The potential of Ca II K observations for solar activity and variability studies

Ermolli, I.; Chatzistergos, Theodosios; Krivova, N. A.; Solanki, S. K.

doi:10.1017/s1743921318001849

Cited by 9 publications

(5 citation statements)

References 19 publications

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“…They also agree on the overall increase of solar activity over the first half of the 20th century and the decrease over recent decades. However, they also show significant inconsistencies, such as large differences in absolute value and in the detected short-and long-term trends (Ermolli et al, 2018;Chatzistergos et al, 2019b). Such differences are seen both between the results derived from different archives and between the same archives but processed and analyzed with different methodologies.…”

Section: Introductionmentioning

confidence: 98%

Delving into the Historical Ca ii K Archive from the Kodaikanal Observatory: The Potential of the Most Recent Digitized Series

et al. 2019

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Full-disc Ca II K photographic observations of the Sun carry direct information as regards the evolution of solar-plage regions for more than a century and are therefore a unique dataset for solar-activity studies. For a long time Ca II K observations were barely explored, but recent digitizations of multiple archives have allowed their extensive analysis. However, various studies have reported diverse results partly due to the insufficient quality of the digitized data. Furthermore, inhomogeneities have been identified within the individual archives, which, at least partly, could be due to the digitization. As a result, some of the archives, e.g. that from the Kodaikanal observatory, were re-digitized. The results obtained by different authors who analyzed the data from the new digitization of the Kodaikanal archive differ from each other and from those derived from the old digitization. Since the data were processed and analyzed using different techniques, it is not clear, however, whether the differences are due to the digitization or the processing of the data. To understand the reasons for such discrepancies, we analyze here the data from the two most recent digitizations of this archive. We use the same techniques to consistently process the images from both archives and to derive the plage areas from them. Some issues have been identified in both digitizations, implying that they are intrinsic characteristics of the data. Moreover, errors in timing of the observations plague both digitizations. Overall, the most recent 16-bit digitization offers an improvement over the earlier 8-bit one. It also includes considerably more data and should be preferred.

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Section: Introductionmentioning

confidence: 98%

Delving into the Historical Ca ii K Archive from the Kodaikanal Observatory: The Potential of the Most Recent Digitized Series

et al. 2019

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“…However, over the recent years, these limitations have been superseded, allowing a more detailed and careful analysis of existing Hα data. Only an accurate and consistent analysis of multiple archives would allow accounting for artefacts in the results derived from the individual datasets and thus a meaningful assessment of the underlying solar processes (Ermolli et al 2009a(Ermolli et al , 2018Chatzistergos et al 2019bChatzistergos et al , 2021.…”

Section: Introductionmentioning

confidence: 99%

Analysis of full-disc Hαobservations: Carrington maps and filament properties in 1909–2022

Chatzistergos,

Ermolli,

Banerjee

et al. 2023

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Context.Full-disc observations of the Sun in the Hαline provide information about the solar chromosphere, and in particular, about the filaments, which are dark and elongated features that lie along magnetic field polarity-inversion lines. This makes them important for studies of solar magnetism. Because full-disc Hαobservations have been performed at various sites since the second half of the 19th century, with regular photographic data having started at the beginning of the 20th century, they are an invaluable source of information on past solar magnetism.Aims.We derive accurate information about filaments from historical and modern full-disc Hαobservations.Methods.We consistently processed observations from 15 Hαarchives spanning 1909–2022. The analysed datasets include long-running ones such as those from Meudon and Kodaikanal, but also previously unexplored datasets such as those from Arcetri, Boulder, Larissa, and Upice. Our data processing includes photometric calibration of the data stored on photographic plates, the compensation for limb-darkening, and the orientation of the data to align solar north at the top of the images. We also constructed Carrington maps from the calibrated Hαimages.Results.We find that filament areas, similar to plage areas in Ca IIK data, are affected by the bandwidth of the observation. Thus, a cross calibration of the filament areas derived from different archives is needed. We produced a composite of filament areas from individual archives by scaling all of them to the Meudon series. Our composite butterfly diagram very distinctly shows the common features of filament evolution, that is, the poleward migration as well as a decrease in the mean latitude of filaments as the cycle progresses. We also find that during activity maxima, filaments cover ∼1% of the solar surface on average. The change in the amplitude of cycles in filament areas is weaker than in sunspot and plage areas.Conclusions.Analysis of Hαdata for archives with contemporaneous Ca IIK observations allowed us to identify and verify archive inconsistencies, which also have implications for reconstructions of past solar magnetism and irradiance from Ca IIK data.

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“…Ribes & Mein 1985;Kariyappa & Pap 1996;Foukal 1996Foukal , 1998Caccin et al 1998;Worden et al 1998;Zharkova et al 2003;Lefebvre et al 2005;Ermolli et al 2009b,a;Tlatov et al 2009;Bertello et al 2010;Dorotovič et al 2010;Sheeley et al 2011;Priyal et al 2014Priyal et al , 2017Chatterjee et al 2016Chatterjee et al , 2017. Overall, there are numerous published results from historical Ca II K images, which agree in some respects, but also show significant differences (see Ermolli et al 2018). A critical aspect of most of these studies is, however, that they used photometrically uncalibrated images and did not analyse the possible systematic changes of the quality of the images recorded by different observers over decades (see the discussion in Ermolli et al 2009b).…”

Section: Introductionmentioning

confidence: 94%

Analysis of full disc Ca II K spectroheliograms. II. Towards an accurate assessment of long-term variations in plage areas

Chatzistergos,

Ermolli,

Krivova

et al. 2019

Preprint

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Context. Reconstructions of past irradiance variations require suitable data on solar activity. The longest direct proxy is the sunspot number, and it has been most widely employed for this purpose. These data, however, only provide information on the surface magnetic field emerging in sunspots, while a suitable proxy of the evolution of the bright magnetic features, specifically faculae/plage and network, is missing. This information can potentially be extracted from the historical full-disc observations in the Ca II K line. Aims. We use several historical archives of full-disc Ca II K observations to derive plage areas over more than a century. Employment of different datasets allows identification of systematic effects in the images, such as changes in instruments and procedures, as well as an assessment of the uncertainties in the results. Methods. We have analysed over 100,000 historical images from 8 digitised photographic archives of the Arcetri, Kodaikanal, McMath-

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The potential of Ca II K observations for solar activity and variability studies

Cited by 9 publications

References 19 publications

Delving into the Historical Ca ii K Archive from the Kodaikanal Observatory: The Potential of the Most Recent Digitized Series

Delving into the Historical Ca ii K Archive from the Kodaikanal Observatory: The Potential of the Most Recent Digitized Series

Analysis of full-disc Hαobservations: Carrington maps and filament properties in 1909–2022

Analysis of full disc Ca II K spectroheliograms. II. Towards an accurate assessment of long-term variations in plage areas

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