Linear and nonlinear properties of the Goldreich-Schubert-Fricke instability in stellar interiors with arbitrary local radial and latitudinal differential rotation

TL;DR

This study analyzes the linear and nonlinear behaviors of the Goldreich-Schubert-Fricke instability in stellar interiors with complex rotation patterns, revealing how it enhances angular momentum transport and turbulence in stars.

Contribution

It provides the first detailed nonlinear three-dimensional analysis of the GSF instability with arbitrary differential rotation orientations in stellar radiative zones.

Findings

Nonlinear development of zonal jets enhances turbulent transport.

Instability with mixed shear transports angular momentum more efficiently.

Transport properties are largely insensitive to box size, allowing extrapolation to stars.

Abstract

We investigate the linear and nonlinear properties of the Goldreich-Schubert-Fricke (GSF) instability in stellar radiative zones with arbitrary local (radial and latitudinal) differential rotation. This instability may lead to turbulence that contributes to redistribution of angular momentum and chemical composition in stars. In our local Boussinesq model, we investigate varying the orientation of the shear with respect to the 'effective gravity', which we describe using the angle $\phi$. We first perform an axisymmetric linear analysis to explore the effects of varying $\phi$ on the local stability of arbitrary differential rotations. We then explore the nonlinear hydrodynamical evolution in three dimensions using a modified shearing box. The model exhibits both the diffusive GSF instability, and a non-diffusive instability that occurs when the Solberg-H\{o}iland criteria are violated. We observe the nonlinear development of strong zonal jets ("layering" in the angular momentum) with a preferred orientation in both cases, which can considerably enhance turbulent transport. By varying $\phi$ we find the instability with mixed radial and latitudinal shears transports angular momentum more efficiently (particularly if adiabatically unstable) than cases with purely radial shear $(\phi = 0)$. By exploring the dependence on box size, we find the transport properties of the GSF instability to be largely insensitive to this, implying we can meaningfully extrapolate our results to stars. However, there is no preferred length-scale for adiabatic instability, which therefore exhibits strong box-size dependence. These instabilities may contribute to the missing angular momentum transport required in red giant and subgiant stars and drive turbulence in the solar tachocline.