Evidence for a long-term variation of the dynamo action responsible for the solar magnetic cycle

By using the sunspot time series as a proxy, we have made a detailed analysis of the mean solar magnetic field over the last two and half centuries, by means of a reconstruction of its phase space. We find evidence of a long-term trend variation of some of the solar physical processes (over a few decades) that might be responsible for the apparent erratic behaviour of the solar magnetic cycle. The analysis is done by means of a careful study of the axisymmetric dynamo model equations, where we show that the temporal counterpart of the magnetic field can be described by a self-regulated two-dimensional dynamic system, usually known as a Van der Pol-Duffing oscillator. Our results suggest that during the last two and half centuries, the velocity of the meridional flow, v(p), and the efficiency of the alpha mechanism responsible for the conversion of toroidal magnetic field into poloidal magnetic field might have suffered variations that can explain the observed variability in the solar cycle.