  (Abridged) We aim to quantitatively understand the dynamical effect and
observational signatures of magnetization of the GRB ejecta on the onset of the
afterglow. We perform ultrahigh-resolution one-dimensional relativistic MHD
simulations of the interaction of a radially expanding, magnetized ejecta with
the interstellar medium. The need of ultrahigh numerical resolution derives
from the extreme jump conditions in the region of interaction between the
ejecta and the circumburst medium. We study the evolution of an
ultrarelativistic shell all the way to a the self-similar asymptotic phase. Our
simulations show that the complete evolution can be characterized in terms of
two parameters, namely, the \xi parameter introduced by Sari & Piran (1995) and
the magnetization \sigma_0. We exploit this property by producing numerical
models where the shell Lorentz factor is \gamma_0 ~ tens and rescaling the
results to arbitrarily large \gamma_0. We find that the reverse shock is
typically very weak or absent for ejecta characterized by \sigma_0 >~ 1. The
onset of the forward shock emission is strongly affected by the magnetization.
On the other hand, the magnetic energy of the shell is transfered to the
external medium on a short timescale (~several times the duration of the
burst). The later forward shock emission does not contain information for the
initial magnetization of the flow. The asymptotic evolution of strongly
magnetized shells, after they have suffred a substantial deceleration,
resembles that of hydrodynamic shells, i.e., they fully enter in the
Blandford-McKee self-similar regime.
