  Polarization has proved an invaluable tool for probing magnetic fields in
relativistic jets. Maps of the intrinsic polarization vectors have provided the
best evidence to date for uniform, toroidally dominated magnetic fields within
jets. More recently, maps of the rotation measure (RM) in jets have for the
first time probed the field geometry of the cool, moderately relativistic
surrounding material. In most cases, clear signatures of toroidal magnetic
field are detected, corresponding to gradients in RM profiles transverse to the
jet. However, in many objects these profiles also display marked asymmetries
which are difficult to explain in simple helical jet models. Furthermore, in
some cases the RM profiles are strongly frequency and/or time dependent. Here
we show that these features may be naturally accounted for by including
relativistic helical motion in the jet model. In particular, we are able to
reproduce bent RM profiles observed in a variety of jets, frequency dependent
RM profile morphologies and even the time dependence of the RM profiles of
knots in 3C 273. Finally, we predict that some sources may show reversals in
their RM profiles at sufficiently high frequencies, depending upon the the
ratio of the components of jet sheath velocity transverse and parallel to the
jet. Thus, multi-frequency RM maps promise a novel way in which to probe the
velocity structure of relativistic outflows.
