The Lyman-alpha line in various solar features. I - Observations

The authors give an overview of the main features of the solar atmosphere as observed in the Lyα line by the Ultraviolet Spectrometer and Polarimeter on the SMM. The authors use high spectral and spatial resolution data as well as broad-band spectroheliograms to characterize the various features, and they present the values which theoretical models have to explain. It is shown that on the disk (quiet Sun and active regions) the central reversal of the line is highly variable in depth and frequently shifted, leading to asymmetric profiles. Observations above the limb reveal the appearance of a dynamic layer (cloud layer) overlying the limb. The authors present its Lyα profile and show that it can be responsible for distorting the background profile of the line and leading to the observed asymmetric profiles.