Nonthermal velocities in the solar transition zone observed with the high-resolution telescope and spectrograph

Abstract

Data obtained during the first rocket flight of the NRL High Resolution Telescope and Spectrograph (HRTS) have been used to study nonthermal velocities for spectral lines primarily covering the temperature range 104 to 2 × 106 K. The high spectral and spatial resolution, combined with an enhanced dynamic intensity range of the reduced data, has enabled us to study the distribution of the nonthermal velocities for quiet and active regions. Average values of the nonthermal velocities peak at about 27 km s−1 at 105 K for the quiet regions, with a wide distribution of nonthermal velocities for each line. The active region nonthermal velocities have a narrower distribution which is weighted towards higher values. The SiIV and C IV line profiles are not well described by a single Gaussian, indicating that high-velocity components (above 30 km s−1) are present in the quiet-Sun spectra. The radiative losses for all plasma above l05 K have been calculated for the quiet Sun, an active region and a coronal hole. These have been compared with the acoustic wave flux inferred from the nonthermal line widths. There appears to be a sufficient flux of waves to heat these regions of the atmosphere.