Confinement of the Sun's interior magnetic field, with implications for lithium burning

Abstract

The simplest interior magnetic field B_i 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 radiative interior. This paper describes new analytical and numerical solutions of the relevant magnetohydrodynamic equations 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~10^{-5}cm/s over the poles is enough to enforce exponential decay of B_i with altitude, in a confinement layer only a fraction of a megameter thick. Downwelling in the polar tachocline is implied both by helioseismic observations, combined with elementary dynamics, and by theoretical arguments about the "gyroscopic pumping" that would spread differential rotation downward in the absence of B_i. 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. We discuss 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 early Sun and solar lithium depletion.