  The formation of relativistic jets in active galactic nuclei (AGN) is related
to accretion on to their central supermassive black holes, and magnetic fields
are believed to play a central role in launching, collimating and accelerating
the jet streams from very compact regions out to kiloparsec or megaparsec
scales. In the presence of helical or toroidal magnetic fields threading the
AGN jets and their immediate vicinity, gradients in the observed Faraday
rotation measures are expected due to the systematic change in the
line-of-sight component of the magnetic field across the jet. We have analysed
total intensity, linear polarization, fractional polarization and Faraday
rotation maps based on Very Long Baseline Array data obtained at four
wavelengths in the 18-22 cm range for six AGN (OJ 287, 3C 279, PKS 1510-089, 3C
345, BL Lac and 3C 454.3). These observations typically probe projected
distances out to tens of parsecs from the observed core, and are well suited
for Faraday rotation studies due to the relatively long wavelengths used and
the similarity of the structures measured at the different wavelengths. We have
identified statistically significant, monotonic, transverse Faraday rotation
gradients across the jets of four of these six sources, as well as a tentative
transverse Faraday rotation gradient across the jet of OJ 287, providing
evidence for the presence of toroidal magnetic fields, which may be one
component of helical magnetic fields associated with these AGN jets.
