Copyright © 1996 by Annual Reviews. All rights reserved
|
| Annu. Rev. Astron. Astrophys. 1996. 34:
155-206 Copyright © 1996 by Annual Reviews. 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.
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.