Space Weather and Space Climate—What the Look from the Earth Tells Us About the Sun

Abstract

Data for geomagnetic activity can be used to derive long-term variations in solar dynamo parameters and to estimate the regime of operation of the dynamo. It is found that the dynamo operates in moderately advection-dominated regime in the upper part of the convection zone: a part of the flux diffuses directly to the tachocline, “short-circuiting” the meridional circulation; another part makes a full circle to the poles, down to the base of the convection zone and equatorward to sunspot latitudes. This provides an explanation of the two peaks in the sunspot cycle due to the two surges of the toroidal field. In the lower part of the convection zone, diffusion is more important than advection, so the sunspot cycle amplitude increases with the speed of the deep meridional circulation. The well-known Waldmeier rule relating the rise time of a cycle with its amplitude is a direct consequence of this dependence. It is found that periods of grand minima are characterized by a change in the regime of operation of the dynamo in the lower part of the convection zone to advection-dominated. The correlations between the dynamo parameters in the solar cycles allow us to conclude that the factor determining the cycle amplitude is the speed of the surface meridional circulation and to find its dependence on planetary tidal forces. From the changing correlation between sunspot and geomagnetic activity the long-term variations in the toroidal and poloidal components of the solar magnetic field can be estimated.