Dynamic Models of Optical Emission in Impulsive Solar Flares

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Abstract
Numerical simulations of the dynamics and radiation in a solar —are loop are presented. The heating processes in the lower atmosphere include nonthermal heating by accelerated electrons and thermal soft X-ray irradiation from the —are-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 de—nite observational signature: an initial gentle phase, characterized by an atmospheric state of near- equilibrium, and a subsequent explosive phase, characterized by large material —ows and strong hydrody- namic 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 signi—- cantly Doppler-shifted, and the pro—les remain symmetric about their nominal line-center frequencies. Emission pro—les generated during the explosive phase can be highly Doppler-shifted and distorted, re—ecting 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 —rst under- goes a slight dimming, followed eventually by a brightening. The continuum behavior is controlled by the trade-oÜ 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 chromo- sphere, the contribution of the thermal soft X-rays to the overall energy balance in the lower atmosphere is negligible. Subject headings: methods: numericalradiative transferSun: chromosphereSun: —ares

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