# RADYNVERSION: Learning to Invert a Solar Flare Atmosphere with Invertible Neural Networks

## TL;DR

This paper introduces an invertible neural network that rapidly inverts observed solar line profiles to infer physical properties of the flaring solar atmosphere, demonstrating accurate results consistent with physical expectations.

## Contribution

The novel invertible neural network approach enables fast, physics-informed inversion of solar flare observations, trained on RADYN simulations, for detailed atmospheric diagnostics.

## Key findings

- Inverted atmospheres match expected density and temperature profiles.
- Bulk velocity gradients align with chromospheric evaporation signatures.
- The method achieves inversion times of approximately 10 microseconds.

## Abstract

During a solar flare, it is believed that reconnection takes place in the corona followed by fast energy transport to the chromosphere. The resulting intense heating strongly disturbs the chromospheric structure, and induces complex radiation hydrodynamic effects. Interpreting the physics of the flaring solar atmosphere is one of the most challenging tasks in solar physics. Here we present a novel deep learning approach, an invertible neural network, to understanding the chromospheric physics of a flaring solar atmosphere via the inversion of observed solar line profiles in H{\alpha} and Ca II {\lambda}8542. Our network is trained using flare simulations from the 1D radiation hydrodynamics code RADYN as the expected atmosphere and line profile. This model is then applied to single pixels from an observation of an M1.1 solar flare taken with SST/CRISP instrument just after the flare onset. The inverted atmospheres obtained from observations provide physical information on the electron number density, temperature and bulk velocity flow of the plasma throughout the solar atmosphere ranging from 0-10 Mm in height. The density and temperature profiles appear consistent with the expected atmospheric response, and the bulk plasma velocity provides the gradients needed to produce the broad spectral lines whilst also predicting the expected chromospheric evaporation from flare heating. We conclude that we have taught our novel algorithm the physics of a solar flare according to RADYN and that this can be confidently used for the analysis of flare data taken in these two wavelengths. This algorithm can also be adapted for a menagerie of inverse problems providing extremely fast ($\sim$10 {\mu}s) inversion samples.