Sunspot Cycle: A Driven Nonlinear Oscillator?

A property of nonlinear oscillators—mutual dependence between their instantaneous amplitude and frequency—is tested in the yearly and monthly records of the sunspot numbers using the histogram-adjusted isospectral surrogate data and the Barnes model as the autoregressive moving average surrogates. The instantaneous amplitudes and frequencies are obtained by means of the analytic signal approach using the discrete Hilbert transform. In several tests the amplitude-frequency correlation has been found significant on levels ranging from $p<\mathrm{}0.03\mathrm{}$ to $p<\mathrm{}0.07,$ which supports the hypothesis of a driven nonlinear oscillator as a mechanism underlying the sunspot cycle.