Mid-term quasi-periodicities in the CaII-K plage index of the Sun and their implications

Abstract

We present results of the fast Fourier transform (FFT), the MEM analysis, and the wavelet analysis (WA), of the temporal variation of the monthly disk integrated “CaII-K plage-area and enhanced network (EN) area” hereafter called ‘plage index’ derived by Tlatov et al. (2009), from spectro-heliograms taken in Ca II K line at Kodaikanal Observatory, from February 1907 to April, 1998, In the range >3 months and < decade, the Fourier transform power spectra reveal solar cycle periodicity ∼122 months, its sub-harmonic ∼61 months, two short quasi-periodicities (>2 and <4 months), and nine ‘intermediate-range’ quasiperiodicities ( 4 mo and <11 yr), in the variation of the plage index. The quasi-periodicities include Rieger, Rieger type and quasi-biennial. The presence of quasi-periodicities and the mean values of the periods are confirmed by the maximum entropy method (MEM). The temporal spans of the quasi-periodicities during different solar cycles are determined from the complex Morlet-wavelet analyses. The Rieger quasi-periodicity (150–160 days) appeared during cycle 21 and cycle 22. Each of the quasi-periodicities in the studied range lies close to one or more planetary periodicities (orbital, or spring tidal, or heliocentric conjunction frequency). We discuss possible interpretations of our results, and those of similar results obtained earlier by other authors, towards understanding the mechanisms of excitation of various quasi-periodicities detected in solar variability parameters.