7.1 Method

We have shown that an intrinsically symmetrical, decelerating relativistic jet model containing simple prescriptions for the velocity field and emissivity with a locally random but anisotropic magnetic field, accounts for the major features revealed by deep VLA imaging of the straight segments of the jet and counter-jet in 3C31. The principal new features of our approach are:

1. the use of three-dimensional (but axisymmetric) parameterized models of velocity, emissivity and field ordering;
2. rigorous calculation of synchrotron emission, including both relativistic aberration and anisotropy in the rest frame;
3. fitting to images with many independent data-points in linear polarization as well as total intensity using a robust optimization algorithm.