NONLINEAR WINDING OF LARGE-SCALE MAGNETIC-FIELDS IN SPIRAL GALAXIES

A new-developed 3D numerical code is applied to an uniform external (p rimordial) magnetic field subject to a complex flow pattern representi ng the situation in a turbulent spiral galaxy. The spiral arms are def ined by the radial-azimuthal profiles of density and the turbulent vel ocity, but they do not yet possess any own large-scale velocity field. No dynamo alpha is assumed to exist, but all the known turbulence eff ects such as eddy diamagnetism and turbulent pumping are involved. Two different models are followed: The (nonaxisymmetric) external magneti c field is considered as an initial-value and/or as a boundary conditi on. 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 i s strongly wound up by the differential rotation. Any amplification of the magnetic energy does not appear. The nonlinear diffusivity quench ing only plays a role for small eddy diffusivity. If the galaxy is emb edded in an external intergalactic magnetic field there is an amplific ation of the magnetic energy by a factor of 10. But very soon the magn etic spirals have been transformed into rings and after about 1.5 Gyr the galaxy is nearly field-free. Our results confirm the idea that pri mordial 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 i n the interarm region. We interpret the peak amplification as being du e to the fact that the turbulence in the interarm regions is assumed a s weak hence the diffusion there is strongly reduced. The differential rotation then amplifies the initial field maximally while the field d ecay is delayed.