed profiles outside line core.
- Four models are made publicly available for further research.

## Abstract

Context. The solar chromosphere and the lower transition region is believed to play a crucial role in the heating of the solar corona. Models that describe the chromosphere (and the lower transition region), accounting for its highly dynamic and structured character are, so far, found to be lacking. This is partly due to the breakdown of complete frequency redistribution in the chromospheric layers and also because of the difficulty in obtaining complete sets of observations that adequately constrain the solar atmosphere at all relevant heights. Aims. We aim to obtain semi-empirical model atmospheres that reproduce the features of the Mg II h&k line profiles that sample the middle chromosphere with focus on a sunspot. Methods. We use spectropolarimetric observations of the Ca II 8542 A spectra obtained with the Swedish 1-m Solar Telescope (SST) and use NICOLE inversions to obtain semi-empirical model atmospheres for different features in and around a sunspot. These are used to synthesize Mg II h&k spectra using RH1.5D code, which we compare with observations taken with the Interface Region Imaging Spectrograph (IRIS). Results. Comparison of the synthetic profiles with IRIS observations reveals that there are several areas, especially in the penumbra of the sunspot, where most of the observed Mg II h&k profiles are very well reproduced. In addition, we find that supersonic hot downflows, present in our collection of models in the umbra, lead to synthetic profiles that agree well with the IRIS Mg II h&k profiles, with the exception of the line core. Conclusions. We put forward and make available four semi-empirical model atmospheres. Two for the penumbra, reflecting the range of temperatures obtained for the chromosphere, one for umbral flashes, and a model representative of the quiet surroundings of a sunspot. These are available in electronic as well as in table formats.