This region was defined initially by the more rapid
spreading of its outer isophotes. Our modelling shows it to be a
region in which several dramatic changes in the other jet characteristics
occur together:
- The jets decelerate rapidly to an on-axis velocity of 0.55
after an initial slow decline from 0.77.
- They maintain a transverse velocity profile in which the edge
velocity drops to approximately 70% of the on-axis value.
- The intrinsic emissivity increases abruptly at the boundary
with the inner region, then declines with distance
from the nucleus,
, as 
in the shear layer and 
in
the spine.
- The emissivity at the edges of the jet drops to about 20% of
that on the jet axis.
- The radial component of the magnetic field in the shear layer
becomes significant, rising from zero at the spine boundary to 90% of the
toroidal and longitudinal components at the outer edge of the layer,
i.e. the field is essentially isotropic at the outer boundary of the
shear layer in this region.
- The ratio of longitudinal to toroidal field strength decreases slightly
from about 1.1 to 0.8, independent of radius in the jet.
The sudden increase in rest-frame emissivity at the flaring point suggests
that there is a discontinuity in the flow, perhaps a stationary
reconfinement shock system. The brightness and polarization structure in
this region cannot be described by a simple adiabatic model. The
transverse velocity profile and the growth of the radial field component
strongly suggest that entrainment across the jet boundary becomes
important.
