Hydrogen Lyman−α and Lyman−β radiances and profiles in polar coronal holes

The hydrogen Lyman-alpha (Ly−α) 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−α profiles obtained by the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER) on the SOHO (Solar and Heliospheric Observatory) spacecraft in coronal holes and quiet Sun. In a subset of these observations, also the H i Lyman-β (Ly−β), Si iii, and Ovi lines were (quasi-) simultaneously recorded. We find that the distances between the two peaks of Ly−α 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−β profiles in the coronal hole have a stronger blue peak, in contrast to those in quiet-Sun regions. Whilst in both regions the Ly−α 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−α and Ly−β decreases towards 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 quiet Sun. Subject headings: Sun: UV radiation—Sun: transition region—Sun: corona— Line: formation—Line: profiles School of earth and space sciences, Peking University, 100871, Beijing, China; [email protected] Max-Planck-Institut für Sonnensystemforschung, 37191, Katlenburg-Lindau, Germany Institut d’Astrophysique Spatiale, Unité Mixte, CNRS-Université de Paris XI, Bat 121, 91405 Orsay, France