The Excitation of Nonaxisymmetric Magnetic Fields in Galaxies

Summary

The nonaxisymmetric (‘bisymmetric spiral’) magnetic field observed in the spiral galaxy M81 presents a challenge for mean field dynamo theory. We discuss several relevant mechanisms, and present simple numerical models to illustrate how a dominant m = 2 dependence of the turbulent coefficients might produce significant m = 1 field structure.

INTRODUCTION

Very naturally, many of the investigations into astrophysical dynamo theory have been directed to explaining the Solar cycle: after all, this is the system for which the most detailed information, both spatial and temporal, is available. The large scale Solar magnetic field appears to be approximately axisymmetric and so it is appropriate to study strictly axisymmetric dynamos. More recently, evidence has accumulated that magnetic fields with a significant nonaxisymmetric component may be present in late type ‘active giant’ stars (see, e.g., the discussion in Moss et al. 1991a, and references therein), and also in one or two spiral galaxies, notably M81 (e.g. Krause et al. 1989; Sokoloff et al. 1992). Thus the conditions under which nonaxisymmetric fields can be excited in astrophysical systems are of current interest. Radler et al. (1990) and Moss et al. (1991a) have recently investigated nonlinear spherical mean field dynamo models in which stable nonaxisymmetric fields may be excited with suitably chosen distributions of alpha effect and differential rotation; see also Stix (1971). Rüdiger & Elstner (1992) considered models where the introduction of an anisotropy in the alpha tensor may have a similar effect; see also Rüdiger (1980).