The rotation-magnetism-convection coupling in the Sun

The rotational and magnetic evolution of a star depends on its initial mass and angular momentum, but not directly on its initial magnetic field if the star has an outer convection zone. The present field is generated by a self-excited dynamo. Rotation influences convection and so generates the two main ingredients of the dynamo process, non-uniform rotation and helicity. Several competing descriptions have been proposed: A two-level picture where first single convective cells are distorted by rotation and then the net effect on the mean flow and field is considered, and a direct numerical approach, including rotational effects, where small-scale motions enter only as isotropic diffusivities of heat, momentum, and field. Difficulties arise in particular from the fact that the magnetic field is concentrated into fibrils of large field strength. Predictions of a quantitative relationship between a star's rate of rotation and its magnetic field amplitude are also difficult, although observations of late-type stars suggest such a relationship. A transition between two modes of dynamo operation may occur as the star's rotation slows down by magnetic braking. (Auth.)