3.6. Seeds of galactic magnetism
A few possible seeds for galactic magnetism have been proposed, from primordial cosmological origins to recent local origins.
3.6.1. Primordial cosmological magnetic seeds in galaxies
The original strength of such a primordial cosmical magnetic field is unknown, but weak. No observational evidence has been found for an uniform primordial field in intergalactic space (e.g., a review in Vallée 1990b), with upper limits of 10-10 gauss being set. This primordial hypothesis has already been shown elsewhere to be difficult, because the primordial magnetic field may diffuse out of the disk through the halo and out into the intergalactic medium. This diffusion arises on a short time scale (~ 108 yrs), shorter than the time scale of the galaxy (Parker, 1971; Parker, 1976), or not quite that short (e.g., Kulsrud and Howard 1997).
Ratra (1992) has argued for a very weak cosmological seed magnetic field, originating in quantum mechanical irregularities at an early epoch. The following rapid expansion of the Universe during the inflation era would stretch this early magnetic seed to reach between 10-60 Gauss and 10-10 Gauss nowadays over a length scale of ~ 20 Mpc (i.e., typical of a supercluster of galaxies).
The initial amplification of such a primordial seed is under debate: inflation, gravitation, and/or dynamo are proposed. Zweibel (1993) showed a picture with a primordial magnetic field being wound up by a rotating galaxy: "This winding up of the field into a bisymmetric spiral leads to reversals in direction on progressively smaller scales over time..." (p.591). Twisting and amplification of this primordial magnetic field, by galactic differential rotation under gravitational forces, is thought to produce a bisymmetric magnetic field distribution. This may not be the only way to produce a bisymmetric distribution.
In complex dynamo models, a weak primordial magnetic field may be necessary (e.g., Kulsrud & Anderson 1992) or may not be necessary (e.g., Field 1995) in the galactic disk. This area of research is being pursued with vigor.