Dynamo models for the structure of the regular mag- netic field in the galaxy NGC 6946 are confronted with recent observations of polarized radio emission at 3:5 cm. The ob- served behaviour of the concentration of regular fields between the gaseous spiral arms (magnetic spiral arms) is achieved via a non-axisymmetric azimuthal distribution of only the cor- relation time corr of interstellar turbulence, determined by the typical size of interstellar clouds or the lifetime of supernova remnants or superbubbles. There is no need for the turbulent gas velocity of gas clouds to vary between arm and interarm regions. Furthermore, a satisfactory agreement between model and observations of the radial distribution of the regular field strength requires that the galaxy rotates differentially at radii beyond a few 100 pc, as indicated by observations of the molec- ular gas. Our model also explains the strong concentration of regular fields within the inner 2 kpc as well as the observed arm/interarm variation of the field pitch angle.

Swing-excited dynamo-induced magnetic fields in spiral galaxies are studied. We investigate the phenomenon of swing excitation with linear and nonlinear 3D numerical simu- lations of the galactic dynamo. The model includes differential rotation, axisymmetric and isotropic -effect and uniform eddy diffusivity. The nonaxisymmetry is introduced via large-scale radial and azimuthal velocity components associated with a spi- ral density-wave perturbation. In a first step we present a linear analysis in order to get insight in basic properties of the swing-excitation phenomenon. Both one and two-armed spiral galaxies are investigated. We find enlarged growth rates for the magnetic energy connected with the parametric resonance conditionp 2 !m (p pattern speed, !m magnetic drift rate), but only if the amplitude of the perturbation exceeds 10 km s 1 . The resonance behavior also appears in the nonlinear regime. The solutions are a mixture of several magnetic field modes. For a two-armed spiral the even modes (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 p, the excitation is weakest at the radius where the differentially rotating gas and the spiral pattern are corotating.

A new-developed 3D numerical code is applied to an uniform external (primordial) magnetic eld subject to a complex flow pattern representing the situation in a turbulent spiralgalaxy.Thespiralarmsaredenedbytheradial-azimuthal proles of density and the turbulent velocity, but they do not yet possess any own large-scale velocity eld. No dynamo alpha is assumed to exist, but all the known turbulence effects such as eddy diamagnetism and turbulent pumping are involved. Two different models are followed: The (nonaxisymmetric) externalmagneticeldisconsideredasaninitial-valueand/oras a boundary condition. In the rst case the decay of the magnetic eld is rather fast. The initial eld cannot survive more than 500 Myr. In its early times the magnetic eld is concentrated between the spirals but later it is strongly wound up by the differential rotation. Any amplication of the magnetic energy does not appear. The nonlinear diffusivity quenching only plays a role for small eddy diffusivity. If the galaxy is embedded in an external intergalactic mag- netic eld there is an amplication of the magnetic energy by a factorof10.Butverysoonthemagneticspiralshavebeentrans- formed into rings and after about 1.5 Gyr the galaxy is nearly eld-free. Our results conrm the idea that primordial magnetic elds in galaxies are unable to become old. If both the gaseous and the magnetic spirals had a common origin, the gaseous spirals are revealed here as young phenomena. Tuningthepatternspeedofthespiralsanexceptionalampli- cation of the magnetic eld is found in case of 'resonance' of the pattern speed and a magnetic drift velocity. Our calculations showthatthemaximaleldthenremainsintheinterarmregion. We interpret the peak amplication as being due to the fact that the turbulence in the interarm regions is assumed as weak hence the diffusion there is strongly reduced. The differential rotation then amplies the initial eld maximally while the eld decay is delayed.

Solutions of the 3D nonlinear induction equation for dynamos in spiral galaxies are presented in this paper. Our model includes diierential rotation, ambipolar diiusion and, based on small-scale turbulence, eddy diiu-sivity and the tensorial {eeect underlying magnetic feedback. The nonaxisymmetric radial-azimuthal spiral pattern and the vertical stratiication of the galaxy are represented in the density and turbulence proole. Our calculations are chieey concerned with the eeect of eld concentration between (or within) the spiral arms. The possible induction of bisymmetric instead of axisym-metric structured elds is also investigated. We distinguish between models with weak and strong turbulence contribution, that is small and large correlation time corr of interstellar turbulence. For small correlation times we nd axisymmetric steady solutions of even parity showing concentration between the spiral arms. The pitch angles are relatively large and increase with enlarged correlation time. Large correlation times are linked with oscillating BSS type solutions of again even parity, but they are clearly concentrated within the spiral arms.

Solutions of the 3D nonlinear induction equation for dynamos in spiral galaxies are presented in this paper. Our model includes differential rotation, ambipolar diffusion and, based on small-scale turbulence, eddy diffusivity and the tensorial -effect underlying magnetic feedback. The nonaxisymmetric radial-azimuthal spiral pattern and the vertical stratification of the galaxy are represented in the density and turbulence profile. Our calculations are chiefly concerned with the effect of field concentration between (or within) the spiral arms. The possible induction of bisymmetric instead of axisymmetric structured fields is also investigated. We distinguish between models with weak and strong turbu- lence contribution, that is small and large correlation time corr of interstellar turbulence. For small correlation times we find axisymmetric steady solutions of even parity showing concen- tration between the spiral arms. The pitch angles are relatively large and increase with enlarged correlation time. Large corre- lation times are linked with oscillating BSS type solutions of again even parity, but they are clearly concentrated within the spiral arms.

We present simulations of the 3D nonlinear induction equation in order to investigate the temporal evolution of large-scale magnetic fields in spiral galaxies. Our model includes differential rotation, ambipolar diffusion and, based on small-scale turbulence, eddy diffusivity and the tensorial α-effect with magnetic feedback. The nonaxisymmetric spiral pattern and – if considered – the vertical stratification of the galaxy are represented in its density and turbulence profile.