The effective magnetic Prandtl number in magnetized and differentially rotating stellar radiation zones

TL;DR

This paper uses linear theory to analyze magnetic and rotational instabilities in stellar radiative zones, revealing that the effective magnetic Prandtl number is high, which influences angular momentum transport and stellar core rotation.

Contribution

It introduces a detailed analysis of anisotropic magnetic diffusivity and viscosity in stellar radiative zones, highlighting the high effective magnetic Prandtl number due to magnetic instabilities.

Findings

Eddy diffusivity in latitudinal direction exceeds radial diffusivity by orders of magnitude.

Effective magnetic Prandtl number reaches values of around 100.

Differential rotation decays faster than the background magnetic field, explaining slow stellar core rotation.

Abstract

With application to inner stellar radiative zones, a linear theory is used to analyze the instability of a dipole-parity toroidal background field, in the presence of density stratification, differential rotation, and realistically small Prandtl numbers. The physical parameters are the normalized latitudinal shear $a$ and the normalized field amplitude $b$. Only the solutions for the wavelengths with the maximal growth rates are considered. If these scales are combined to the radial values of velocity, one finds that the (very small) radial velocity only depends slightly on $a$ and $b$, so that it can be used as the free parameter of the eigenvalue system. The resulting instability-generated tensors of magnetic diffusivity and eddy viscosity are highly anisotropic. The eddy diffusivity in latitudinal direction exceeds the eddy diffusivity in radial direction by orders of magnitude. Its latitudinal profile shows a strong concentration toward the poles which is also true for the effective viscosity which has been calculated via the angular momentum transport of the instability pattern. The resulting effective magnetic Prandtl number reaches values of $O(10^2)$, so that the differential rotation decays much faster than the toroidal background field, which is {the} necessary condition to explain the observed slow rotation of the early red-giant and sub-giant cores by means of magnetic instabilities.