Stability of axially symmetric magnetic fields in stars

Abstract

The magnetic fields observed in Ap-stars, white dwarfs, and neutron stars are known to be stable for long times. However, the physical conditions inside the stellar interiors that allow these states are still a matter of research. It has been formally demonstrated that both purely toroidal and purely poloidal magnetic fields develop instabilities at some point in the star. On the other hand, numerical simulations have proved the stability of roughly axisymmetric magnetic field configurations inside stably stratified stars. These configurations consist of mutually stabilizing toroidal and poloidal components in a twisted torus shape. Previous studies have proposed rough upper and lower bounds on the ratio of the magnetic energy in the toroidal and poloidal components of the magnetic field. With the purpose of mapping out the parameter space under which such configurations remain stable, we used the Pencil Code to perform 3D magnetohydrodynamic simulations of the evolution of the magnetic field in non-rotating, non-degenerate stars in which viscosity is the only dissipation mechanism, both for stars with a uniform (barotropic) and radially increasing (stably stratified) specific entropy. Furthermore, we considered different conditions regarding the degree of stable stratification and the magnetic energy in each component, roughly confirming the previously suggested stability boundaries for the magnetic field.

9 pages, 9 figures

References in corpus (9)

• The strongest cosmic magnets: Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars
• Weak magnetic fields in Ap/Bp stars: Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy
• Modelling magnetically deformed neutron stars
• CCOs and the hidden magnetic field scenario
• Magnetic-field evolution in a plastically-failing neutron-star crust
• Instability of Magnetic Equilibria in Barotropic Stars
• Axisymmetric and stationary structures of magnetized barotropic stars with extremely strong magnetic fields deep inside
• On magnetic equilibria in barotropic stars
• On the free-precession candidate PSR B1828-11: Evidence for increasing deformation

Cited by in corpus (8)

• Magnetism in High-Mass Stars
• Tayler-Spruit dynamo in stably stratified rotating fluids: Application to proto-magnetars
• Linking the Interiors and Surfaces of Magnetic Stars
• Oscillations of red giant stars with magnetic damping in the core. I. Dissipation of mode energy in dipole-like magnetic fields
• Strong toroidal magnetic fields sustained by the elastic crust in a neutron star
• Validating and improving two-fluid simulations of the magnetic field evolution in neutron star cores
• Standing torsional Alfvén waves as the source of the rotational period variation in magnetic early-type stars
• Exploring the probing power of gamma-Dor's inertial dip for core magnetism: case of a toroidal field