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

We study the emission of the hydrogen Lyman-a line in the quiet Sun, its center-to-limb variation (CLV), and its radiance distribution. We also compare quasi-simultaneous Ly-a and Ly-b line profiles. 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-a and Ly-b at Sun center in sit-and-stare mode. We infer the flow characteristic in the Ly-a map from variations in the calibrated 1206 Si III line centroids. We present the average profile of Ly-a, its radiance distribution, its CLV behaviour, and the signature of flows on the line profiles. Little CLV and no limb brightening are observed in the profiles of the Ly-a line. In contrast to all other lines of the Lyman series, which have a stronger red-horn, Ly-a has a 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 CLV in the profiles may be more indicative of an isotropic field than a radial flow. It appears that the ubiquitous hydrogen behaves similar to a filter that dampens all signatures of the line formation by processes in the chromosphere and transition region.