# Modelling Mg II During Solar Flares. I. Partial Frequency Redistribution, Opacity, and Coronal Irradiation

## TL;DR

This paper investigates the physics necessary for accurate Mg II line modeling during solar flares, emphasizing the importance of partial frequency redistribution, non-LTE effects, and coronal irradiation in radiation transfer simulations.

## Contribution

It systematically compares modeling approaches for Mg II lines in flares, highlighting the physics required for realistic forward modeling and modifications to the RH code for including irradiation effects.

## Key findings

- PRD remains essential during flare simulations.
- Full angle-dependent PRD influences solutions but is computationally intensive.
- Including Mg I NLTE has negligible effect on Mg II lines but affects the NUV continuum.

## Abstract

The Interface Region Imaging Spectrograph (IRIS) has routinely observed the flaring Mg II NUV spectrum, offering excellent diagnostic potential and a window into the location of energy deposition. A number of studies have forward modelled both the general properties of these lines and specific flare observations. Generally these have forward modelled radiation via post-processing of snapshots from hydrodynamic flare simulations through radiation transfer codes. There has, however, not been a study of how the physics included in these radiation transport codes affects the solution. A baseline setup for forward modelling MgII in flares is presented and contrasted with approaches that add or remove complexity. It is shown for Mg II: (1) PRD is still required during flare simulations despite the increased densities, (2) using full angle-dependent PRD affects the solution but takes significantly longer to process a snapshot, (3) including Mg I in NLTE results in negligible differences to the Mg II lines but does affect the NUV quasi-continuum, (4) only hydrogen and Mg II need to be included in NLTE, (5) ideally the non-equilibrium hydrogen populations, with non-thermal collisional rates, should be used rather than the statistical equilibrium populations, (6) an atom consisting of only the ground state, h & k upper levels, and continuum level is insufficient to model the resonance lines, and (7) irradiation from a hot, dense flaring transition region can affect the formation of Mg II. We discuss modifications to the RH code allowing straightforward inclusion of transition region and coronal irradiation in flares.