The PI will build on previous work improving the understanding of the role of photospheric magnetic activity in determining solar irradiance variation. The main objectives are: (1) improving empirical models of variations in total irradiance using the digital, full-disc CaK images produced from the Mount Wilson daily spectroheliograms obtained between 1915 and 1985; (2) using these digital CaK data to produce the first empirical models of long-term solar spectral irradiance variations in the ultraviolet and extreme ultraviolet, extending back to 1915; (3) extending digitization of the Mount Wilson data to include plates of different format obtained between 1905 and 1915; and (4) extending previous study of the behavior of plages and spots at times of highest solar activity.
