Long-term Evolution of Chromospheric Plage Areas from Kodaikanal Observatory Ca ii K Images and Their Relation to Solar Activity Indices

Abstract

This study investigates the correlation and phase relationships between chromospheric Ca ii K plage areas and key solar activity indices—including sunspot number, sunspot area, and 10.7 cm radio flux—across Solar Cycles 14 – 22. Using homogenized data from the Kodaikanal Solar Observatory, we demonstrate robust correlations (r > 0.85) across all parameters and confirm that plage areas adhere to the Gnevyshev-Ohl rule and exhibit Gnevyshev gaps during several cycle maxima. Through Morlet wavelet analysis and wavelet coherence (WTC), we identify significant intermediate-term quasi-periodicities, specifically Rieger-type periods (130 – 190 days) and quasi-biennial oscillations (QBOs; 1 – 4 years). While these signals show notable cycle-to-cycle variability and frequent phase asynchrony at intermediate scales, a stable phase synchrony persists within the fundamental 9 – 12 year periodicity belt, indicating a tightly coupled decadal evolution of chromospheric and photospheric magnetic manifestations. These results, which align with theories of tachocline instabilities and magnetic Rossby waves, provide critical empirical constraints for solar dynamo models and enhance our understanding of the long-term evolution of the Sun’s multi-layered magnetic environment.