Scrutinising the relationship between plage areas and sunspot areas and numbers

Abstract

Context. Studies and reconstructions of past solar activity require data on all magnetic regions on the surface of the Sun (i.e. on dark sunspots as well as bright faculaeplage and network). Such data are also important for understanding the magnetic activity and variability of the Sun and Sun-like stars. The longest available direct faculaeplage datasets are white-light facular and Ca ii K observations going back to 1874 and 1892, respectively. Prior to that time the only direct data available are for sunspots. Aims. We reassess the relationship between plage areas and sunspot records (areas and numbers) since 1892, to allow reconstructions of facularplage areas which can be employed for studies going further back in time to the period when solely sunspot observations are available. Methods. We use the plage areas derived from 38 consistently processed Ca ii K archives as well as the plage area composite based on these archives. The considered archives include both the well-known observatories (e.g. Coimbra, Kodaikanal, Meudon, Mt Wilson), and the less explored observatories (e.g. Kharkiv, Mees, and Upice). These data allow us to study the relationship between plage area and sunspot records (areas and number) over a period of 12 solar cycles and for dierent bandpasses. Results. We nd the relationship between plage and sunspot areas to be well represented by a power-law function; the relationship between the plage areas and the sunspot number is also best t with a power-law function. We further nd that the relationships depend on the bandwidth and the solar cycle strength. The reconstructions with a power-law relationship are in good agreement with the original plage area series, whereas employment of a cycle-strength-dependent relationship improves the reconstructions only marginally. We also estimate the error in the plage areas reconstructed from the sunspot areas or numbers. Performing the same analysis on other previously published plage area series, usually derived from a single archive with diverse processing techniques, returns dierent results when using dierent time series. This highlights the importance of applying a consistent processing to the various archives and demonstrates the uncertainties introduced by using previously published series for studies of past solar activity, including irradiance reconstructions. Conclusions. Our results have implications for past solar activity and irradiance reconstructions and for stellar activity studies, which sometimes assume a linear dependence between plage and sunspot areas.

Full-disc Ca ii K observations—A window to past solar magnetism

“…Past studies, in general, suggest a quadratic relation between plage and sunspot areas for daily values. A linear relationship is typically favored for annual sunspot values, however Chatzistergos et al (2022) reported a, in general, non linear relationship even for annual values. For sunspot numbers the reported relation is typically linear when annual values are considered, but non-linearities have been reported for daily values (Chatzistergos et al, 2022).…”

“…A linear relationship is typically favored for annual sunspot values, however Chatzistergos et al (2022) reported a, in general, non linear relationship even for annual values. For sunspot numbers the reported relation is typically linear when annual values are considered, but non-linearities have been reported for daily values (Chatzistergos et al, 2022). No qualitative difference has been reported when alternative sunspot number series, such as those by Svalgaard and Schatten (2016); Usoskin et al (2016); Chatzistergos et al (2017); Willamo et al (2017), were used for the analysis.…”

“…The relation between plage areas and sunspot areas or numbers has also been studied (see Foukal and Vernazza, 1979;Schatten et al, 1985;Lawrence, 1987;Steinegger et al, 1996b;Chapman et al, 1997, Chapman et al, 2011Fligge and Solanki, 1998;Bertello et al, 2016;Mandal et al, 2017a;Chatzistergos et al, 2018a;Chatzistergos et al, 2022;Yeo et al, 2020a). Past studies, in general, suggest a quadratic relation between plage and sunspot areas for daily values.…”

“…No qualitative difference has been reported when alternative sunspot number series, such as those by Svalgaard and Schatten (2016); Usoskin et al (2016); Chatzistergos et al (2017); Willamo et al (2017), were used for the analysis. Chatzistergos et al (2022) also reported that the bandwidth of the Ca II K observations affects the relationship between plage areas and sunspot data.…”

Investigating the Effect of Solar Ambient and Data Characteristics on Ca ii K Observations and Line Profile Measurements

“…The literature concerning solar and stellar observations at the Ca II K line is extensive and suggestive of plenty of data available at that radiation. In fact, full-disk spatially resolved solar observations at the Ca II K line have continuously been performed since 1892 with various telescopes operating at the Ca II K line with bandwidths in the range [0.09,10] Å centered at the line core, and spatial resolution larger than 1″ (e.g., Chatzistergos et al 2018Chatzistergos et al , 2019aChatzistergos et al , 2020Chatzistergos et al , 2022a. Most prominent archives of historical Ca II K data are, e.g., those of the Meudon (Malherbe et al 2022), Kodaikanal (Chatzistergos et al 2019c), Mt Wilson (Lefebvre et al 2005), and Coimbra (Lourenço et al 2019) Observatories, while the ones for modern data are, e.g., those of the Rome (Ermolli et al 2022) and Kanzelhöhe (Pötzi et al 2021) Observatories.…”