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

Abstract

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-

Disentangling the solar activity–solar wind predictive causality at Space Climate scales

“…physical solar activity indicator. This emission index has been demonstrated to be a reliable proxy for the magnetic flux density at the Sun (Schrijver et al 1989;Ortiz and Rast 2005;Chatzistergos et al 2019b) and it has also been proven to trace long-term variations in solar activity (e.g., Judge 2006;Bertello et al 2016;Chatzistergos et al 2019a).…”