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

5.2. The Primordial Origin of Galactic Magnetic Fields

We now assess the possibility that the large-scale magnetic field observed in galaxies is merely a result of the twisting of a cosmological magnetic field by galactic differential rotation (see e.g. Kulsrud 1986). Aiming at conservative estimates, we neglect any magnetic field dissipation. An isotropic contraction of the protogalaxy with a frozen-in magnetic field, from an intergalactic density _IG 10^-29 g cm^-3 up to an interstellar density 10^-24 g cm^-3, results in amplification of the primordial magnetic field by a factor of 2 × 10³. Differential rotation results in an amplification of the magnetic field in a young galaxy by the number of galactic rotations in 10¹⁰ yr, which is N ~ 30. Altogether, a conservative upper limit on the field in the galactic disk resulting from a primordial field is

(10)

where the more favorable constraint (7) has been used. A primordial field wound up by differential rotation ultimately decays: In a region with closed streamlines (a galaxy in this case) this effect is known as flux expulsion (Moffatt 1978).