Nonlinear winding of large-scale magnetic fields in spiral galaxies

A new-developed 3D numerical code is applied to an uniform external (primordial) magnetic field subject to a complex flow pattern representing the situation in a turbulent spiral galaxy. The spiral arms are defined by the radial-azimuthal profiles of density and the turbulent velocity, but they do not yet possess any own large-scale velocity field. No dynamo alpha is assumed to exist, but all the known turbulence effects such as eddy diamagnetism and turbulent pumping are involved. Two different models are followed: The (nonaxisymmetric) external magnetic field is considered as an initial-value and/or as a boundary condition. In the first case the decay of the magnetic field is rather fast. The initial field cannot survive more than 500 Myr. In its early times the magnetic field is concentrated between the spirals but later it is strongly wound up by the differential rotation. Any amplification of the magnetic energy does not appear. The nonlinear diffusivity quenching only plays a role for small eddy diffusivity. If the galaxy is embedded in an external intergalactic magnetic field there is an amplification of the magnetic energy by a factor of 10. But very soon the magnetic spirals have been transformed into rings and after about 1.5 Gyr the galaxy is nearly field-free. Our results confirm the idea that primordial magnetic fields in galaxies are unable to become old. If both the gaseous and the magnetic spirals had a common origin, the gaseous spirals are revealed here as young phenomena. Tuning the pattern speed of the spirals an exceptional amplification of the magnetic field is found in case of 'resonance' of the pattern speed and a magnetic drift velocity. Our calculations show that the maximal field then remains in the interarm region. We interpret the peak amplification as being due to the fact that the turbulence in the interarm regions is assumed as weak hence the diffusion there is strongly reduced. The differential rotation then amplifies the initial field maximally while the field decay is delayed.