|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
8.2. Large-Scale Azimuthal Patterns
Even the simplest asymptotic kinematic models of the mean-field dynamo in a thin disk have the promising property that only m = 0 modes are excited in those galaxies where the field is observed to be axisymmetric (M31 and IC 342), whereas the m = 1 mode is also excited (if it is not the fastest growing) in the galaxies with a dominant bisymmetric or mixed magnetic structure (e.g. M33, M51, and M81) (see Krasheninnikova et al 1990, Ruzmaikin et al 1988a, b for reviews). The thinner the disk, the more readily the m = 1 mode can be maintained (Ruzmaikin et al 1988a Section VII.8; Moss & Brandenburg 1992). Weaker differential rotation is favorable for bisymmetric field generation. Even higher azimuthal modes might survive in galactic disks, e.g. the m = 2 mode (Starchenko & Shukurov 1989, Vallée 1992), which has a four-armed spiral pattern. An admixture of the m = 2 mode may arise as a distortion of an m = 0 field by a two-armed spiral pattern. An m = 2 mode superimposed on a m = 0 mode of similar amplitude would produce a pattern of the type possibly observed in NGC 6946 (Section 3.4).
The dominance of a bisymmetric field requires additional physical mechanisms to be invoked, as discussed in Sections 6.2 and 6.3; it seems, however, that these mechanisms are efficient only under certain conditions that can occur only in rare cases. Therefore, a general prediction of the galactic dynamo theory is that normally either axisymmetric magnetic structures (in the galaxies where only the m = 0 mode is excited) or a superposition of m = 0 and m = 1 modes (where both are maintained) should be found. The former situation is encountered in M31 and IC 342, whereas the latter is represented by M51. An admixture of even higher m-modes cannot be excluded, as possibly seen in NGC 6946. Only in those galaxies that provide a suitable environment for a fine tuning of the dynamo (Sections 6.2 and 6.3) should a dominant bisymmetric field be expected, as exemplified by M81. An important factor in maintaining BSS seems to be tidal interaction with a companion galaxy (Section 6.2).
In general, this picture is reasonably consistent with observations that most galactic fields do not have simple structures. Note that a superposition of even two or three azimuthal modes may give an appearance of a rather irregular large-scale magnetic field. So far, observations of only a few galaxies have been interpreted with allowance for such superpositions. We expect that new observations and analyses will extend the list of galaxies hosting mixed spiral structure.
A primordial magnetic field twisted by differential rotation is strongly dominated by the S1 or A0 modes (Section 3). An S0 field can arise only if it is assumed that the magnetic field had a very strong inhomogeneity across the protogalaxy (Sofue et al 1986), which appears to be a rather artificial requirement.