Magnetic field structure due to the global velocity field in spiral galaxies

We present a set of global, self-consistent N-body/smoothed particle hydrodynamic (SPH) simulations of the dynamic evolution of galactic discs with gas, including magnetic fields. We have implemented a description to follow the evolution of magnetic fields with the ideal induction equation in the SPH part of the vine code. Results from a direct implementation of the field equations are compared to a representation by Euler potentials, which pose a del center dot B-free description, a constraint not fulfilled for the direct implementation. All simulations are compared to an implementation of magnetic fields in the gadget code which also includes cleaning methods for del center dot B. Starting with a homogeneous seed field, we find that by differential rotation and spiral structure formation of the disc the field is amplified by one order of magnitude within five rotation periods of the disc. The amplification is stronger for higher numerical resolution. Moreover, we find a tight connection of the magnetic field structure to the density pattern of the galaxy in our simulations, with the magnetic field lines being aligned with the developing spiral pattern of the gas. Our simulations clearly show the importance of non-axisymmetry for the evolution of the magnetic field.