MAGNETIC BUOYANCY, VORTICITY, AND 3-DIMENSIONAL FLUX-TUBE FORMATION

Sunspots and other intense flux elements making up active regions are formed from loops of magnetic held that erupt through the Sun's surfac e; the field is believed to originate in the stable overshoot region b elow the convection zone. It is likely that these structures arise thr ough magnetic buoyancy. We have conducted the first numerical simulati ons of the three-dimensional evolution of an unstable horizontal layer of magnetic field. The initial development is almost two-dimensional, with little variation along the field lines, but then the held become s markedly three-dimensional, forming arched structures. A separate ca lculation with an isolated rising flux tube shows no such arching; the three-dimensional behavior of the layer is in fact due to a nonlinear interaction between adjacent but oppositely directed vortex tubes ind uced by the rising field. These vorticity-driven modes, absent from ea rlier flux-tube models, seem likely to be of importance in the process of sunspot formation.