HYDROGEN Ly alpha AND Ly beta RADIANCES AND PROFILES IN POLAR CORONAL HOLES

The hydrogen Lya plays a dominant role in the radiative energy transport in the lower transition region, and is important for the studies of transition-region structure as well as solar wind origin. We investigate the Ly alpha profiles obtained by the Solar Ultraviolet Measurement of Emitted Radiation spectrograph on the Solar and Heliospheric Observatory spacecraft in coronal holes and the quiet Sun. In a subset of these observations, the H I Ly beta, Si III, and O VI lines were also (quasi-)simultaneously recorded. We find that the distances between the two peaks of Ly alpha profiles are larger in coronal holes than in the quiet Sun, indicating a larger opacity in coronal holes. This difference might result from the different magnetic structures or the different radiation fields in the two regions. Most of the Ly beta profiles in the coronal hole have a stronger blue peak, in contrast to those in quiet-Sun regions while in both regions the Ly alpha profiles are stronger in the blue peak. Although the asymmetries are likely to be produced by differential flows in the solar atmosphere, their detailed formation processes are still unclear. The radiance ratio between Ly alpha and Ly beta decreases toward the limb in the coronal hole, which might be due to the different opacity of the two lines. We also find that the radiance distributions of the four lines are set by a combined effect of limb brightening and the different emission level between coronal holes and the quiet Sun.