Numerical simulations of impulsively heated solar flares

Abstract

We present the results of a series of numerical simulations which detail the response of a model solar atmosphere to heating by an electron beam. The electron beam flux spectrum is a power law with a low-energy "knee"; it rises linearly with time to a peak at 30 s, then falls linearly to zero at 60 s. We have calculated model flare atmospheres with "knee" energies ranging from 10 to 20 keV, spectral indices of 4 and 6, and peak beam fluxes from 10 10 to 10 11 ergs cm-2 s-1. When the beam parameters are such that more of the energy is deposited in the chromospheric layers of the model, the coronal temperatures and densities tend to be lower, their rate of increase as the flare evolves is slower, and the upflow velocity of the plasma moving into the corona is smaller. The reduced temperature is due to the reduced coronal heating, while the other two effects are caused by radiation in the chromospheric layers of the model removing energy that would otherwise be available to heat strongly, and hence evaporate, mass in the preflare chromosphere. Thus higher peak electron beam fluxes, lower low-energy "knees," and larger spectral indices all move the atmospheric response toward greater enhancements of the parameters in the coronal regions of the atmosphere. Conversely , therefore, the coronal response-as evidenced, for example, in soft X-ray line profiles-can be used as a diagnostic of the parameters of the electron beam.