Full-disc Ca II K observations -- a window to past solar magnetism

TL;DR

This paper reviews a century of Ca II K solar observations, highlighting their importance for studying solar magnetism and climate, while addressing data challenges and processing techniques to unlock their full potential.

Contribution

It provides a comprehensive overview of historical Ca II K data archives, discusses data inhomogeneities, and reviews processing methods and key scientific results.

Findings

Ca II K data span over a century from 40+ sites

Data inhomogeneities hinder full scientific utilization

Proper processing enhances understanding of solar magnetism

Abstract

First such observations were made in 1892 and since then various sites around the world have carried out regular observations, with Kodaikanal, Meudon, Mt Wilson, and Coimbra being some of the most prominent ones. By now, Ca II K observations from over 40 different sites allow an almost complete daily coverage of the last century. Ca II K images provide direct information on plage and network regions on the Sun and, through their connection to solar surface magnetic field, offer an excellent opportunity to study solar magnetism over more than a century. This makes them also extremely important, among others, for solar irradiance reconstructions and studies of the solar influence on Earth's climate. However, these data also suffer from numerous issues, which for a long time have hampered their analysis. Without properly addressing these issues, Ca II K data cannot be used to their full potential. Here, we first provide an overview of the currently known Ca II K data archives and sources of the inhomogeneities in the data, before discussing existing processing techniques, followed by a recap of the main results derived with such data so far.