A simplemodel for the solar cycle

A simple nonlinear model which describes the 11-year solar cycle can be derived from the usual alpha-omega dynamo theory in the form of a Van der Pol equation. Solar activity displays also small-scale inter-cycle persistent stochastic oscillations with a Hurst exponent of the order of Hsimilar or equal to0.76+/-0.01. The results obtained from the Van der Pol oscillator superimposed on a fractional Brownian motion which describes the stochastic fluctuations are presented.