Full-disc Ca II K observations -- a window to past 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.

47 pages, 13 figures, accepted for publication in Frontiers in Astronomy and Space Sciences

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Cited by in corpus (9)

• Re-calibration of the Sunspot Number: Status Report
• Understanding Sun-as-a-star variability of solar Balmer lines
• Analysis of full-disc H alpha observations: Carrington maps and filament properties in 1909-2022
• Investigating the effect of solar ambient and data characteristics on Ca II K observations and line profile measurements
• Solar photospheric spectrum microvariability II. Observed relations to magnetic activity and radial-velocity modulation
• Revisiting the SATIRE-S irradiance reconstruction: Heritage of Mt Wilson magnetograms and Ca II K observations
• Variations of the Ca II K Line Profile Parameters with Solar Latitude and Time Observed from Kodaikanal Solar Observatory
• Solar Irradiance Reconstruction over the Telescopic Era Using a Revised Photospheric Magnetic Field Model
• Reconstruction of total solar irradiance variability as simultaneously apparent from Solar Orbiter and Solar Dynamics Observatory