| Abstract
| - Aims. We extend earlier work by studying the line profiles of the hydrogen Lyman- α and Lyman- β lines in the quiet Sun. They were obtained quasi-simultaneously in a raster scan with a size of about 150″ $\times$ 120″ near disk center. Methods. The self-reversal depths of the Ly- α and Ly- β profiles. we are quantified by measuring the maximum spectral radiances of the two horns and the minimum spectral radiance of the central reversal. The information on the asymmetries of the Ly- α and Ly- β profiles is obtained through calculating the 1st and 3rd-order moments of the line profiles. By comparing maps of self-reversal depths and the moments with radiance images of the Lyman lines, photospheric magnetograms, and Dopplergrams of two other optically thin lines, we studied the spatial distribution of the Ly- α and Ly- β profiles with different self-reversal depths, and investigated the relationship between profile asymmetries and flows in the solar atmosphere. Results. We find that the emissions of the Lyman lines tend to be more strongly absorbed in the internetwork, as compared to those in the network region. Almost all of the Ly- α profiles are self-reversed, while about 17% of the Ly- β profiles are not reversed. The ratio of Ly- α and Ly- β intensities seems to be independent of the magnetic field strength. Most Ly- α profiles are stronger in the blue horn, whereas most Ly- β profiles are stronger in the red horn. However, the opposite asymmetries of Ly- α and Ly- β are not correlated pixel-to-pixel. We also confirm that when larger transition-region downflows are present, the Ly- α and Ly- β profiles are more enhanced in the blue and red horns, respectively. The first-order moment of Ly- β, which reflects the combined effects of the profile asymmetry and motion of the emitting material, strongly correlates with the Doppler shifts of the Si iii and O vi lines, while this correlation is much weaker for Ly- α. Our analysis shows that both Ly- α and Ly- β might be more redshifted if stronger transition-region downflows are present. We also find that the observed average Ly- β profile is redshifted with respect to its rest position.
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