5 - Turbulence in the tachocline

Summary

Helioseismic inversions suggest that the tachocline straddles the base of the convection zone, incorporating the overshoot region and extending into the stably stratified radiative interior. Thus, the upper tachocline is dominated by penetrative convection while the lower tachocline is a stably stratified shear flow under the influence of rotation and magnetism. We review the nature of the turbulence that is likely to exist in these two disparate regions, focusing on the interaction between turbulence and differential rotation. It is argued that turbulent angular momentum transport is likely to be poleward throughout the tachocline, tending to suppress the latitudinal differential rotation maintained by turbulent stresses in the overlying convective envelope. Meanwhile, vertical angular momentum transport in the lower tachocline may be anti-diffusive, tending to amplify the vertical shear. The turbulent alignment of convective plumes may also drive an equatorward meridional circulation in the upper tachocline where it overlaps with the overshoot region.

Introduction

The solar tachocline lies near the base of the solar convection zone. This is a well-known result of course, but it is essential to establish precisely what near means in this context. Helioseismic structure inversions reveal a stiff transition between the nearly adiabatic stratification of the convection zone and the strongly subadiabatic stratification of the radiative interior, mediated by only a narrow region of convective overshoot. As others have argued in this volume, tachocline dynamics is very sensitive to where the rotational shear occurs relative to this structural transition.