Investigation of the Long-term Variation of Solar Ca II K Intensity. II. Reconstruction of Solar UV Irradiance

Abstract

Reconstruction of long-term solar UV variations during the entire 20th century is reported. The sunspot number has been used for this purpose so far. By using the full-disk Ca K intensity as an additional solar UV proxy, the range of allowed values for the reconstructed UV irradiance becomes more restricted. We use long-term archival data of the photographic Ca K plates digitized at the Kodaikanal Solar Observatory. The photographic calibration method developed in our previous paper (Paper I) is applied. Various long-term proxy data of solar activity have been used to estimate past UV irradiance. In light of this context, some issues using the historical Ca K data are commented on.