Spatio-temporal non-localities in a solar-like mean-field dynamo

Abstract

The scale separation approximation, which is in the base of the solar mean field dynamo models, can be hardly justified both by observations and theoretical applications to astrophysical dynamos.{ The general expression for the mean turbulent electromotive force can be written in integral form with convolution of the turbulent effects and mean magnetic field variations over scales of the turbulent flows and global scales of the mean field dynamo. Following results of DNS, which had been reported earlier, we take the Lorentzian form of the integral convolution kernels as an experimental fact. }It allows us to approximate the governing equation for the mean electromotive force by the reaction--diffusion type equation. Solution of the eigenvalue problem reveals a few curious properties of the dynamo model with the nonlocal mean electromotive force. We find a decrease of the critical dynamo instability threshold, and an increase the dynamo periods of the unstable modes, as reported in earlier studies. Simultaneously, the nonlocal model shows substantially lower growth rate of the unstable dynamo modes in proximity of the critical threshold than the model which employ the scale separation approximation. We verify these findings using the nonlinear solar dynamo model. For the supercritical regime, when the $\alpha$ effect magnitude is about twice of the instability threshold, the model shows the Parker's dynamo wave solutions with the wave propagating from the mid latitude at the bottom of the convection zone toward the solar equator at the surface. In the weakly nonlinear regime, when the $\alpha$ effect magnitude is near the instability threshold, the interference of the dynamo modes of different spatial localization shows the Grand activity cycles of a period about 300 years.