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Chatzistergos T, Ermolli I, Krivova NA, Solanki SK, Banerjee D, Barata T, Belik M, et al. (July 2020). "Analysis of full-disc Ca II K spectroheliograms – III. Plage area composite series covering 1892–2019". Astronomy and Astrophysics. 639: A88. arXiv:2005.01435. Bibcode:2020A&A...639A..88C. doi:10.1051/0004-6361/202037746. S2CID218487277.
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Chatzistergos T, Ermolli I, Krivova NA, Solanki SK, Banerjee D, Barata T, Belik M, et al. (July 2020). "Analysis of full-disc Ca II K spectroheliograms – III. Plage area composite series covering 1892–2019". Astronomy and Astrophysics. 639: A88. arXiv:2005.01435. Bibcode:2020A&A...639A..88C. doi:10.1051/0004-6361/202037746. S2CID218487277.