Astron. Astrophys. 343, 120-131 (1999)

Galactic dynamos driven by magnetic buoyancy

David Moss ¹, Anvar Shukurov ² and Dmitry Sokoloff <sup>2,3

1</sup> Department of Mathematics, University of Manchester, Manchester, M13 9PL, UK
² Department of Mathematics, University of Newcastle, Newcastle on Tyne, NE1 7RU, UK
³ Physics Department, Moscow University, 19899 Moscow, Russia

Received 28 July 1998 / Accepted 13 November 1998

Abstract

We consider a mean field dynamo model for the generation of galactic magnetic fields, in which the driving mechanism for the alpha-effect is the buoyancy-driven Parker instability, rather than the conventional cyclonic turbulence. The dominant quenching mechanism is a buoyant velocity, which lifts field away from the disc plane. Our modified alpha-effect is essentially nonlinear, as its magnitude depends on the magnetic field strength. The dynamo thus exhibits threshold behaviour - the marginal dynamo number depends on the seed field geometry and strength. We investigate a basic one-dimensional `slab' model both analytically and numerically, and also present results from a three dimensional calculation. In some ways our steady solutions resemble conventional galactic dynamo solutions, but the fields do not decrease so markedly away from the disc midplane, remaining strong out to the boundary of the dynamo region. The latter property is quite natural for a buoyancy driven system and may possibly be related to strong horizontal magnetic fields detected in the halos of some spiral galaxies.

Key words: galaxies: interactions galaxies: magnetic fields galaxies: ISM ISM: magnetic fields Magnetohydrodynamics (MHD) magnetic fields

Send offprint requests to: D. Moss

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Contents

• 1. Introduction
• 2. The alpha effect due to magnetic buoyancy
• 3. The basic model
• 4. A local analysis
  • 4.1. An approximate solution
  • 4.2. Slab surrounded by vacuum
  • 4.3. Slab surrounded by a passive halo
  • 4.4. Threshold effects
  • 4.5. Other calculations
• 5. A three-dimensional dynamo model
  • 5.1. The three-dimensional solutions
• 6. Discussion
• 7. Conclusions
• Acknowledgements
• Appendix A
• Appendix A: ofanthexplicite formapproximate steady state solution
• Appendix B: the marginal dynamo number
• References

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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