A&a Manuscript No. Swing Excitation of Magnetic Elds in Trailing Spiral Galaxies?

Swing-excited dynamo-induced magnetic elds in spiral galaxies are studied. We investigate the phenomenon of swing excitation with nonlinear 3D numerical simulations of the galactic dynamo. In a rst step we present a linear analysis in order to get inside in basic properties of the swing-excitation phenomenon in one and two-armed spiral galaxies. The model includes diierential rotation, axisymmetric and isotropic-eeect as well as uniform eddy diiusivity. The nonaxisymmetry is introduced via large-scale radial and azimuthal velocity components associated with a spiral density-wave perturbation. The solutions are a mixture of several magnetic eld modes; for a two-armed spiral the even modes (m = 0, 2, ...) are preferred. The contribution of higher magnetic eld modes to the solution is largest if the parametric resonance condition p 2 ! m ((p pattern speed, ! m magnetic drift rate) is fulllled. The eld geometry is strongly aaected. We also nd enlarged growth rates for the magnetic energy connected with the parametric resonance condition, but only if the amplitude of the perturbation exceeds 10 km s ?1 .