The Digital Database of Long-Term Solar Chromospheric Variation

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.

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Analysis of full-disc Ca II K spectroheliograms

Chatzistergos

Ermolli

Krivova

et al. 2020

A&A

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Historical solar Ca II K observations at the Kyoto and Sacramento Peak observatories

Chatzistergos

Ermolli

Krivova

et al. 2020

J. Phys.: Conf. Ser.

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Archives of full-disc solar Ca II K observations covering even short periods of time can offer important data to fill observation gaps and to allow a better transition from historical to modern data. Two examples of such archives are those from the Kyoto and Sacramento Peak observatories. As most other historical Ca II K data, they suffer from artefacts that need to be accounted for to derive accurate plage areas. Here we present the results of our analysis of these archives of solar Ca II K images.

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Full-disc Ca ii K observations—A window to past solar magnetism

Chatzistergos

Krivova

Ermolli

2022

Front. Astron. Space Sci.

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Full-disc observations of the Sun in the Ca ii K line provide one of the longest collections of solar data. First such observations were made in 1892 and since then various sites around the world have carried out regular observations, with Kodaikanal, Meudon, Mt Wilson, and Coimbra being some of the most prominent ones. By now, Ca ii K observations from over 40 different sites allow an almost complete daily coverage of the last century. Ca ii K images provide direct information on plage and network regions on the Sun and, through their connection to solar surface magnetic field, offer an excellent opportunity to study solar magnetism over more than a century. This makes them also extremely important, among others, for solar irradiance reconstructions and studies of the solar influence on Earth’s climate. However, these data also suffer from numerous issues, which for a long time have hampered their analysis. Without properly addressing these issues, Ca ii K data cannot be used to their full potential. Here, we first provide an overview of the currently known Ca ii K data archives and sources of the inhomogeneities in the data, before discussing existing processing techniques, followed by a recap of the main results derived with such data so far.

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The Digital Database of Long-Term Solar Chromospheric Variation

Cited by 3 publications

References 2 publications

Analysis of full-disc Ca II K spectroheliograms

Analysis of full-disc Ca II K spectroheliograms

Historical solar Ca II K observations at the Kyoto and Sacramento Peak observatories

Full-disc Ca ii K observations—A window to past solar magnetism

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