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

¹ Institute of Theoretical Astrophysics, University of Oslo PO Box 1029 Blindern 0315 Oslo, Norway
² Rosseland Centre for Solar Physics, University of Oslo PO Box 1029 Blindern 0315 Oslo, Norway

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 July 2025
Accepted: 18 November 2025

Abstract

Context. Much of a solar flare’s energy is thought to be released in the continuum. The optical continuum, white light (WL), is of special interest since it can be observed from the ground.

Aims. We aim to investigate the prevalence of WL emissions in simulations of purely electron beam-driven solar flares, what determines the occurrence of these enhancements, and the underlying causes.

Methods. 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 WL intensity. Furthermore, we calculated the 6684 Å continuum intensities as well as the Balmer and the Paschen ratios. Finally, we analyzed two particular cases, one with a high 6684 Å intensity and one with a large Balmer ratio, to determine the dominant mechanisms in these simulations.

Results. Of the 84 flares included in the F-CHROMA grid, 33 show WL 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%, which is significantly lower than observational values. The total energy (which is correlated with the maximum beam flux) is the main factor for deciding whether excess WL emissions will be detectable or not. 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 in these electron beam-driven atmospheres. 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. We confirm the inability to reproduce the characteristics of type II WL flares, namely, the appearance of photospheric continuum enhancements without prior chromospheric emission increases and the lack of a clear Balmer jump, with the F-CHROMA grid.