On the interpretation of the apparent existence of a preferred magnetic polarity in extragalactic jet sources

TL;DR

This paper discusses possible explanations for observed magnetic polarity patterns in extragalactic jets, suggesting that Hall currents in accretion discs could influence Faraday rotation measurements, challenging previous interpretations.

Contribution

It proposes an alternative explanation involving Hall currents affecting Faraday rotation, contrasting with prior models linking magnetic polarity to cosmic magnetic field origins.

Findings

Faraday rotation may be dominated by centrifugally driven winds

Hall currents could influence magnetic field polarity in accretion discs

Further observations are needed to confirm the relationship

Abstract

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.