White-light radiation from semi-empirical flare models

Abstract

We show that some recently published semi-empirical models for solar flares predict a significant flux of visible continuum radiation, due to bound-free radiation from hydrogen atoms and H⁻ ions in the chromospheric flare. The ratio of the emergent intensity in one flare model to that in the quiet Sun is more than 100% at the head of the Balmer continuum for a flare close to the limb, and 8% at disk centre. The predicted flare spectrum has a relatively strong Balmer jump. We compare the theoretical flare continuum with observations and find disagreement in several important respects. The main disagreements are: (1) the fact that few flares are observed to emit a white-light continuum, while the models suggest that they should do so; (2) the prediction of a strong Balmer jump, which is not observed in most white-light flares; and (3) the absence of a ‘blue continuum’ in the theoretical prediction. We conclude that observations of flare continua provide useful constraints on semi-empirical models, and that at present the models do not satisfy these constraints.