The Sources of Solar Ultraviolet Variability between 2765 and 2885 A: Mgi, Mgii, Sii, and Continuum

Abstract

The variation of the solar spectrum between 2000 and 4000 A is a major component of the total irradiance variability of the Sun. Recent analyses suggest that variations in the solar ultraviolet flux at these wavelengths account for about 30% of the total solar irradiance variability. Most estimates of solar spectral irradiance variability in the ultraviolet are based on the ratio of the intensity of features such as solar plages and sunspots to the intensity of the quiet Sun. These ratios are referred to as contrast factors. To a large degree, contrast factors at ultraviolet wavelengths have not been measured. We present measurements of the average intensities of plage, sunspot, and quiet-Sun regions in the spectral range between 2765 and 2885 A, derived from high spatial and spectral resolution spectra obtained during the ninth rocket flight of the High Resolution Telescope and Spectrograph in 1995. From these average spectra, plage and sunspot contrast factors are determined. To our knowledge, these are the first contrast factors at these wavelengths derived from measured solar intensities. These spectra show a large contrast in the Mg I, Mg II, and Si I Fraunhofer lines and a much smaller contrast (1.01 ± 0.015 to 1.04 ± 0.04) in the line-blanketed continuum. Contrast factors are also determined for three intensity levels of the quiet Sun as well as for a single sunspot. Many fine spectral features in the contrast factors can be attributed to weak Fraunhofer lines of Cr II, Fe I, Fe II, and Mg I.