The Radiative Hydrodynamics of Flare Loops Heated by Impulsive Bursts of Energetic Electrons

I have modeled the hydrodynamic and radiative response of a preflare solar loop atmosphere to a short (5 second) burst of energy in the form of energetic nonthermal electrons. Energy fluxes in my calculations range over values suggested by observations. I have improved on previous hydrodynamic flare calculations by taking into account optically thick losses in the flare chromosphere, by spatially resolving the flare transition region, and by self-consistently accounting for conductive flux saturation. My major conclusions are: (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 about 2.35c(,s), where c(,s) is the sound speed in the evaporated material. (3) A simple analytic "gasbag" model for the temporal variation of velocity in the explosively evaporated plasma successfully reproduces my own numerical results, as well as those of MacNeice et al (1983). (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 10('5)K is directly proportional to the pressure in the flare transition region. In the case of explosive evaporation, a short powerful pulse of Extreme ultraviolet radiation is emitted from temperatures near 10('5)K during the adjustment to this equilibrium. (5) The plasma driven downward by explosive evaporation is cool and dense in comparison with the chromospheric material ahead of it. This "chromospheric condensation" is the inevitable consequence of compression of a thermally stable heated plasma. (6) Simple models for the propagation of both radiative-acoustic waves and chromospheric condensations in the flare chromosphere, based on quasisteady equilibrium between flare heating and radiative losses, agree well with my numerical results.