1111.6894 (P. J. Käpylä et al.)

Oscillatory large-scale dynamos from Cartesian convection simulations    [PDF]

P. J. Käpylä, M. J. Mantere, A. Brandenburg

We present results from compressible Cartesian convection simulations with and without imposed shear. In the former case the dynamo is expected to be of alpha^2 Omega type which is likely to be relevant for the Sun, whereas the latter case refers to alpha^2 dynamos which are more likely to occur in more rapidly rotating stars. The latter exhibit oscillatory large-scale magnetic fields. We perform a parameter study where the shear flow is kept fixed and the rotational influence is varied in order to probe the relative importance of both modes. We find that in cases with shear, the streamwise component of the magnetic field is between 9 and 20 times greater than the cross-streamwise one, depending on the rotation rate. Oscillatory solutions are preferred either when the shear is relatively strong or weak in comparison to rotation, but not when the two are comparable to each other. However, exceptions to these rules also appear and in many cases the solution is oscillatory only in the kinematic regime whereas in the nonlinear stage the mean fields are stationary. 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 strength of total and the large-scale component of the magnetic field at the saturated state, however, are sensitive to the chosen boundary conditions.

View original: http://arxiv.org/abs/1111.6894