  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.
