VELOCITY-TEMPERATURE DISTRIBUTION IN THE EVAPORATING PLASMA DURING THE IMPULSIVE PHASE OF SOLAR-FLARES

In order to study the velocity-temperature distribution in the plasma evaporating from the chromosphere during the impulsive phase of solar flares, we have performed a combined analysis of the Ca XIX and Fe XXV spectra, obtained with the bent crystal spectrometer operating on the Solar Maximum Mission satellite. This analysis on data obtained during the impulsive phase of 11 flares of class M and X has shown that, in half the events, higher evaporation velocities are observed in the Fe XXV spectra. The velocity distributions derived from the Ca XIX spectra are characterized by a maximum velocity of 500 km s-1 (value which represents an average over the data set). The velocity distributions derived from Fe XXV extend to higher values with a maximum of 800 km s-1, on average. A direct measure of the electron temperature as a function of velocity shows that the evaporating plasma is not in isothermal conditions, and that the material flowing at higher velocity is characterized by higher temperatures. The simultaneity of high-velocity, high-temperature evaporation and the peak of the main hard X-ray burst indicates that high-velocity evaporation occurs when the energy injection in the chromosphere is at its maximum during the flare. A comparison of observed and simulated velocity distributions suggests that high evaporation velocities are caused by chromospheric heating with an energy flux which exceeds 10 ergs cm-2s-1.