Structure of magnetic fields in spiral galaxies: global properties of the turbulent dynamo

Abstract

General considerations are given to the global modes of magnetic fields generated by the galactic dynamo. The standard disc dynamo equations are extended to include the generation of azimuthal field components via the α-effect (i.e. the α2-dynamo), which is important in central regions of some galaxies (e.g. M51) where the shear of galactic rotation is weak. We focus our attention on the non-axisymmetric modes (m = 1 modes) to compare them with the observed bisymmetric magnetic fields. Although growing m = 1 modes do exist, they tend to grow more slowly than the axisymmetric modes with m = 0. Their eigenfunctions are confined within narrow regions in the radial direction of a disc. Thus, using the standard galactic dynamo, it is still difficult to explain the dominance of the bisymmetric fields and to reproduce their coherent structure extending over a disc by a single m = 1 mode. The growth rate of m = 1 modes depends on the thickness of the disc; it is higher in a thin disc than in a thick disc. As a general property of the field configuration generated by the galactic dynamo, the field lines show a more tightly wound appearance in the outer region of the disc, in spite of the decrease of shear strength in the galactic rotation.