Three-dimensional dynamos in spiral galaxies

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 α–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 turbulence contribution, that is small and large correlation time τcorr of interstellar turbulence. For small correlation times we find 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.