The Stability of Prendergast Magnetic Fields

Convection is thought to generate large scale, core magnetic fields, which persist in stars after they evolve off the main sequence. It's thought that the remnants of these dynamo fields may take the form of the Prendergast magnetic field (Prendergast 1956), a combination of poloidal and toroidal field components which are expected to stabilize each other. Previous analytic and numerical stability calculations have suggested this magnetic field is stable. We present long timescale numerical calculations which show a linear instability of this magnetic field, and determine the effect of changing diffusivity, Brunt–Väisälä frequency, azimuthal wavenumber, and boundary conditions on the instability. We show that the instability persists over a large parameter space and is sufficiently rapid to destabilize the magnetic field on timescales shorter than the stellar evolution time.