  The role of magnetic fields in gamma-ray burst (GRB) flows remains
controversial. The study of the early afterglow phases and, in particular, of
the reverse shock dynamics and associated emission offers a promising probe of
the magnetization of the ejecta. In this paper, we derive the conditions for
the existence of a reverse shock in arbitrarily magnetized ejecta that
decelerate and interact with the circumburst medium. Both constant and
wind-like density profiles are considered. We show, in contrast to previous
estimates, that ejecta with magnetization larger than unity are not crossed by
a reverse shock for a large fraction of the parameter space relevant to GRB
flows. Allowing for shell spreading, there is always a relativistic or mildly
relativistic reverse shock forming in sigma_o < 0.3 ejecta. From this, we
conclude that the paucity of optical flashes, believed to be a distinctive
signature of a reverse shock, may be explained by the existence of dynamically
important magnetic fields in the ejecta.
