HIGH-RESOLUTION CENTER-TO-LIMB VARIATION OF THE QUIET SOLAR SPECTRUM NEAR Mg II

The accurate determination of the center-to-limb variation of the quiet solar spectrum is of fundamental importance to our understanding of both spatially resolved solar spectral radiance as well as full-disk spectral irradiance. Previous studies have examined the center-to-limb variation at various spectral resolutions using both observations and calculations. Here we derive the center-to-limb variation near Mg II at 2800 angstrom from observations made by the HRTS-9 rocket-spectrograph at both high spatial (1 '') and spectral (0.2 angstrom) resolution. This region of the solar spectrum is important because of its impact on the terrestrial atmosphere and its use in generating the Mg II index. The initial part of this paper presents the correction methods used to determine the center-to-limb variation, including the determination of the vignetting properties of the HRTS-9 instrument. This latter correction accounts for the intensity variation along the slit axis of the observed spectrogram. The resulting center-to-limb variation is employed in a model of solar spectral irradiance near Mg II. Using these results and calibrated solar spectra, the absolute intensity calibration is determined for the HRTS-9 spectra. An important aspect of the high-resolution center-to-limb variation is the presence of many spectral features illustrating the differences between the wings and cores of the numerous Fraunhofer lines in the 2765-2875 angstrom region. Also, both the Mg I and Mg II lines in this spectral region have broad, smooth wings that vary differently than the nearby line-blanketed continuum. The results of this study will provide a good source of comparison for detailed models of the quiet solar spectrum.