Temperature Dependence of Ultraviolet Line Average Doppler Shifts in the Quiet Sun

Abstract

The existence of prevailing redshifts in the UV lines formed in the solar transition region raises an important question concerning its physical origin and its role in the mass and energy balance of the outer solar atmosphere. A series of UV spectral lines observed by SUMER has been analyzed to obtain the spatial average of Doppler shifts in the quiet Sun as a function of temperature. The UV lines used for the analysis cover temperatures ranging from 104 to 106 K. The wavelength calibration has been done in reference to the coolest chromospheric lines such as neutral lines of silicon and sulfur. The positioning of the line center in blended lines has been made by employing a constrained multi-Gaussian fitting technique. The error in the measured average of the Doppler shifts is estimated to be smaller than 1 km s-1.