Turbulent Confinement of Large-Scale, Poloidal Magnetic Fields to Stellar Radiative Zones

Abstract:

Motivated by stars with outer convection zones overlying radiative zones like the Sun, we study the dynamics stemming from the interaction of convective overshooting motions with a large-scale, poloidal magnetic field embedded in a stable region. We have run a series of 3D Boussinesq numerical simulations in a spherical shell with a convective region sitting on top of a stable region that initially compactly contains a weak dipole field. We study the effect of convective turbulent transport on the overall evolution of the magnetic field. In particular here, we vary the degree of convective driving through the Rayleigh number Ra_o, in a parameter regime where there is no dynamo action. We find that at lower Rayleigh numbers (Ra_o≤10⁸), an outward turbulent diffusion due to convection and overshooting dominates and leads to a faster removal of the dipole field than a purely diffusive process would achieve. However, at a higher Rayleigh number (Ra_o=10⁹), inward turbulent pumping becomes more efficient and compensates for the turbulent diffusion, resulting in a local accumulation of the dipole field below the overshoot region. Although still decaying, a higher degree of spatial confinement of the field to the stable region is found (in a fractional sense). These findings are of interest for certain models of the Sun which require a large-scale poloidal magnetic field to be confined in the solar radiative zone in order to explain simultaneously both the thinness of the tachocline and the almost uniform rotation of the stable region.