SUMER observations of Doppler shift in the quiet Sun and in an active region

Abstract

The UV spectral lines formed at transition region temperatures in the solar atmosphere, show a prevailing redshifted emission. Using the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft, we measure the amount of line shift as a function of the temperature for several spectral lines formed in the range between 10(4) and 10(6) \ K. We analyze spectrograms relative to the quiet Sun and to the active region NOAA\ 7946. The velocities derived are increasing from a redshift of ~ \ 0 \ km ; s(-1) at ~ 20000\ K to 10\ km ; s(-1) at 1.9\ 10(5) \ K for the quiet Sun, and to ~ 15 \ km ; s(-1) at 10(5) \ K for the active region. At higher temperature an opposite behaviour is observed. In the quiet Sun a blueshift of ~ \ -2 \ km ; s(-1) is observed at the Ne viii formation temperature (6.3\ 10(5) \ K), while in the active region, a blue-shifted value around -8\ km ; s(-1) is observed for the same spectral line. The finding of blueshift in Ne viii is due to the adoption of a new rest wavelength of 770.428 Ä. By 10(6) \ K the blueshift is ~ \ -10 \ km ; s(-1) in the active region as measured by Fe xii 1242.