THE NONLINEAR GALACTIC DYNAMO .1. FIELD-STRENGTH AND VERTICAL PARITY

The mean-field dynamo equation is solved with a 2D timestepping code f or a given flow system with the aim to investigate the equatorial symm etry of the induced magnetic fields. The turbulence pattern is assumed to be uniform along the radius but variable in the vertical direction up to the galactic halo where the value of the rms velocity is fixed. The midplane turbulence intensity is the free parameter in the theory , while the density is assumed constant. The (magnetically quenched) t urbulent electromotive force is then derived from the given flow patte rn. The resulting magnetic fields are generally concentrated in the di sk. If, however, the molecular halo diffusivity is small (''hot plasma '') and the inner rigidly rotating core is small (thick disks), large toroidal fields are generated in the halo - which are not observed, ho wever. The real galactic dynamo, therefore, works with either vacuum o r strong turbulence outside the disk. The calculated magnetic fields r esemble the observed ones only if the midplane turbulent velocity slig htly exceeds the halo values. Both observed magnetic field strengths a nd pitch angles are reproduced by the model. The fields increase with thickness of the disk while the pitch angles are proportional to the c orrelation time of the turbulence (greater-than-or-equal-to 10(7) yr). If the midplane rms velocity is drastically increased the dynamo regi me tends to switch to the alpha2-type with increasing magnetic fields and a sudden change to dipolar parity occurs. Even in that case the ma gnetic fields do not exceed greatly the equipartition values.