Dynamic models of optical emission in impulsive solar flares

Numerical simulations of the dynamics and radiation in a solar hare loop are presented. The heating processes in the lower atmosphere include nonthermal heating by accelerated electrons and thermal soft X-ray irradiation from the flare-heated transition region and corona. Important transitions of hydrogen, helium, and singly ionized calcium and magnesium are treated in non-LTE. The principal results of the analysis are the following: 1. An impulsive event can be described as having two distinct dynamic phases, each with a definite observational signature: an initial gentle phase, characterized by an atmospheric state of near-equilibrium, and a subsequent explosive phase, characterized by large material flows and strong hydrodynamic waves and shocks. The amount of time the atmosphere remains in the gentle phase is determined by the level of nonthermal heating. Excess line emission produced during the gentle phase is not significantly Doppler-shifted, and the profiles remain symmetric about their nominal line-center frequencies. Emission profiles generated during the explosive phase can be highly Doppler-shifted and distorted, reflecting the material motion and steep velocity gradients present at their depths of formation. 2. Hydrogen recombination radiation, from a high-temperature plateau formed during the gentle phase and from a chromospheric condensation formed during the explosive phase, is the primary cause of the white-light continuum brightening produced in the simulations. 3. A time lag is found between the brightening of the Paschen continuum and the brightening of the near wings of Ha, in the sense that the line wings brighten immediately, but the continuum first undergoes a slight dimming, followed eventually by a brightening. The continuum behavior is controlled by the trade-off between increased photoionization from enhanced populations of the excited states of hydrogen and increased electron density, which promotes recombination. 4. During the impulsive phase, when the accelerated electrons are directly heating the upper chromosphere, the contribution of the thermal soft X-rays to the overall energy balance in the lower atmosphere is negligible.