The Interplay of Magnetic Fields, Turbulence and Vorticity in Core-Collapse Supernovae

Abstract

The convective turbulent motion of the fluid below the shock in a core-collapse supernova stretches and amplifies the magnetic field of the progenitor star. The energy contained in the field is sourced from the work done by the fluid on the field which comes at the expense of the fluid's kinetic and internal energy. In addition to the energy exchange with the fluid, the magnetic field also has a back-reaction upon the fluid via a contribution to the baroclinic vector and thus affects the fluid vorticity. In this paper we explore the interaction of the magnetic field with the fluid in core-collapse supernovae using the ELEPHANT code with solar-metallicity stars of 15 and 20 \msun zero-age main sequence mass and purely toroidal initial magnetic fields of $B_0 = 0, 10^{10}, and 10^{12}$~G. We find that the magnetic field in the gain region does not become so large that it alters the global dynamics above the 10% level in the early post-bounce evolution. The turbulent kinetic energy of the fluid in the gain region is smaller in simulations with a strong initial magnetic field, but the exact amount of reduction is uncertain due to limitations of the methods for measuring turbulent kinetic energy. The structure of the field in the simulations quickly becomes a tangled mass of flux ropes as soon as convection begins, which leads to a large magnetic field contribution to the baroclinic vector that dominates over the hydrodynamic contribution. Although governed by very similar transport equations to the magnetic field, the fluid vorticity and magnetic field are always close to being randomly aligned at every stage of the evolution. The enstrophy, which is seen to be closely associated with the turbulent kinetic energy, is found to be reduced in simulations with a strong initial magnetic field supporting the inference that the magnetic field reduces the amount of turbulence in the fluid.