Magnetic Cycles in a Convective Dynamo Simulation of a Young Solar-type Star

Young solar-type stars rotate rapidly and many are magnetically active; some undergo magnetic cycles similar to the 22-year solar activity cycle. We conduct simulations of dynamo action in rapidly rotating suns with the 3D MHD anelastic spherical harmonic (ASH) code to explore dynamo action achieved in the convective envelope of a solar-type star rotating at 5 times the current solar rotation rate. Striking global-scale magnetic wreaths appear in the midst of the turbulent convection zone and show rich time-dependence. The dynamo exhibits cyclic activity and undergoes quasi-periodic polarity reversals where both the global-scale poloidal and toroidal fields change in sense on a roughly 1500 day time scale. These magnetic activity patterns emerge spontaneously from the turbulent flow and are more organized temporally and spatially than those realized in our previous simulations of the solar dynamo. We assess in detail the competing processes of magnetic field creation and destruction within our simulations that contribute to the global-scale reversals. We find that the mean toroidal fields are built primarily through an $\Omega$-effect, while the mean poloidal fields are built by turbulent correlations which are not necessarily well represented by a simple $\alpha$-effect. During a reversal the magnetic wreaths propagate towards the polar regions, and this appears to arise from a poleward propagating dynamo wave. The primary response in the convective flows involves the axisymmetric differential rotation which shows variations associated with the poleward propagating magnetic wreaths. In the Sun, similar patterns are observed in the poleward branch of the torsional oscillations, and these may represent poleward propagating magnetic fields deep below the solar surface. [abridged]