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

Abstract

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-αand 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-αand Ly-βat the Sun center in sit-and-stare mode. We infer the flow characteristic in the Ly-α map from variations in the calibrated β 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-αhas 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. © 2008 ESO.