The SUMER Ly-α line profile in quiescent prominences

Aims. As the result of a novel observing technique, we publish for the first time SoHO-SUMER observations of the true spectral line profile of hydrogen Lyman-α in quiescent prominences. Because SoHO is not in Earth orbit, our high-quality data set is free of geocoronal absorption. We studied the line profile to complement earlier observations of the higher Lyman lines and to substantiate recent model predictions. Methods. We applied the reduced-aperture observing mode to two prominence targets and did a statistical analysis of the line profiles in both data sets. In particular, we investigated the shape of the profile, the radiance distribution, and the line shape-to-radiance interrelation. We also compared Ly-α data to co-temporal λ 1206 Si iii data. Results. We find that the average profile of Ly-α has a blue-peak dominance and is reversed more if the line-of-sight is perpendicular to the field lines. The contrast of Ly-α prominence emission rasters is very low, and the radiance distribution differs from the lognormal distribution of the disk. Features in the Si iii line are not always co-spatial with Ly-α emission. Conclusions. Our empirical results support recent multi-thread models, which predict that asymmetries and depths of the self-reversal depend on the orientation of the prominence axis relative to the line-of-sight.