Inevitability of a magnetic field in the Sun's radiative interior

The gas in the convective outer layers of the Sun rotates faster at the equator than in the polar regions, yet deeper inside (in the radiative zone) the gas rotates almost uniformly1,2,3. There is a thin transition layer between these zones, called the tachocline4. This structure has been measured seismologically1,2,3, but no purely fluid-dynamical mechanism can explain its existence. Here we argue that a self-consistent model requires a large-scale magnetic field in the Sun's interior, as well as consideration of the Coriolis effects in the convection zone and in the tachocline. Turbulent stresses in the convection zone induce (through Coriolis effects) a meridional circulation, causing the gas from the convection zone to burrow downwards, thereby generating the horizontal and vertical shear that characterizes the tachocline. The interior magnetic field stops the burrowing, and confines the shear, as demanded by the observed structure of the tachocline. We outline a dynamical theory of the flow, from which we estimate a field strength of about 10−4 tesla just beneath the tachocline. An important test of this picture, after numerical refinement, will be quantitative consistency between the predicted and observed interior angular velocities.