The potential of Ca II K observations for solar activity and variability studies

TL;DR

This paper reviews historical and modern Ca II K solar observations, emphasizing their importance for studying long-term solar activity and the challenges in data processing and merging for reliable analysis.

Contribution

It provides a comprehensive overview of Ca II K observations, highlighting data quality issues and recent analytical results over the past decade.

Findings

Ca II K data reveal long-term solar magnetic variations.

Data processing and merging are crucial for accurate analysis.

Recent studies have improved understanding of solar activity cycles.

Abstract

Several observatories around the globe started regular full-disc imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. These observations are continued today at a few sites with either old spectroheliographs or modern telescopes equipped with narrow-band filters. The Ca II K time series are unique in representing long-term variations of the Sun's chromospheric magnetic field. However, meaningful results from their analysis require accurate processing of the available data and robust merging of the information stored in different archives. This paper provides an overview of the historical and modern full-disc Ca II K observations, with focus on their quality and the main results obtained from their analysis over the last decade.