Hilbert–Huang transform analysis of periodicities in the last two solar activity cycles

We investigated periodicities associated with the last two-and-a-half solar activity cycles with the novel Hilbert–Huang transform (HHT) method. Raw data signals of five different observational proxies: the 10.7 cm radio flux intensity, the helioseismic frequency shift, and the sunspot area signals recorded from the whole solar disc, and separately from the Northern and Southern hemispheres, were ...