3.6.2. Battery origin for magnetic seeds in galaxies

The original strength of such a battery-type magnetic seed is very small. In a moving ionized gas, there are thermal and inertial battery effects, capable of creating a weak magnetic field - this is now referred to as the Biermann battery (e.g., Biermann 1950; Kempt 1982). Battery effects in a protogalaxy could create a seed protogalactic magnetic field B ~ 10 - 20 Gauss.

The initial amplification of such a battery-type seed field is under debate: gravitational collapse, strong flows, or dynamos. In one model, the seed field of 10^-20 Gauss could then be enhanced by a factor ~ 10⁴ due to galactic collapse under gravitational forces, and it could be subsequently amplified by a factor ~ 10⁸ in strong non-axisymmetric flows from bars and spirals (e.g., Lesch & Chiba 1995). Dynamo amplification would then follow. Unfortunately, these very weak magnetic fields are difficult to amplify and to order by a dynamo alone over a time scale ~ 10⁸ years (e.g., Kulsrud and Anderson, 1992; Balbus, 1993).

Battery effects around a central massive black hole (~ 10⁶ M) in the galactic center could also create a seed field by the rotation of an aspherical cloud of ionized gas. It is envisioned to be transported out to the galactic disk by jets, twin outflows, slow winds (e.g., Chakrabarti et al. 1994; Daly & Loeb 1990; Akasofu & Hakamada 1982). This magnetic field transport in the galactic disk could have occurred in an initial burst a long time ago, but would be unlikely to occur routinely now under current galactic disk conditions. For ejection along the galactic plane, radial resistance may be encountered in the spiral arms and the denser parts of the interstellar medium, stopping the ejections from the galactic nucleus at a radius r_stop. Within the distance r_stop, any previously ordered magnetic field distribution will be removed (e.g., Vallée 1982a). Beyond the distance r_stop, cosmic-ray particles may not move rapidly across pre-existing horizontal, azimuthal magnetic field lines, possibly drawn there by differential rotation (e.g., Vallée 1995a).