Quasi-biennial oscillation of the Ca ii K plage area.

This article focuses on analyzing solar quasi-biennial oscillations (QBOs) and the Gnevyshev gap using a composite dataset of monthly Ca ii K plage area (PA) observations from 38 stations spanning 1892 to 2023. Employing synchrosqueezed wavelet transform (SWT) techniques, the study identifies multiple periodic modes in the PA data, including the ~40-year, 22-year Hale, 11-year Schwabe, quasi-six-year, and several QBO modes ranging from about 1.2 to 3.4 years. The research finds that the Gnevyshev gap—a multi-peak structure observed in solar cycles—is likely produced by the modulation of solar QBOs by the 11-year Schwabe cycle and coincides temporally with solar polar magnetic field reversals, suggesting both mechanisms contribute to its occurrence. These findings provide insights into the complex periodicities of solar activity and support the role of solar QBOs in understanding the solar dynamo and magnetic field dynamics.