Ca ii K Polar Network Index of the Sun: A Proxy for Historical Polar Magnetic Field

• DOI: 10.3847/1538-4357/adb3a8
• Publication Date: 2025-03-20T06:16:42Z
• Authors: Dibya Kirti Mishra; Bibhuti Kumar Jha; Theodosios Chatzi...; Ilaria Ermolli; Dipankar Banerjee; Lisa A. Upton; M. Saleem Khan
• Keywords: Proxy (statistics); Polar cap

Abstract The Sun’s polar magnetic field is pivotal in understanding solar dynamo processes and forecasting future cycles. However, direct measurements of the have only been available since 1970s. chromospheric Ca ii K network index (PNI; fractional area regions above a certain latitude) has recently emerged as reliable proxy for fields. In this study, we derive PNI estimates from newly calibrated, rotation-corrected observations Kodaikanal Solar Observatory (1904–2007) modern data Rome Precision Photometric Telescope (2000–2022). We use both those archives to identify with an automatic adaptive threshold segmentation technique calculate PNI. obtained shows significant correlation measured Wilcox (Pearson coefficient r > 0.93) derived based on Advective Flux Transport Model ( 0.91). series also faculae counts Mount Wilson 0.87) data. Finally, reconstruct over 119 yr long period, which includes last 11 cycles (Cycles 14–24). obtain relationship between amplitude (in 13 month smoothed sunspot number) strength reconstructed at preceding cycle minimum validate prediction ongoing cycle, Cycle 25.