|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
Only the dynamo theory for galactic magnetic fields has been developed sufficiently to provide models of magnetic fields in particular galaxies that can be confronted with observations. Therefore, our discussion below is inevitably more detailed in the case of the dynamo theory. Wherever possible, we also mention inferences from the primordial field theory, ignoring the conceptual difficulties discussed in Section 5.
8.1. The Parity
It is generally believed that galactic magnetic fields have an even parity. As discussed in Section 3.8.1, the field parity near the Sun most plausibly is even. There is some evidence for an even symmetry of the regular magnetic field in the edge-on galaxy NGC 253 (Beck et al 1994b). In mildly inclined galaxies, Faraday rotation measures for even and odd fields of equal strengths would differ only by a factor of 2 (Krause et al 1989a), which makes it difficult to distinguish between the two configurations. All conventional dynamo models indicate that the quadrupole parity must be dominant in galactic disks.
A uniform primordial magnetic field trapped by a protogalaxy, with arbitrary inclination to the rotation axis, produces an S1 component from the action of the radial gradient of the angular velocity on r (which is then even in z and nonaxisymmetric) and an A0 field from the action of / z on z (which is odd and axisymmetric). Since | / r| >> | / z|, at least during late stages of galactic evolution, the S1 field will become tightly wound and quickly decay because of reconnection. The resulting symmetry of a fossil field is then A0 or, possibly, a superposition of S1 and A0 configurations.