Swing excitation of magnetic fields in trailing spiral galaxies?

Swing-excited dynamo-induced magnetic fields in spiral galaxies are studied . We investigate the phenomenon of swing excitation with linear and nonline ar 3D numerical simulations of the galactic dynamo. The model includes diff erential rotation, axisymmetric and isotropic alpha-effect and uniform eddy diffusivity. The nonaxisymmetry is introduced via large-scale radial and a zimuthal velocity components associated with a spiral density-wave perturba tion. In a first step we present a linear analysis in order to get insight in bas ic properties of the swing-excitation phenomenon. Both one and two-armed sp iral galaxies are investigated. We find enlarged growth rates for the magne tic energy connected with the parametric resonance condition Omega(P) appro ximate to 2 omega(m) (Omega(P) pattern speed, omega(m) magnetic drift rate) , but only if the amplitude of the perturbation exceeds 10 km s(-1). The re sonance behavior also appears in the nonlinear regime. The solutions are a mixture of several magnetic field modes. For a two-armed spiral the even mo des (m = 0, 2,...) are preferred. The contribution of higher magnetic field modes to the solution is largest if the parametric resonance condition is fulfilled. The field geometry depends strongly on the pattern speed Omega(P ), the excitation is weakest at the radius where the differentially rotatin g gas and the spiral pattern are corotating.