Grand minima in a buoyancy-driven solar dynamo

Numerical simulations of a 2D mean-field model are presented which show that grand minima, typical for the long-term behaviour of solar magnetic activity, can be produced by a dynamo that features an alpha effect based on the buoyancy instability of magnetic fluxtubes. The buoyancy-driven alpha effect functions only if the magnetic field strength exceeds a minimal value necessary for instability. It opens the possibility of dynamo action within the solar overshoot layer, where a strong magnetic field, B~ 10⁵ G, is thought to be stored. The existence of a magnetic threshold for dynamo action can lead to interruptions of the magnetic cycle, similar to the grand minima of solar activity. Transitions across the instability threshold are triggered by magnetic-flux injections from the convection zone. This is modelled by allowing for a small-scale kinematic alpha effect in the convection zone, and convective downdrafts that penetrate the overshoot layer.