On Limiting the Thickness of the Solar Tachocline

TL;DR

This study uses axisymmetric simulations to explore how the Sun's tachocline thickness is limited, revealing that meridional circulation cells, rather than magnetic fields, primarily control its extent.

Contribution

It demonstrates that meridional circulation cells can halt tachocline spread independently of magnetic fields, challenging previous assumptions about magnetic influence.

Findings

Meridional circulation cells develop in the tachocline and limit its spread.

Magnetic fields do not significantly transfer angular momentum between regions.

Tachocline penetration varies with latitude and circulation patterns.

Abstract

We present axisymmetric simulations of the coupled convective and radiative regions in the Sun in order to investigate the angular momentum evolution of the radiative interior. Both hydrodynamic and magnetohydrodynamic models were run. We find an initial rapid adjustment in which the dif- ferential rotation of the convection zone viscously spreads into the radiative interior, thus forming a "tachocline". In polar regions the subsequent spread of the tachocline is halted by a counter-rotating meridional circulation cell which develops in the tachocline. Near the equator such a counter-rotating cell is more intermittent and the tachocline penetration depth continues to increase, albeit more slowly than previously predicted. In the magnetic models we impose a dipolar field initially confined to the radiative interior. The behavior of the magnetic models is very similar to their non-magnetic counter- parts. Despite being connected to the convection zone, very little angular momentum is transferred between the convective and radiative regions. Therefore, while it appears that a magnetic field is not necessary to stop the tachocline spread, it also does not promote such a spread if connected to the convection zone.