Extension of the F 10 . 7 index to 1905 using Mt . Wilson Ca K spectro h eliog ra ms

The F10.7 index provides a daily record of solar microwave emissions, which vary in rough proportion to the projected area of bright magnetic structures called plages and network, and also sunspots, on the sun's disk. The daily observations used to form the index only began in 1947. Recently, we digitized the archive of daily Ca K spectroheliograms obtained at Mt. Wilson Observatory between 1905-1984, and measured the area variations of plages and enhanced network, on these photographic plates. We calibrated these variations against he F10.7 index between 1947-1984, so we are able to construct a full-disk proxy of F10.7 extending back to 1905. The behavior of this extended index indicates that UV irradiance levels achieved near the peaks of sunspot cycles 15, 16, and 17 between 1915-1945, were 25-40% higher than would be estimated from behavior of the Zurich sunspot number, Rz.