The spherical-harmonic-Fourier (SHF) analysis of the sun's magnetic field inferred from the Greenwich sunspot data is refined and extended to include the full length (1874-1976) of the data on the magnetic tape provided by H. Balthasar. Perspective plots and grey level diagrams of the SHF power spectra for the odd and the even degree axisymmetric modes are presented. Comparing these with spectra obtained from two simulated data sets with random redistribution within the wings in the butterfly diagrams, it is concluded that there is no clear evidence for the existence of any relation between the harmonic degree and the temporal frequency of the power concentrations of the inferred field. Apart from the power 'ridge' in the narrow frequency band at about 1/21.4/y, and low ridges at odd multiples of this frequency, there are no other spectral features. This strongly suggests that the solar magnetic cycle consists of some global oscillations of the sun 'forced' at a frequency about 1/21.4/y and, perhaps, weak resonances at its odd harmonics.
Gokhale M. H., Javaraiah J., Narayanankutty Karuppath, and Varghese B. A., “Global modes constituting the solar magnetic cycle. I - Search for dispersion relations”, Solar Physics, pp. 138: 35–47, 1992.