Astron. Astrophys. 347, 860-865 (1999)

Swing excitation of magnetic fields in trailing spiral galaxies?

R. Rohde, G. Rüdiger and D. Elstner

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany

Received 10 December 1998 / Accepted 19 May 1999

Abstract

Swing-excited dynamo-induced magnetic fields in spiral galaxies are studied. We investigate the phenomenon of swing excitation with linear and nonlinear 3D numerical simulations 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 spiral 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 condition ( pattern speed, 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 , the excitation is weakest at the radius where the differentially rotating gas and the spiral pattern are corotating.

Key words: turbulence ISM: magnetic fields galaxies: ISM galaxies: magnetic fields galaxies: spiral

Send offprint requests to: R. Rohde

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Contents

• 1. Introduction
• 2. The mean-field electrodynamics
• 3. The model
• 4. Growth rates and magnetic field drift
  • 4.1. Two-armed spiral galaxies
  • 4.2. One-armed spiral galaxies
• 5. The nonlinear regime
• 6. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999
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