Magnetic Fields generated after Recombination

When dealing with cosmic magnetic fields in present day astrophysical problems, it is customary to fully assume hypotheses that are accepted in magnetohydrodynamics. It is in fact assumed that magnetic fields can be modified, amplified and even be subject to diffusion or reconnection, but that they cannot be created. However, the three above mentioned mechanisms are able to create magnetic fields out of nothing. There are also more classical mechanisms of net creation of magnetic fields, with the well known Biermann's battery providing a clear example (Biermann, 1950; Biermann and Schleuter, 1951). Another battery mechanism was proposed by Mishustin and Ruzmaikin (1973), in which the CMB radiation differentially interacts via Compton scattering with protons and electrons, thus establishing a weak electric field and weak electrical currents that in turn are able to originate weak magnetic fields.

In a protogalactic cloud, the conditions are similar to those needed for the classical Biermann's battery, mainly a combination of gravitational field with differential rotation. Lesch and Chiba (1995) showed that magnetic field strengths in the range 10^-13 - 10^-16G can be produced at early stages in the protogalactic cloud. This seed may be exponentially amplified by non-axisymmetric instabilities during the disk formation epoch, so that magnetic fields of the order of 1$\mu$G can be reached in less than 1-2 Gyr, as observed in recently born galactic systems (Chiba and Lesch, 1994). Kulsrud et al. (1997) and Howard and Kulsrud (1996) also demonstrated that protogalactic magnetic fields can be created without any seed after Recombination.

Kronberg, Lesch and Hopp (1998) have proposed that superwinds of dwarf galaxies of the M82-type, which eject great quantities of matter and magnetic fields, have effectively seeded the intergalactic medium with magnetic fields, in a first generation of (z $\sim$ 10) galaxies. The seeding would have been accomplished by z $\sim$ 6. Under this hypothesis, pre-Recombination fields would not be required, at least to understand galactic fields.