On the interpretation of the apparent existence of a preferred magnetic
  polarity in extragalactic jet sources
  Contopoulos et al. recently argued that there is observational evidence for a
preferred sense of the Faraday rotation-measure gradients across jets from
active galactic nuclei (AGNs). Such behaviour could arise if there were a
deterministic relationship between the polarity of the poloidal magnetic field
that threads the outflow and the sense of rotation of the outflow's source.
Based on this interpretation, Countopoulos et al. suggested that their finding
supports a model for the origin of cosmic magnetic fields in a
Poynting-Robertson process operating in AGN accretion discs. Here I point out
that an alternative explanation of such a relationship could be that the Hall
current plays a key role in the magnetohydrodynamics of the underlying disc. In
this picture, the measured Faraday rotation is dominated by the contribution of
a centrifugally driven wind that is launched from the weakly ionized outer
region of the disc. Additional observations are, however, needed to verify the
claimed behaviour.
