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Paper: Solar Activity and Irradiance Studies with Ca II Spectroheliograms: Potential and Problems
Volume: 368, The Physics of Chromospheric Plasmas
Page: 533
Authors: Ermolli, I.; Tlatov, A.; Solanki, S.K.; Krivova, N.A.; Singh, J.
Abstract: Various observatories around the globe carried out synoptic fulldisk observations of the Sun since the beginning of the 20th century. The archives created by these observations, especially those including Ca II spectroheliograms, have the potential of providing far more detailed information on solar activity than the indices usually used to study activity variations, solar cycle and irradiance changes. However, these data suffer significantly from various problems including numerous defects in the photographic plates, missing or inaccurate calibration of the blackening curve, changes in the positioning of the exit slit with respect to the spectral line and variable seeing. Here we discuss the quality of images obtained by the digitization of three historic Ca IIK time series, specifically those stored by the Arcetri, Kodaikanal and Mt Wilson Observatories. The aim of this work is to evaluate the potential value of these data for studies of solar activity and variability. It also shows the importance of the detailed and accurate image processing technique, in order to obtain uniform and trustable results from images coming from different historic archives.
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