  Homan & Lister (2006) have recently published circular-polarization (CP)
detections for 34 objects in the MOJAVE sample - a set of bright, compact AGN
being monitored by the Very Long Baseline Array at 15 GHz. We report the
detection of 15-GHz parsec-scale CP in two more AGN (3C345 and 2231+114), and
confirm the MOJAVE detection of CP in 1633+382. It is generally believed that
the most likely mechanism for the generation of this CP is Faraday conversion
of linear polarization to CP. A helical jet magnetic-field (B-field) geometry
can facilitate this process - linearly polarized emission from the far side of
the jet is converted to CP as it passes through the magnetised plasma at the
front side of the jet on its way toward the observer. In this case, the sign of
the generated CP is essentially determined by the pitch angle and helicity of
the helical B field. We have determined the pitch-angle regimes and helicities
of the helical jet B fields in 8 AGN for which parsec-scale CP has been
detected, and used them to predict the expected CP signs for these AGN if the
CP is generated via conversion in these helical fields. We have obtained the
intriguing result that our predictions agree with the observed signs in all
eight cases, provided that the longitudinal B-field components in the jets
correspond to South magnetic poles. This clearly non-random pattern
demonstrates that the observed CP in AGN is directly associated with the
presence of helical jet B fields. These results suggest that helical B fields
are ubiquitous in AGN jets.
