Stability of Toroidal Magnetic Fields in Rotating Stellar Radiation Zones

Abstract

The questions of how strong magnetic fields can be stored in rotating stellar radiative zones without being subjected to pinch-type instabilities and how much radial mixing is produced if the fields are unstable are addressed. Linear equations are derived for weak disturbances of magnetic and velocity fields which are global in horizontal dimensions but short--scaled in radius. The equations are solved to evaluate the stability of toroidal field patterns with one or two latitudinal belts under the influence of a rigid basic rotation. Hydrodynamic stability of latitudinal differential rotation is also considered. The magnetic instability is essentially three--dimensional. It does not exist in a 2D formulation with strictly horizontal disturbances on decoupled spherical shells. Only stable (magnetically modified) r-modes are found in this case. The instability recovers in 3D. The most rapidly growing modes for the Sun have radial scales smaller than 1 Mm. The finite thermal conductivity makes a strong destabilizing effect. The marginal field strength for the onset of the instability in the upper part of the solar radiative zone is about 600 G. The toroidal field can only slightly exceed this critical value for otherwise the radial mixing produced by the instability would be too strong to be compatible with the observed lithium abundance. Also the threshold for hydrodynamic instability of differential rotation which exists in 2D is lowered in 3D. When radial displacements are included, the value of 28% for critical shear is reduced to 21%.