Astron. Astrophys. 333, 27-30 (1998)

1. Introduction

It is known from radio polarization observations that spiral galaxies own large-scale magnetic fields of different, but in the most cases spiral geometry (Beck et al. 1996). Two basic field properties are of interest respecting the spiral field geometry: i) The field concentration between spiral arms as it is observed in the case of NGC 6946 (Beck & Hoernes 1996); ii) The appearance of axisymmetric (ASS) and bisymmetric spiral structured fields (BSS). The latter seems to be rather seldom and is observed in e.g. M 81 (Beck et al. 1996). The mechanisms concerning such properties of galactic magnetic fields are still not satisfactorily investigated. The basic ansatz for those investigations is a nonaxisymmetry in the underlying galaxy model, whereas the discussed mechanisms are of different origin (Beck et al. 1996; Moss 1997a). Therefore a 3D analysis is unavoidable.

So far applied 2D (Moss 1996, 1995) and 3D simulations (Moss 1997b; Otmaniowska-Mazur & Chiba 1995; Moss et al. 1993; Panesar & Nelson 1992) were mostly concerned with mechanisms leading to a nonaxisymmetric gas velocity, whereby restrictions were used as neglecting vertical stratifications, discussing linear theory or evaluating solutions restricted to symmetry conditions. Further turbulence effects as diamagnetism and buoyancy were then neglected. This situation is also found in simulations which investigate nonaxisymmetric turbulence coefficients (Moss 1996; Moss et al. 1993).

Since most investigations (Moss et al. 1993; Moss 1995, 1996) chiefly discussed the induction of BSS type fields we lay in this paper emphasis on the effect of field concentration between spiral arms, respecting the observations made for NGC 6946; but we also touch the discussion about inducing ASS versus BSS type fields.

In our models the built-in nonaxisymmetry is attached to the density profile and the turbulence contribution. The density stratification is taken from observational results and is used as a base for the calculated vertical turbulence profile in order to represent the galactic properties most realistic in a consistent manner. The -tensor includes a consistent feedback mechanism of the magnetic field (Rüdiger & Kitchatinov 1993); but we neglect the feedback onto the eddy diffusivity. We do not follow the concept of solving both the induction and the Navier-Stokes equation (Barker & Moss 1994) as we neglect influence of the magnetic field onto the gas velocity. Our 3D time-stepping code (Elstner et al. 1990; Rohde et al. 1998) uses cylindrical polar coordinates and allows a nonlinear treatment without restrictions onto the symmetry of the solutions.

© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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