Estimation of polar magnetic fields using Ca II K polar network as a proxy

Abstract

The polar magnetic field plays a crucial role in the solar dynamo model and contributes to predicting future solar cycles. However, continuous and direct measurements of this polar field have been available only since 1976, with data provided by the Wilcox Solar Observatory (WSO). Recent findings suggest that the Ca II K Polar Network Index (PNI) can serve as a promising proxy for estimating the polar field of the Sun. In this study, we aim to reconstruct the polar field for the pre-1976 period by leveraging Ca II K data from the Kodaikanal Solar Observatory (KoSO; 1904 ─ 2007) and modern Ca II K observations from the Rome Precision Solar Photometric Telescope (Rome-PSPT; 2000 ─ 2022). We employ an automatic adaptive threshold technique to detect polar networks and calculate PNI values. Then, we calibrate these PNI values with the WSO polar field to reconstruct the polar field over 119 years.