OBSERVED REDSHIFTS IN OV AND DOWNFLOWS IN THE SOLAR TRANSITION REGION

Solar UV observations reveal a redshifted emission at transition region temperatures, commonly interpreted as a net downflow of plasma. In earlier investigations the magnitude of the redshift has been found to increase with temperature, reaching a maximum at T = 10(5) K, and then to decrease toward higher temperatures. These observations, mostly from Skylab, suggested no significant shift of the O V line at 1218 angstrom formed at 2.4 x 10(5) K. The variation of the downflow velocity with temperature is, however, uncertain since there are few reliable observations of lines formed at higher temperatures. Using spectrograms from the High Resolution Telescope and Spectrograph (HRTS) we find an average net redshift of the O V lines at 1218 and 1371 angstrom at all locations extending from disk center to solar limb. A discrepancy between the observed flow velocity in the two lines is probably caused by uncertainty in the available laboratory wavelength of the intercombination line at 1218 angstrom (2s2 S-1(0)-2s2p P-3(1)). Thus, a short review of the laboratory measurements of this line is given. The observed shift in O V is compared with measurements of the O IV line at 1401 angstrom and the Si IV line at 1402 angstrom. Large variations in the shift are found along the instrument slit. Thus, blueshifts are also observed with the sites of the largest upflow located in the sunspot umbrae and in a quiet region close to an active region. Several workers have compared their numerical models of flows in the solar transition region with the earlier observations suggesting no significant shift in the O V at 1218 angstrom. Thus, our result could put new constraints on such models and will also be of importance for modeling stellar atmospheres. Similar observations should now be possible with the High Resolution Spectrograph on the Hubble Space Telescope.