The Chromosphere in Coronal Holes and the Quiet-Sun Network: an HE I (584 Å) Full-Disk Scan by SUMER/SOHO

Abstract

A raster scan by Solar Ultraviolet Measurements of Emitted Radiation/Solar and Heliospheric Observatory (SOHO) of the full solar disk containing over 2 million individual spectra of the He I line at 584 Å provides the possibility to study small- and large-scale variations of intensity, line shift, and line width. New striking features are the enhanced line width and the blueshifts in the coronal holes as well as the long-known reduced intensity there. The relation of intensity versus line shift and the variation of the line shift in the coronal holes indicate that the blueshifts in the holes are not consistent with a uniform outflow. The observations suggest that optical depth effects might be responsible for the observed blueshifts and line widths. These results motivate new observations by SOHO and ground-based facilities.