TEMPERATURE DEPENDENCE OF NONTHERMAL MOTIONS IN SOLAR FLARE PLASMAS OBSERVED WITH THE FLAT CRYSTAL SPECTROMETER ON SMM

摘要

We have analyzed the intensities and profiles of a number of soft X-ray lines, O VIII, Ne IX, Mg XI, Si XIII, S XV, Ca XIX, and Fe xxv, which have been observed during solar flares with the Flat Crystal Spectrometer (FCS) of the Solar Maximum Mission. These lines are emitted in the temperature range from 3 x 10~6 K to 5 x 10~7 K. The temperature distribution of the flare plasma, φ(T), computed on basis of the FCS line intensities, consists of two plasma components at different temperatures, 5-8 x 10~6 K and 16-25 x 10~6 K, respectively, in agreement with previous studies. Significant nonthermal motions in the flare plasma are inferred from the presence of nonthermal broadenings in the observed lines. The broadening is deduced by comparing observed profiles to synthetic ones derived using the differential emission measure calculated from a series of FCS line intensities. The Doppler temperature T_D, inferred from the observed line width is found to exceed systematically the average temperature of line formation, T_e, characterizing the width of the reconstructed thermal lines. Lines formed predominantly in the lower temperature, "quasi-hot" plasma component yield an average nonthermal velocity v_(nt) = 64 ± 3 km s~(-1), which is independent, within the statistical errors, of the temperature of line formation. The nonthermal motions found in the higher temperature, "hot" flare plasma are characterized by a higher velocity with a value which is an increasing function of temperature. There is a tendency for the temperature dependence of the nonthermal velocities to be stronger earlier in the flare and to decrease during the flare decay. We also find that the increase of nonthermal motions with temperature is a more general characteristic of the solar atmosphere. The law v_(nt) = 0.41 x T~(0.32) km s~(-1) is valid from the chromosphere to the corona, including all temperature regimes up to 10~7 K. The established relation between velocity of the nonthermal plasma motions and temperature, indicating a positive correlation both locally in flare plasmas and throughout the solar atmosphere, supports the idea that nonthermal velocities are very likely a manifestation of the heating process.