The middle plot is the hypothetical PSD of a time varying process which, when the absolute value is taken, produces an output similar to the SSN. The periods associated with the natural frequencies of this two mode model are P1 = 20 and P2 = 23.6 years. When the data are squared, the PSD convolves with itself, producing 4 peaks with associated periods at P2*P1/(P2+P1) = 10.8 years, P2*P1/(P2-P1) = 131 years, P1/2 = 10 years, and P2/2 = 11.8 years.
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