ON THE MHD STRUCTURE OF THE SOLAR TACHOCLINE - STEADY AND DYNAMICAL SOLUTIONS

Using recent helioseismology measurements, we carry out a parameter st udy to investigate the steady and unsteady configurations of the solar tachocline (hereafter ST) between the differentially rotating convect ion zone and the underlying solid-body rotating core. It is shown that in the case of steady solutions, a poloidal magnetic field of quadrup ole type topology generates toroidal flux tubes (hereafter B-T) that a re three times stronger than in the dipole case. In both cases however , B-T acquires values that are far below those required to overcome di stortion via convective turbulence. These values remain rather sensiti ve to the conditions imposed on the boundaries and on the underlying v iscosity prescription. They appear however to have a weak dependence o n the width of the ST. For example, imposing a 50 G poloidal field or stronger on the outer boundary yields unsteady solutions only. Using t he above-obtained steady configurations as initial conditions, we carr ied out several calculations to follow the evolution of the flow over several hundred dynamical time scales. It appears that the final dynam ical solutions obtained differ significantly from their corresponding initial profiles. Moreover, a large scale polewards oriented motion al ways results independent of whether magnetic effects are in- or exclud ed. The numerical results indicate that the ST is likely to be governe d by purely hydrodynamical or weakly magnetized flows in order to re-p roduce Omega-profiles that are consistent with the helioseismology mea surements. The storage of a strong B-T in the near-equator region as a possible scenario for the evolution of the sun spots is numerically i nvestigated as well. (C) 1998 Elsevier Science B.V. All rights reserve d.