The average I and P profiles of 3C353's jets are modeled well
by the emission from a thick, constant-depth, boundary layer containing a
random magnetic field with no component 
perpendicular to
the jet flow. We associate this layer with a region where
is suppressed by velocity shear (presumably faster flow at the center
of the jet and slower flow at its surface). The roughly constant
layer depth and degree of linear polarization along the jet, plus the absence
of , imply that the radiating layer does not contain large-scale
turbulent eddies associated with entrainment of ambient
material (De Young 1996).
The model has a simple relation to the decelerating relativistic-jet
model for the first few kpc of FRI (plumed) sources
(Laing 1993, Laing 1996, Laing & Bridle, in preparation).
The field configuration (
) in the outer layer is
similar to that inferred by Laing & Bridle in the outer layer of
the rapidly-spreading jets in the FRI radio galaxy 3C31,
but in 3C353 the layer has constant depth and the jets spread slowly.
The apparently low emissivity of 3C353's jet spine
could also be due to Doppler "hiding'' of a faster (relativistic)
flow near the jet axis.
If parts of 3C353's jets move
relativistically, we have modeled their appearance in our rest frame.
Relativistic aberration modifies the angle of the fields to
the line of sight, so our best fit may correspond to to a different
ratio of 
to 
in the jet's frame, depending on the velocity field in the outer layer.
The modification will be within our uncertainties if the velocities
in the outer layer are not too high.
For example,
the parameter ranges that fit our data to within their
errors imply that we could fit the observed profiles with
a jet at 
to the sky, v < 0.5c and 
everywhere in the outer layer, and v > 0.8c in the spine (to produce
the Doppler hiding of this region).
The spine field geometry
is poorly constrained by our data, except that it cannot be dominated
by 
.
If 
everywhere in the outer layer, then Doppler
favoritism would also produce a 2:1 jet-counterjet intensity asymmetry.
As the observed counterjet in 3C353 is not just a faint replica of the
main jet, some intrinsic asymmetry must also be present. Parts, or all,
of the outer-layer flow may therefore have v < 0.5c.
A relativistic-jet scenario in which the fields in the jet's frame might differ substantially from our model is that of a high-velocity (v >~ 0.8c) outer-layer flow close enough to the plane of the sky that Doppler favoritism is small (to keep a 2:1 intensity ratio between the jets) but in which the aberration is large.