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Galactic spiral patterns and dynamo action - II. Asymptotic solutions

Lookup NU author(s): Professor Anvar Shukurov

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Abstract

The exploration of mean-field galactic dynamos affected by a galactic spiral pattern, begun in Chamandy et al. (Paper I) with numerical simulations, is continued here with an asymptotic solution. 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). The effect of the spiral pattern on the dynamo considered is the enhancement of the alpha-effect in spiral-shaped regions (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. Good qualitative agreement is obtained between the asymptotic solution and numerical solutions of Paper I for a global, rigidly rotating material spiral (density wave). At all galactocentric distances except for the corotation radius, we find magnetic arms displaced in azimuth from the alpha-arms, 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 (related to the turbulence correlation time) is to phase shift the magnetic arms in the direction opposite to the galactic rotation even at the corotation radius. This mechanism can be used to explain the phase shifts between magnetic and material arms observed in some spiral galaxies.


Publication metadata

Author(s): Shukurov A; Chamandy L; Subramanian K

Publication type: Article

Publication status: Published

Journal: Monthly Notices of the Royal Astronomical Society

Year: 2013

Volume: 433

Issue: 4

Pages: 3274-3289

Print publication date: 01/08/2013

ISSN (print): 0035-8711

ISSN (electronic): 1365-2966

Publisher: Oxford University Press

URL: http://dx.doi.org/10.1093/mnras/stt967

DOI: 10.1093/mnras/stt967


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