  In models for gamma ray burst afterglows, it is normally assumed that the
external shock strongly amplifies the magnetic field and that this field
maintains a steady value throughout the shocked region. We discuss the effects
of modifying this (probably simplistic) assumption by allowing for a short
decay time. The observations are incompatible with a post-shock field that
decays too rapidly. However if the field pervades only a few percent of the
total thickness of the shocked shell (and the electrons undergo only inverse
Compton losses in the remainder) the model could be compatible with data. This
would suggest a strong dependence of model parameters on the uncertainties of
shock physics, therefore calling for independent external density estimates. We
claim that afterglow emission should be instead seen as a laboratory where we
can understand relativistic shock physics. The model we propose here together
with afterglow observations in all wavebands could help to pin down the field
structure.
