Flare Loop Radiative Hydrodynamics - Part Six - Chromospheric Evaporation due to Heating by Nonthermal Electrons

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Abstract

The response of the solar chromosphere to flare heating by nonthermal electrons is examined. A number of interesting phenomena appear in our numerical solutions of the equations of hydrodynamics and radiative transfer. Here we discuss one aspect of these results: the phenomenon of chromospheric evaporation. We present results for a range of heating fluxes and show how these may be understood in simple terms. Our major conclusions are as follows: (1) There is an energy flux threshold for "explosive" evaporation. Explosive evaporation occurs when the upper chromosphere is unable to radiate the flare energy deposited there, and is therefore heated rapidly to coronal temperatures. Energy fluxes less than this threshold produce "gentle" evaporation, in which the chromosphere is eaten away by conduction at a much slower rate. (2) The expansion velocity of explosively evaporated plasma cannot exceed ∼ 2.35c_s , where c_s is the sound speed in the evaporated material. (3) We derive a simple analytic model for the temporal variation of velocity in explosively evaporated plasma. This "gasbag" model, based on isothermal expansion of an impulsively heated mass of plasma, is used successfully to reproduce our own numerical results, as well as those of MacNeice et al. (1984). (4) The lower transition region, in both gentle and explosive evaporation, quickly reaches a quasisteady balance between conduction and radiation, so that the conductive flux at 105 K is given by 3.42 x 105 P ergs cm -2 s-1, where P(dyn cm-2) is the pressure in the flare transition region. In the case of explosive evaporation, a short powerful pulse of EUV radiation is emitted from plasma with temperatures near 105 K during the adjustment to this equilibrium.

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