ANALYSIS OF TRANSITION-REGION EMISSION-LINE PROFILES FROM FULL-DISK SCANS OF THE SUN USING THE SUMER INSTRUMENT ON SOHO

摘要

We examine statistical properties of line profiles seen in full-disk observations with the UV spectro- graph 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 A), Ne VIII (770 A), C III (977 A) and C IV (1548 A) emission lines. In this paper we will concentrate on C IV and Ne VIII. Collec- tively 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 par- ticular, these data reveal the center-to-limb behavior of line shifts and line widths for the first time in a 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 VIII. 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 varia- tion 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 discu