Delving into the Historical Ca II K Archive from the Kodaikanal Observatory: the Potential of the Most Recent Digitised Series

Abstract

Full-disc Ca II K photographic observations of the Sun carry direct information about 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 digitisations of multiple archives have allowed their extensive analysis. However, various studies have reported diverse results partly due to the insufficient quality of the digitised data. Furthermore, inhomogeneities have been identified within the individual archives, which, at least partly, could be due to the digitisation. As a result, some of the archives, e.g. that from the Kodaikanal observatory, were re-digitised. The results obtained by different authors who analysed the data from the new digitisation of the Kodaikanal archive differ from each other as well as from those derived from the old digitisation. Since the data were processed and analysed using different techniques, it is not clear, however, whether the differences are due to the digitisation or the processing of the data. To understand the reasons for such discrepancies, we analyse here the data from the two most recent digitisations 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 digitisations, implying that they are intrinsic characteristics of the data. Moreover, errors in timing of the observations plague both digitisations. Overall, the most recent 16-bit digitisation offers an improvement over the earlier 8-bit one. It also includes considerably more data and should be preferred.

arXiv comment28 pages, 17 figures, accepted for publication in solar physics

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Cited by in corpus (9)

• Analysis of full disc Ca II K spectroheliograms III. Plage area composite series covering 1892-2019
• Full-disc Ca II K observations -- a window to past solar magnetism
• Reconstructing solar irradiance from historical Ca II K observations. I. Method and its validation
• Modelling solar irradiance from ground-based photometric observations
• Scrutinizing the relationship between plage areas and sunspot areas and numbers
• Time-Latitude Distribution of Prominences for 10 Solar Cycles: A study using Kodaikanal, Meudon and Kanzelhohe Data
• Analysis of full-disc H alpha observations: Carrington maps and filament properties in 1909-2022
• Determining the Variations of Ca-K index and Features using a Century Long Equal Contrast Images from Kodaikanal Observatory
• Investigating the effect of solar ambient and data characteristics on Ca II K observations and line profile measurements