The invariant Twist of Magnetic Fields in the Relativistic Jets of
  Active Galactic Nuclei
  The origin of cosmic magnetic (B) fields remains an open question. It is
generally believed that very weak primordial B fields are amplified by dynamo
processes, but it appears unlikely that the amplification proceeds fast enough
to account for the fields presently observed in galaxies and galaxy clusters.
In an alternative scenario, cosmic B fields are generated near the inner edges
of accretion disks in Active Galactic Nuclei (AGNs) by azimuthal electric
currents due to the difference between the plasma electron and ion velocities
that arises when the electrons are retarded by interactions with photons. While
dynamo processes show no preference for the polarity of the (presumably random)
seed field that they amplify, this alternative mechanism uniquely relates the
polarity of the poloidal B field to the angular velocity of the accretion disk,
resulting in a unique direction for the toroidal B field induced by disk
rotation. Observations of the toroidal fields of 29 AGN jets revealed by
parsec-scale Faraday rotation measurements show a clear asymmetry that is
consistent with this model, with the probability that this asymmetry came about
by chance being less than 1%. This lends support to the hypothesis that the
Universe is seeded by B fields that are generated in AGN via this mechanism and
subsequently injected into intergalactic space by the jet outflows.
