The Ly-α profile and center-to-limb variation of the quiet Sun

Aims. We study the emission of the hydrogen Lyman-α line in the quiet Sun, its center-to-limb variation, and its radiance distribution. We also compare quasi-simultaneous Ly-α and Ly-β line profiles. Methods. We used the high spectral and spatial resolution of the SUMER spectrometer and completed raster scans at various locations along the disk. For the first time, we used a method to reduce the incoming photon flux to a 20%-level by partly closing the aperture door. We also performed a quasi-simultaneous observation of both Ly-α and Ly-β at the Sun center in sit-and-stare mode. We infer the flow characteristic in the Ly-α map from variations in the calibrated λ 1206 Si iii line centroids. Results. We present the average profile of Ly-α, its radiance distribution, its center-to-limb behaviour, and the signature of flows on the line profiles. Little center-to-limb variation and no limb brightening are observed in the profiles of the Ly-α line. In contrast to all other lines of the Lyman series, which have a red-horn asymmetry, Ly-α has a robust and – except for dark locations – dominating blue-horn asymmetry. There appears to be a brightness-to-asymmetry relationship. A similar and even clearer trend is observed in the downflow-to-asymmetry relationship. This important result is consistent with predictions from models that include flows. However, the absence of a clear center-to-limb variation in the profiles may be more indicative of an isotropic field than a mainly radial flow. Conclusions. It appears that the ubiquitous hydrogen behaves in a similar way to a filter that dampens all signatures of the line formation by processes in both the chromosphere and transition region.