Multiscale Evolutionary Characteristics of Solar Chromosphere from Century-long Ca II K Plage Observations

Abstract The solar chromosphere is a key interface layer for understanding magnetic energy transport and multiscale magnetic structures, yet its multi-scale periodic variations from days to a century remain poorly understood. The longest available composite time series of Ca II K plage areas from 1907 February 1 to 2023 December 31 is used to investigate these characteristics by applying the methods of ensemble empirical mode decomposition (EEMD) and continuous wavelet transform. The data are decomposed by EEMD into 14 intrinsic mode functions, which represent the inherent periodicities of the plages. Periodicity analysis reveals a wide range of statistically significant periods. The periods for short-term components at 2.64, 8.63, 26.62, 27.80, and 77.50 days are associated with magnetic flux emergence and solar rotation. The midterm periods of 154.99 days (Rieger period), 358.15 days (annual variation), and 2.26 yr (quasi-biennial oscillations) are modulated by different harmonics of magnetic Rossby waves at the base of the convection zone. The long-term periods of 10.32 and 11.25 yr correspond to the Schwabe cycle; the 21.48 yr period is associated with the magnetic polarity reversal cycle; and the 49.76 yr period is interpreted as the first subharmonic of the Gleissberg cycle. Overall, chromospheric variations on timescales from days to several decades are tightly modulated by solar magnetic activity, and the century-long ground-based Ca II K observations provide a valuable resource for understanding the long-term behavior of the Sun.