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

Abstract

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. We have analysed over 100,000 historical images from 8 digitised photographic archives of the Arcetri, Kodaikanal, McMath-Hulbert, Meudon, Mitaka, Mt Wilson, Schauinsland, and Wendelstein observatories, as well as one archive of modern observations from the Rome/PSPT. The analysed data cover the period 1893--2018. We first performed careful photometric calibration and compensation for the centre-to-limb variation, and then segmented the images to identify plage regions. This has been consistently applied to both historical and modern observations. The plage series derived from different archives are generally in good agreement with each other. However, there are also clear deviations that most likely hint at intrinsic differences in the data and their digitisation. We showed that accurate image processing significantly reduces errors in the plage area estimates. Accurate photometric calibration also allows precise plage identification on images from different archives without the need to arbitrarily adjust the segmentation parameters. Finally, by comparing the plage area series from the various records, we found the conversion laws between them. This allowed us to produce a preliminary composite of the plage areas obtained from all the datasets studied here. This is a first step towards an accurate assessment of the long-term variation of plage regions.

30 pages, 22 figures, accepted in A&A

References in corpus (8)

• Evolution of the solar irradiance during the Holocene
• Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI and SDO/HMI observations
• Solar Irradiance Variability is Caused by the Magnetic Activity on the Solar Surface
• The Mount Wilson Ca II K index
• Modelling total solar irradiance since 1878 from simulated magnetograms
• Analysis of full disc Ca II K spectroheliograms. I. Photometric calibration and CLV compensation
• Sunspot Sizes and The Solar Cycle: Analysis Using Kodaikanal White-light Digitized Data
• Long-term Variations in the Intensity of Plages and Networks as Observed in Kodaikanal Ca-K Digitized Data

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