SPIRAL-ARM RESONANCE IN GALACTIC DYNAMOS

We study further properties of the resonance of galactic dynamos under the influence of galactic density waves. Two-dimensional numerical calculations are performed to elucidate the characteristic structure and evolution of the excited magnetic fields in a galactic disc. Models subject to spiral-arm perturbations of the alpha-effect are considered as well as those with perturbations of the gaseous velocity fields. It is shown that in the state of complete resonance, the magnetic fields exhibit a steady growth of energy without any oscillations. The whole pattern of magnetic fields remains unchanged with time, but propagates just with the imposed disturbances parallel to the galactic plane. The field structure excited is found to be largely influenced by the spatial form of disturbances in the velocity fields and alpha-effects; for example, the wavelength of excited fields is just twice that of density waves.