Quantifying the effect of passband on observations in the Ca II K line
Abstract
Full-disk observations of the Sun in the Ca II K line have been carried out since the late 19th century at various observatories worldwide. These long-term records of solar activity are crucial for reducing discrepancies among solar irradiance reconstructions and for advancing our understanding of the solar dynamo. To construct a consistent composite record, data from different observatories must be cross-calibrated to account for variations in spectral passband and spatial resolution, which are the primary sources of discrepancies between archives. In this study, we use high spectral and spatial resolution observations in the Ca II K line from the state-of-the-art Sunrise III mission to emulate different passbands and derive empirical contrast-contrast relationships between them. We find that these relationships are well described by a power law and provide coefficients for different combinations of passband widths in the range 0.1--9 Angstroms and spatial resolutions between 1 arcsec and 6 arcsec. Applying such a relationship to observations from two major Ca II K archives demonstrates its potential to improve their cross-calibration. The results provide a foundation for the construction of a consistent, century-long time series of solar activity from historical and modern Ca II K observations.
Comments: Accepted for publication in The Astrophysical Journal Letters
Cite
@article{arxiv.2605.29982,
title = {Quantifying the effect of passband on observations in the Ca II K line},
author = {Ajay Kumar Yadav and Theodosios Chatzistergos and Natalie Krivova and Sami K. Solanki and Francisco A. Iglesias and Ilaria Ermolli and Andreas Lagg and Achim Gandorfer and Jose Carlos del Toro Iniesta and Yukio Katsukawa and Pietro Bernasconi and Thomas Berkefeld and Alex Feller and Tino L. Riethmüller and Alberto Álvarez-Herrero and Masahito Kubo and H. N. Smitha and David Orozco Suárez and Bianca Grauf and Michael Carpenter and Alexander Bell and Valentín Martínez Pillet and Laurent Gizon and Johannes Hoelken and Francisco Javier Bailén and Julian Blanco Rodríguez and Juan Sebastián Castellanos Durán and Edvarda Harnes and Ryohtaroh T. Ishikawa and Yusuke Kawabata and Takuma Matsumoto and Takayoshi Oba and Azaymi L. Siu-Tapia and Hanna Strecker and Dušan Vukadinović and Yasuhito Narita},
journal= {arXiv preprint arXiv:2605.29982},
year = {2026}
}