SUMER Measurements of Nonthermal Motions: Constraints on Coronal Heating Mechanisms

We have determined nonthermal velocities in the quiet Sun at temperatures between 104 K and 2 ] 106 K by measuring the widths of a number of EUV and far-ultraviolet (FUV) lines taken with SUMER on board the SOHO spacecraft. The broadenings owing to the SUMER instrument and the nite opacity in each line have been carefully examined. The nonthermal velocity at temperatures below 2 ] 104 K is smaller than 10 km s~1. The velocity increases with temperature, reaches a peak value of 30 km s~1 around 3 ] 105 K, and then decreases with the temperature. The coronal nonthermal velocity is about 20 km s~1. There exists a strong correlation between intensity and nonthermal velocity at temperatures 2 ] 1041 ] 105 K. The correlation at higher temperatures weakens as temperature increases. Furthermore, there is a spatial correlation between the nonthermal velocities inferred from a set of any two lines with temperatures below 2 ] 105 K. Neither signicant center-to-limb variation nor meaningful dependence on the integration time was found from the measured nonthermal velocities. We have discovered the existence of high-velocity components in the observed S VI j933.4 line proles. The average nonthermal velocity and intensity fraction of this S VI line high-velocity component are found to be 55 km s~1 and 0.25, respectively.