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

We have analyzed the intensities and profiles of a number of soft X-ra y lines, O VIII, Ne IX, Mg XI, Si XIII, S XV, Ca XIX, and Fe XXV, whic h have been observed during solar flares with the Flat Crystal Spectro meter (FCS) of the Solar Maximum Mission. These lines are emitted in t he temperature range from 3 x 10(6) K to 5 x 10(7) K. The temperature distribution of the flare plasma, phi(T), computed on basis of the FCS line intensities, consists of two plasma components at different temp eratures, 5-8 x 10(6) K and 16-25 x 10(6) K, respectively, in agreemen t with previous studies. Significant nonthermal motions in the flare p lasma are inferred from the presence of nonthermal broadenings in the observed lines. The broadening is deduced by comparing observed profil es to synthetic ones derived using the differential emission calculate d from a series of FCS line intensities. The Doppler temperature T(D), inferred from the observed line width is found to exceed systematical ly 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 aver age nonthermal velocity upsilonBAR(nt) = 64 +/- 3 km s-1, which is ind ependent, within the statistical errors, of the temperature of line fo rmation. The nonthermal motions found in the higher temperature, ''hot '' flare plasma are characterized by a higher velocity with a value wh ich 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 mor e general characteristic of the solar atmosphere. The law upsilon(nt) with 0.41 x T0.32 km s-1 is valid from the chromosphere to the corona, including all temperature regimes up to 10(7) K. The established rela tion 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 vel ocities are very likely a manifestation of the heating process.