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Description
Studies of long-term solar activity and variability require knowledge of the past evolution of the solar surface magnetism. An important source of such information are the archives of full-disc CaII K observations performed more or less regularly at various sites since 1892. We derive the plage area evolution over the last 12 solar cycles employing data from all CaII K archives available publicly in digital form known to us, including several as yet unexplored CaII K archives. We analyse more than 290000 full-disc CaII K observations from 43 datasets spanning the period 1892-2019. All images were consistently processed with an automatic procedure that performs the photometric calibration (if needed) and the limb-darkening compensation. The processing also accounts for artefacts plaguing many of the images, including some very specific artefacts such as bright arcs found in Kyoto and Yerkes data. The employed methods have previously been tested and evaluated on synthetic data and found to be more accurate than other methods used in the literature to treat a subset of the data analysed here. We have produced a plage area time-series from each analysed dataset. We found that the differences between the plage areas derived from individual archives are mainly due to the differences in the central wavelength and the bandpass used to acquire the data at the various sites. We have empirically cross-calibrated and combined the results obtained from each dataset to produce a composite series of plage areas. "Backbone" series are used to bridge all the series together. We have also shown that the selection of the backbone series has little effect on the final plage area composite. We have quantified the uncertainty of determining the plage areas with our processing due to shifts in the central wavelength and found it to be less than 0.01 in fraction of the solar disc for the average conditions found on historical data. We also found the variable seeing conditions during the observations to slightly increase the plage areas during activity maxima. We provide the so far most complete time series of plage areas based on corrected and calibrated historical and modern CaII K images. Consistent plage areas are now available on 88% of all days from 1892 onwards and on 98% from 1907 onwards.
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