Analysis of full-disc Ca II K spectroheliograms

III. Plage area composite series covering 1892–2019^⋆

¹ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: chatzistergos@mps.mpg.de
² Max Planck Institute for Solar System Research, Justus-von-Liebig-weg 3, 37077 Göttingen, Germany
theodosios.chatzistergos@inaf.it
³ School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, Republic of Korea
⁴ Indian Institute of astrophysics, Koramangala, Bangalore 560034, India
⁵ Aryabhatta Research Institute of Observational Sciences, Nainital, 263 001 Uttarakhand, India
⁶ Univ Coimbra, CITEUC - Center for Earth and Space Research of the University of Coimbra, Geophysical and Astronomical Observatory, 3040-004 Coimbra, Portugal
⁷ Observatory Upice, U lipek 160, 542 32 Úpice, Czech republic
⁸ Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos 3004, 18080 Granada, Spain
⁹ National Astronomical Observatory of Japan 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ Astronomical Institute of Kharkiv V.N. Karazin National University, 35 Sumskaya St., Kharkiv 61022, Ukraine
¹¹ LESIA, Observatoire de Paris, 92195 Meudon, France
¹² PSL Research University, Paris, France
¹³ Kislovodsk Mountain Astronomical Station, Central Astronomical Observatory, Russian Academy of Sciences, Kislovodsk, Russia

Received: 14 February 2020
Accepted: 4 May 2020

Abstract

Context. Studies of long-term solar activity and variability require knowledge of the past evolution of the solar surface magnetism. The archives of full-disc Ca II K observations that have been performed more or less regularly at various sites since 1892 can serve as an important source of such information.

Aims. We derive the plage area evolution over the last 12 solar cycles by employing data from all Ca II K archives that are publicly available in digital form, including several as-yet-unexplored Ca II K archives.

Methods. We analysed more than 290 000 full-disc Ca II K observations from 43 datasets spanning the period between 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 affecting many of the images, including some very specific artefacts, such as bright arcs found in Kyoto and Yerkes data. Our 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.

Results. We 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 empirically cross-calibrated and combined the results obtained from each dataset to produce a composite series of plage areas. The ’backbone’ approach was used to bridge the series together. We have also shown that the selection of the backbone series has little effect on the final composite of the plage area. We 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 the activity maxima.

Conclusions. We provide the most complete so far time series of plage areas based on corrected and calibrated historical and modern Ca II K images. Consistent plage areas are now available on 88% of all days from 1892 onwards and on 98% from 1907 onwards.