  Strong downstream magnetic fields of order of $\sim 1$G, with large
correlation lengths, are believed to cause the large synchrotron emission at
the afterglow phase of gamma ray bursts (GRBs). Despite of the recent
theoretical efforts, models have failed to fully explain the amplification of
the magnetic field, particularly in a matter dominated scenario. We revisit the
problem by considering the synchrotron emission to occur at the expanding shock
front of a weakly magnetized relativistic jet over a magnetized surrounding
medium. Analytical estimates and a number of high resolution 2D relativistic
magneto-hydrodynamical (RMHD) simulations are provided. Jet opening angles of
$\theta = 0^{\circ} - 20^{\circ}$, and ambient to jet density ratios of
$10^{-4} - 10^2$ were considered. We found that most of the amplification is
due to compression of the ambient magnetic field at the contact discontinuity
between the reverse and forward shocks at the jet head, with substantial
pile-up of the magnetic field lines as the jet propagates sweeping the ambient
field lines. The pile-up is maximum for $\theta \rightarrow 0$, decreasing with
$\theta$, but larger than in the spherical blast problem. Values obtained for
certain models are able to explain the observed intensities. The maximum
correlation lengths found for such strong fields is of $l_{\rm corr} \leq
10^{14}$ cm, $2 - 6$ orders of magnitude larger than the found in previous
works.
