  One possible origin for polarization structures across jets of Active
Galactic Nuclei (AGNs) with a central "spine" of orthogonal magnetic field and
a "sheath" of longitudinal magnetic field along one or both edges of the jet is
the presence of a helical jet magnetic field. Simultaneous Very Long Baseline
Array (VLBA) polarization observations of AGN displaying partial or full
spine--sheath polarization structures were obtained at 4.6, 5.0, 7.9, 8.9, 12.9
and 15.4 GHz, in order to search for additional evidence for helical jet
magnetic fields, such as transverse Faraday rotation gradients (due to the
systematic change in the line-of-sight magnetic-field component across the
jet). Results for eight sources displaying monotonic transverse Faraday
rotation gradients with significances $\geq 3\sigma$ are presented here.
Reversals in the directions of the transverse RM gradients with distance from
the core or with time are detected in three of these AGNs. These can be
interpreted as evidence for a nested helical magnetic field structure, with
different directions for the azimuthal field component in the inner and outer
regions of helical field. The results presented here support the idea that many
spine--sheath polarization structures reflect the presence of helical magnetic
fields being carried by these jets.
