  Parsec-scale multi-wavelength VLBA polarization observations can be used to
study the magnetic-field structures of Active Galactic Nuclei (AGN) based on
Faraday Rotation (FR) gradients. A number of transverse FR gradients have been
found, and interpreted as corresponding to helical magnetic fields wrapped
around the jets; the gradients reflect the systematic change in the
line-of-sight component of a toroidal or helical magnetic field across the jet
(e.g Gabuzda, Murray & Cronin 2004). Our observations of a sample of BL Lac
objects at six wavelengths near 2, 4 and 6 cm have also revealed a previously
undetected phenomena: these transverse gradients sometimes change their
direction with distance from the core. We have observed this behaviour in at
least five sources, which display gradients in their VLBI core region opposite
to those in the jet. We suggest that this may be linked to magnetic tower
models. In magnetic tower models, the field lines go outward with the jet and
return and close in the accretion disk (or vice versa); differential rotation
of the accretion disk winds up the inner and outer field lines into two helices
(the inner helix "nested" in the outer helix). The total observed FR gradient
is a sum of the effect of these two helical fields. It may be that gradients
detected relatively far from the core correspond to the outer helix, while
gradients detected in the core region correspond to dominance of the inner
helix. This provides tentative evidence for the unification of helical magnetic
fields and magnetic tower models, which could provide crucial new information
for understanding AGN jets. Further VLBI studies with resolution sufficient to
reliably detect these gradients in the cm-wavelength core and inner jet will be
important for further investigations of this phenomena.
