Fraunhofer Lines in the Center-to-Limb Transition over the Solar Disk

Abstract

Two-dimensional spectra of the Sun (from the center to the limb of the disk) in wavelength ranges of λ = 532.0–532.8 and 539.1–539.9 nm were registered with the ATsU-5 telescope (the Russian abbreviation ATsU corresponds to the Astronomical Coelostat Mounting) of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (MAO NASU) after replacing the regular main mirror by a short-focus one (with a focal length of 1 m). An advantage of the applied method is that the spectrum is registered simultaneously for different heliocentric positions on the solar disk. The observational data were reduced for the influence of light scattered in the spectrograph, the atmospheric-stray light, the spectrograph instrumental profile, and some aberrations. The center-to-limb ratios of the spectra are compared to the data available in the literature. The data on variations in the profiles of eleven spectral lines in transition from the center to the limb of the solar disk were obtained. The revealed nonmonotonicity of these changes is explained by inhomogeneities in physical conditions on the surface of the Sun. In general, the depths of the studied Fe I lines demonstrate the decreasing tendency in the strength of lines when passing toward the limb. For most lines, the full widths at half maximum increase toward the disk edge. The equivalent widths show differently directed variations. The parameters of the line Mn I λ 539.4 nm behave in a different way: all three analyzed parameters grow toward the disk edge; however, at the extreme limb, the depth and the equivalent width also start to decrease. The limb-effect of lines was measured; it exhibits the highest value when the core positions in weak lines are compared. For strong lines, the limb-effect is strongest while comparing the middle parts of the line bisectors.