Quasi-biennial oscillation of the Ca <scp>ii</scp> K plage area

Abstract Solar quasi-biennial oscillations (QBOs) are crucial not only for comprehending the solar dynamo mechanism but also for forecasting space weather. In this present work, composite monthly Ca ii K plage area (PA) data were utilized, derived from cross-calibrated Ca ii K spectra and filtered maps collected at 38 stations from 1892 to 2023 December. The analysis, covering 14–25 solar cycles, employes synchrosqueezed wavelet transforms (SWT) to examine the periodicity of the plage area data. The following prominent results are found: (1) There are many periodic modes with different characteristics in plage area (PA) data, including the $40\:$yr periodic mode, the $22\:$yr magnetic periodic mode, the $11\:$yr Schwabe periodic mode, a medium-term periodic mode, a quasi-sexennial oscillation period mode, and the QBOs periodic modes; (2) the reconstructed solar QBOs exhibit intermittence with signs of stochasticity; (3) the Gnevyshev gap is observed in solar cycles 15–24, and triple peaks are observed in solar cycles 16, 17, and 19. There are two possible mechanisms by which the Gnevyshev gap may occur; one is as a result of the modulation of solar QBOs by the $11\:$yr Schwabe cycle, and the other is as a result of the reversal of the solar magnetic field.