Polar confinement of the Sun's interior magnetic field by laminar magnetostrophic flow

TL;DR

This paper demonstrates that a weak downwelling flow can confine the Sun's interior magnetic field below the convection zone, explaining observed differential rotation and providing insights into solar lithium depletion.

Contribution

It introduces axisymmetric nonlinear MHD solutions showing magnetic confinement by laminar magnetostrophic flow, incorporating realistic stratification and diffusivity effects.

Findings

Magnetic confinement layer is approximately 10^{-3} of the Sun's radius.

Downwelling flow of about 10^{-5} cm/s effectively confines the magnetic field.

Solutions are magnetostrophic, balancing Lorentz, Coriolis, pressure, and buoyancy forces.

Abstract

The global-scale interior magnetic field needed to account for the Sun's observed differential rotation can be effective only if confined below the convection zone in all latitudes, including the polar caps. Axisymmetric nonlinear MHD solutions are obtained showing that such confinement can be brought about by a very weak downwelling flow U~10^{-5}cm/s over each pole. Such downwelling is consistent with the helioseismic evidence. All three components of the magnetic field decay exponentially with altitude across a thin "magnetic confinement layer" located at the bottom of the tachocline. With realistic parameter values, the thickness of the confinement layer ~10^{-3} of the Sun's radius. Alongside baroclinic effects and stable thermal stratification, the solutions take into account the stable compositional stratification of the helium settling layer, if present as in today's Sun, and the small diffusivity of helium through hydrogen, chi. The small value of chi relative to magnetic diffusivity produces a double boundary-layer structure in which a "helium sublayer" of smaller vertical scale is sandwiched between the top of the helium settling layer and the rest of the confinement layer. Solutions are obtained using both semi-analytical and purely numerical, finite-difference techniques. The confinement-layer flows are magnetostrophic to excellent approximation. More precisely, the principal force balances are between Lorentz, Coriolis, pressure-gradient and buoyancy forces, with relative accelerations and viscous forces negligible. This is despite the kinematic viscosity being somewhat greater than chi. 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 into the longstanding enigma of solar lithium depletion.