  Afterglow jets are Rayleigh-Taylor unstable and therefore turbulent during
the early part of their deceleration. There are also several processes which
actively cool the jet. In this letter, we demonstrate that if cooling
significantly increases the compressibility of the flow, the turbulence
collides with the forward shock, destabilizing and corrugating it. In this
case, the forward shock is turbulent enough to produce the magnetic fields
responsible for synchrotron emission via small scale turbulent dynamo. We
calculate light curves assuming the magnetic field is in energy equipartition
with the turbulent kinetic energy and discover that dynamic magnetic fields are
well-approximated by a constant magnetic-to- thermal energy ratio of 1%, though
there is a sizeable delay in the time of peak flux as the magnetic field turns
on only after the turbulence has activated. The reverse shock is found to be
significantly more magnetized than the forward shock, with a
magnetic-to-thermal energy ratio of order 10%. This work motivates future
Rayleigh-Taylor calculations using more physical cooling models.
