Luke Chamandy, Kandaswamy Subramanian, Anvar Shukurov
The exploration of mean-field galactic dynamos affected by a galactic spiral pattern, begun in Chamandy et al. (2013, hereafter Paper I) with numerical simulations, is continued here with an asymptotic solution. The effect of the spiral pattern on the dynamo considered is the enhancement of the {\alpha} effect in spiral arms (which may overlap the gaseous spiral arms or be located in the interarm regions). The axisymmetric and enslaved non-axisymmetric modes of the mean magnetic field are studied semi-analytically to clarify and strengthen the numerical results. The mean-field dynamo model used generalizes the standard theory to include the delayed response of the mean electromotive force to variations of the mean magnetic field and turbulence parameters (the temporal non-locality, or {\tau} effect). Good qualitative agreement is obtained between the asymptotic solution presented and numerical solutions of Paper I for a global rigidly rotating material spiral (density wave). We find magnetic arms displaced from the {\alpha}-arms away from the co-rotation radius, so that the ridges of magnetic field strength are more tightly wound than the {\alpha}-arms. Moreover, the effect of a finite dynamo relaxation time {\tau} is to phase-shift the magnetic arms in the direction opposite to the galactic rotation. This mechanism can be used to explain the observed phase shifts between magnetic and material arms in some galaxies.
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http://arxiv.org/abs/1301.4761
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