Plage areas from seven historical Ca II K archives

Understanding Sun's effect on Earth's climate requires a long and accurate record of solar irradiance measurements. Direct observations of solar irradiance exist only since 1978 and hence do not allow an evaluation of any possible longer-term trends in the irradiance variations. Besides, to reconstruct solar irradiance further back in time models have used data describing the evolution of the solar surface magnetism. Such data, however describe only the sunspots or provide indirect information on the solar surface magnetic field. Ca II K observations have the potential of providing the missing information about the plage contribution to irradiance variations for the entire 20th century and thus can improve the irradiance reconstructions. However, direct use of such data for irradiance reconstructions has been hindered due to the lack of photometric calibration and image defects.

We have developed an automatic procedure to overcome most of the issues affecting such images and to perform the photometric calibration. Using synthetic data we showed that this method performed better than other methods that have been presented in the literature. We will present results of applying our method on 7 historical and 1 modern Ca II K archives. Applying consistent processing on all archives allows us to assess and discuss inconsistencies among the various archives. Finally we will present preliminary composite series of plage areas covering the period 1907-1999.