Global simulations of Tayler instability in stellar interiors: a long-time multistage evolution of the magnetic field

Abstract

ABSTRACT Magnetic fields are observed in massive Ap/Bp stars and are presumably present in the radiative zone of solar-like stars. To date, there is no clear understanding of the dynamics of the magnetic field in stably stratified layers. A purely toroidal magnetic field configuration is known to be unstable, developing mainly non-axisymmetric modes. Rotation and a poloidal field component may lead to stabilization. Here we perform global MHD simulations with the EULAG-MHD code to explore the evolution of a toroidal magnetic field located in a layer whose Brunt-Väisälä frequency resembles the lower solar tachocline. Our numerical experiments allow us to explore the initial unstable phase as well as the long-term evolution of such field. During the first Alfven cycles, we observe the development of the Tayler instability with the prominent longitudinal wavenumber, m = 1. Rotation decreases the growth rate of the instability and eventually suppresses it. However, after a stable phase, energy surges lead to the development of higher-order modes even for fast rotation. These modes extract energy from the initial toroidal field. Nevertheless, our results show that sufficiently fast rotation leads to a lower saturation energy of the unstable modes, resulting in a magnetic topology with only a small fraction of poloidal field, which remains steady for several hundreds of Alfven traveltimes. The system then becomes turbulent and the field is prone to turbulent diffusion. The final toroidal–poloidal configuration of the magnetic field may represent an important aspect of the field generation and evolution in stably stratified layers.