Predicted white-light solar flare emission from the F-CHROMA grid of models

TL;DR

This study uses the F-CHROMA grid of solar flare simulations to analyze white-light emission mechanisms, finding that purely electron beam-driven models underpredict observed WL enhancements and identifying key factors influencing WL intensity.

Contribution

It provides the first detailed analysis of white-light emission in a comprehensive grid of flare simulations, highlighting the limitations of current models in reproducing observed WL enhancements.

Findings

Purely electron beam-driven models produce less than 4% WL enhancement.

Total energy correlates with the likelihood of detectable WL emission.

Hydrogen ionization and recombination dominate WL emission during maximum enhancement.

Abstract

Much of a solar flare's energy is thought to be released in the continuum. The optical continuum (white light) is of special interest due to the ability to observe it from the ground. We aim to investigate the prevalence of white-light (WL) emissions in simulations of purely electron beam-driven solar flares, what determines the occurrence of these enhancements, and the underlying causes. We utilized the F-CHROMA grid of flare simulations created using the radiative hydrodynamics code RADYN. We probed the spectral index, total energy, and low-energy cutoff to draw conclusions about their relationships to the white-light intensity. Furthermore, we calculated the 6684 {\AA} continuum intensities, the Balmer, and the Paschen ratios. Finally, we analyzed two particular cases, one with high 6684 {\AA} intensity and one with a large Balmer ratio, to determine the dominant mechanisms in these simulations. 33 of the 84 flares included in the F-CHROMA grid show white-light intensity enhancements that exceed 0.1% relative to the pre-flare level. We conclude that, with the parameters presented in the F-CHROMA grid, purely electron beam-driven simulations of solar flares are not able to reproduce observed WL enhancements, as the maximum enhancements in the grid are below 4%. The total energy (which is correlated with the maximum beam flux) is the main factor for deciding whether excess white-light emissions will be detectable. There is a linear relationship between the Balmer (and Paschen) ratio and the relative continuum increase. Both case studies show that during the time of maximum WL excess, hydrogen ionization and subsequent recombination in an optically thin medium is the dominant mechanism for WL continuum emission enhancements. Increased H$^-$ emission in the photosphere as a result of radiative backwarming becomes dominant during the declining phase of WL emissions in both case studies.