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.