OBSERVED REDSHIFTS IN THE SOLAR TRANSITION REGION ABOVE ACTIVE AND QUIET REGIONS

Solar UV spectral observations show a redshifted emission at temperatu res between the chromosphere and the corona. We have measured the magn itude of the redshift as a function of the temperature using solar spe ctrograms from the High Resolution Telescope and Spectrograph-HRTS, Th e velocity derived from the average redshift is found to increase up t o a temperature T approximate to 1.35 x 10(5) K in both quiet and acti ve regions, then decrease with increasing temperature, with the rate o f decrease depending critically on the laboratory wavelengths adopted for the transition region lines. This result illustrates the need for improved laboratory measurements. We find that the differential redshi ft between an active region and the surroundings increases smoothly wi th temperature, reaches a maximum velocity difference of 7 km s(-1) at 1.35 x 10(5) K and falls abruptly to zero at 2.3 x 10(5) K. This obse rvation is independent of the laboratory wavelengths. Suggestions rega rding the origin of the redshift are confronted with the results.