Confinement of the Sun’s Interior Magnetic Field

The simplest interior magnetic field Bi that can explain the observed uniform rotation of the Sun’s radiative envelope is an axial dipole stabilized by a deep toroidal field. It can explain the uniform rotation only if confined in the polar caps. The field must be prevented from diffusing up into the high-latitude convection zone, whose slower rotation must remain decoupled from the interior. This paper presents new analytical and numerical solutions showing that such confinement and decoupling is dynamically possible, by means of a laminar “magnetic confinement layer” at the bottom of the tachocline. With realistic values of the microscopic diffusivities, a weak laminar downwelling flow U ∼ 10cm s over the poles is enough to enforce exponential decay of |Bi| with altitude, in a confinement layer only a fraction of a megameter thick. The reality of downwelling in the polar tachocline is implied by helioseismic observations, taken together with elementary dynamics. Our confinement-layer solutions are the first to take account of all the relevant physical effects in a self-consistent mathematical model. The effects include magnetic diffusion, baroclinicity and stable stratification (thermal and compositional), Coriolis effects, and thermal relaxation. All these effects have essential roles in the dynamics. The flow is magnetostrophic to excellent approximation. We hypothesize how the confinement layers at each pole might fit into a global dynamical picture of the solar tachocline. That picture, in turn, suggests a new insight into the lithium depletion problem. Subject headings: MHD — Sun: abundances — Sun: interior — Sun: magnetic fields — Sun: rotation