Hydrogen Lyman-α and Lyman-β spectral radiance profiles in the quiet Sun

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'' × 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.