Heating in the Lower Atmosphere and the Continuum Emission of Solar White-Light Flares

Observationally, there is a small fraction of solar white-light flares (WLFs), the so-called type II WLFs, showing an increased visible continuum but no significant Balmer jump and less strong chromospheric line emission in comparison with type I WLFs. The classical point of view, that the flare energy is initially released in the corona and then transported downward, can hardly explain WLFs of this kind. In this paper we explore the possibility that type II WLFs originate from a deeper layer. Assuming an in situ energy release, in particular in the form of high-energy particles, in a region around the temperature minimum, the continuum emission is computed in different time stages during the flare evolution. At first, nonthermal excitation and ionization of hydrogen atoms caused by bombarding particles result in a decline of the visible continuum. Later on, the lower atmosphere is gradually heated through radiative transfer, mitigating the continuum decline. In the final stage, when the particle bombardment stops while the atmosphere still keeps a heated state, we obtain a positive continuum contrast without an obvious Balmer jump. This meets the condition required for type II WLFs. The presence or absence of a continuum decline in the early stage of the flare provides a diagnostic tool for nonthermal processes in the lower atmosphere.