The non - linear galactic dynamo II . Oscillatory vs . steady solutions

In order to compute the turbulent electromotive force, the vertical momentum equation for the galactic gas is solved. The only input is the density profile of the gaseous disk. The resulting-effect, the advection terms as well as the eddy diffusivity tensor are quenched by the feedback of the magnetic field. The only free parameters are the factor in the vertical structure equation and a " mixing-time" factor c for the correlation time relation. The mean-field dynamo equation is solved with a 2D time-stepping code for a flow system with the turbulence and a given galactic rotation (Elstner et al. 1990). The free parameter c strongly influences the characteristics of the resulting magnetic field. Short correlation times provide steady quadrupoles while longer correlation times provide oscillating dipoles. Only the latter yield the observed magnitude of the (negative) pitch angle. The magnetic field energy is in strict balance with the turbulence energy (" equipartition"). If — as it must be — the magnetic feedback also concerns the eddy diffusivity tensor, only the oscillating solutions remain realistic. The non-oscillating modes diverge. Now the fields are in slight super-equipartition and the pitch angles are reduced. The oscillation periods are of order of 1 Gyr, very similar to the overall growth times.