Analysis of transition-region emission-line profiles from full-disk scans of the sun using the SUMER instrument on SOHO

We examine statistical properties of line profiles seen in full-disk observations with the UV spectrograph SUMER on board SOHO. In the SUMER data archive,full-disk data with complete spectral information are available only for wavelength regions including the He I (584 Angstrom), Ne VIII (770 Angstrom), C III (977 Angstrom) and C IV (1548 Angstrom) emission lines. In this paper we will concentrate on C IV and Ne VIII. Collectively these data provide us with the unique opportunity to study the properties of line profiles in the lower and upper solar transition-region beyond what could be achieved with earlier instruments. In particular, these data reveal the center-to-limb behavior of line shifts and line widths for the first time ina statistically meaningful way. For C IV these data show the well-known redshift: of the transition-region lines in the quiet Sun and a clear correlation of the Doppler shift and the nonthermal broadening to the intensity as a characteristic of the network structure. This correlation is not found in the coronal holes. No indications for the network can be found in Ne vm. For Ne VIII we find a center-to-limb variation of the line shift opposite to C IV, which leads to the conclusion that Ne VIII must be blueshifted at disk center. This also shows the need for a remeasurement of the wavelength of Ne VIII in the laboratory. The center-to-limb variation of the line width leads to the conclusion that the nonresolved motions are not isotropic with a preference for the vertical component. Both lines show a clear signal for an outflow in the polar coronal holes. We discuss the implications of these and other observations for models of the transition-region and corona. The line shift behavior of Ne VIII and other lines from the upper transition-region deserves deeper investigation and probably has pivotal importance in our understanding of the solar transition-region. We are pursuing such work.