  The dynamics of a standing shock front in a Poynting-flux dominated
relativistic flow is investigated by using a one-dimensional, relativistic,
two-fluid simulation. An upstream flow containing a circularly polarized,
sinusoidal magnetic shear wave is considered, mimicking a wave driven by an
obliquely rotating pulsar. It is demonstrated that this wave is converted into
large amplitude electromagnetic waves with superluminal phase speeds by
interacting with the shock when the shock-frame frequency of the wave exceeds
the proper plasma frequency. The superluminal waves propagate in the upstream,
modify the shock structure substantially, and form a well-developed precursor
region ahead of a subshock. Dissipation of Poynting flux occurs in the
precursor as well as in the downstream region through a parametric instability
driven by the superluminal waves. The Poynting flux remaining in the downstream
region is carried entirely by the superluminal waves. The downstream plasma is
therefore an essentially unmagnetized, relativistically hot plasma with a
non-relativistic flow speed, as suggested by observations of pulsar wind
nebulae.
