Oscillatory large-scale dynamos from Cartesian convection simulations

Abstract

We present results from compressible Cartesian convection simulations with and without imposed shear. In the former case the dynamo is expected to be of $α^2\varOmega$ type which is generally expected to be relevant for the Sun, whereas the latter case refers to $α^2$ dynamos which are more likely to occur in more rapidly rotating stars whose differential rotation is small. We perform a parameter study where the shear flow and the rotational influence are varied to probe the relative importance of both types of dynamos. Oscillatory solutions are preferred both in the kinematic and saturated regimes when the negative ratio of shear to rotation rates, $q\equiv -S/\varOmega$, is between 1.5 and 2, i.e., when shear and rotation are of comparable strengths. Other regions of oscillatory solutions are found with small values of $q$, i.e., when shear is weak in comparison to rotation, and in the regime of large negative $q$s, when shear is very strong in comparison to rotation. However, exceptions to these rules also appear so that for a given ratio of shear to rotation, solutions are non-oscillatory for small and large shear, but oscillatory in the intermediate range. Changing the boundary conditions from vertical field to perfect conductor ones changes the dynamo mode from oscillatory to quasi-steady. Furthermore, in many cases an oscillatory solution exists only in the kinematic regime whereas in the nonlinear stage the mean fields are stationary. However, the cases with rotation and no shear are always oscillatory in the parameter range studied here and the dynamo mode does not depend on the magnetic boundary conditions. The strengths of total and large-scale components of the magnetic field in the saturated state, however, are sensitive to the chosen boundary conditions.