VERTICAL MAGNETIC-FIELDS ABOVE THE DISKS OF SPIRAL GALAXIES

We investigate the magnetic fields above the discs of spiral galaxies in the framework of axisymmetric nonlinear mean-field dynamo models for a disc surrounded by a spherical halo, using realistic rotation curves. We consider, in particular, NGC 4631 and NGC 89 1, and include turbulent diamagnetism, an anisotropy of the alpha effect and a galactic wind. In these models magnetic field is generated in a disc of scale height 1.5 kpc and distorted by the wind in the halo. For typical wind velocities of 50 ... 200 km/s in the halo we find good qualitative agreement between the observed polarisation maps and those synthesised from the magnetic fields of our models: Poloidal synthesised fields dominate in the halo of NGC 4631, and horizontal fields in that of NGC 89 1. In some cases, a galactic wind can enhance dynamo action, contrary to previous expectations. Our results indicate that turbulent diamagnetism plays an important role in galactic dynamos.