Symmetry of the magnetic fields in galactic dynamo and the material arms

E. A. Mikhailov

Faculty of Physics, Moscow Lomonosov University, 2-1 Leninskie gori, 119991 Moscow, Russia

Abstract
It is well known that some of the galaxies have magnetic fields of several microgauss. From the observational point of view, they are usually studied using Faraday rotation measurements. First works in this field supposed that the magnetic field was associated with material arms. This was supported with some theoretical ideas presuming an amount of the ionized medium which is a driver of the field growth. However, nowadays there are observations of different objects, where a large-scale magnetic field can be assumed approximately axisymmetric and does not have strong correlation with the material arms. Sometimes the magnetic field can be concentrated even between the arms. Here we describe the reasons of this phenomenon based on the mean field dynamo mechanism. The field is studied using the no-z approximation which was introduced for thin discs. We construct eigenfunctions which can describe a magnetic field corresponding to the axial symmetry field and higher modes. For each mode, an eigenvalue is given and the typical timescale is estimated. Also, the role of different effects, such as turbulent diffusion and other terms in the equations which can make the features of the field decay, are studied. Typical plots of the magnetic field evolution are presented which demonstrate that the regular field during a long time becomes independent on the azimuthal angle. It is shown that non-axisymmetric fields are much more difficult to generate, so the difference between the material arms and the field structure can be quite realistic, which supports the observation data. Figs 2, Refs 49.