Hydrogen Lyman-alpha and Lyman-beta radiances and profiles in polar coronal holes

TL;DR

This study analyzes hydrogen Lyman-alpha and Lyman-beta line profiles in coronal holes and quiet Sun regions, revealing differences in opacity, asymmetries, and radiance ratios that inform understanding of solar atmospheric structures and dynamics.

Contribution

It provides new observational insights into the differences in Lyman-alpha and Lyman-beta line profiles between coronal holes and quiet Sun regions, highlighting the effects of magnetic structures and radiation fields.

Findings

Larger peak separation in Ly-alpha profiles in coronal holes.

Stronger blue peak in Ly-beta profiles in coronal holes.

Radiance ratio of Ly-alpha to Ly-beta decreases towards the limb in coronal holes.

Abstract

The hydrogen Lyman-alpha plays a dominant role in the radiative energy transport in the lower transition region, and is important for the stud- ies of transition-region structure as well as solar wind origin. We investigate the Ly-alpha profiles obtained by SUMER in coronal holes and quiet Sun. In a subset of these observations, also the Hi Lyman-beta, Si iii, and O vi lines were (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. Whilst 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 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.