The Tayler Instability in the Anelastic Approximation

TL;DR

This paper derives a new criterion for the Tayler instability within the anelastic approximation, revealing stability differences from fully compressible models and analyzing unstable modes relevant for stellar magnetic field simulations.

Contribution

It introduces an anelastic Tayler instability criterion, extending previous fully compressible results to more realistic stellar simulation conditions.

Findings

Some configurations unstable in compressible models are stable in anelastic models.

Numerical calculations identify and characterize unstable modes.

Implications for non-linear stellar magnetic field simulations are discussed.

Abstract

The Tayler instability (TI) is a non-axisymmetric linear instability of an axisymmetric toroidal magnetic field in magneto-hydrostatic equilibrium (MHSE). Spruit (1999, 2002) has proposed that in a differentially rotating radiative region of a star, the TI drives a dynamo which generates magnetic fields that can efficiently transport angular momentum; a parameterized version of this dynamo has been implemented in stellar structure and evolution codes and shown to be important for determining interior spin. Numerical simulations, however, have yet to definitively demonstrate the operation of the dynamo. A criterion for the MHSE to develop the TI was derived using fully-compressible magneto-hydrodynamics, while numerical simulations of dynamical processes in stars frequently use an anelastic approximation. This motivates us to derive a new anelastic Tayler instability (anTI) criterion. We find that some MHSE configurations unstable in the fully-compressible case, become stable in the anelastic case. We find and characterize the unstable modes of a simple family of cylindrical MHSE configurations using numerical calculations, and discuss the implications for fully non-linear anelastic simulations.