  We present the first radio emission simulations from high resolution three
dimensional relativistic hydrodynamic jets, which allow for a study of the
observational implications of the interaction between the jet and external
medium. This interaction gives rise to a stratification of the jet where a fast
spine is surrounded by a slow high energy shear layer. The stratification, and
in particular the large specific internal energy and slow flow in the shear
layer largely determines the emission from the jet. If the magnetic field in
the shear layer becomes helical (e.g., resulting from an initial toroidal field
and an aligned field component generated by shear) the emission shows a cross
section asymmetry, in which either the top or the bottom of the jet dominates
the emission. This, as well as limb or spine brightening, is a function of the
viewing angle and flow velocity, and the top/bottom jet emission predominance
can be reversed if the jet changes direction with respect to the observer, or
presents a change in velocity. The asymmetry is more prominent in the polarized
flux, because of field cancellation (or amplification) along the line of sight.
Recent observations of jet cross section emission asymmetries in the blazar
1055+018 can be explained assuming the existence of a shear layer with a
helical magnetic field.
