Active Region Magnetic Fields Below the Visible Surface
Active regions contain the largest concentrations of magnetic flux observed on the Sun and are thought to be the result of emerging loops of magnetic flux that have their origin at or near the base of the solar convection zone (see " About the Sun II"). If magnetic loops are to emerge through the visible surface as distinct bipolar entities (as is observed), then these loops must maintain a certain amount of cohesion during their buoyant rise through the turbulent interior.
A 3D MHD simulation (in the anelastic approximation) of the decay of a magnetic flux rope (column 1) and layer (column 2) in a gravitationally stratified model convection zone (from Abbett et al. 2004).
Yet there are many factors that contribute to the fragmentation of an isolated flux rope: for example, the hydrodynamic forces due to the interaction of vortices that develop during the rope's buoyant ascent, and the advection of flux away from the rope's axis as a result of convective flows. There are also a number of forces that prevent the fragmentation of a magnetic flux rope: for example, the magnetic tension that results from the presence of fieldline twist about the rope's central axis, and the non-inertial Coriolis force due to solar rotation. With George Fisher and Yuhong Fan, I have performed a number of theoretical studies that focus on the connection between active region magnetic fields observed at the visible surface and magnetic fields that exist near the base of the convection zone: