  We propose a theory to describe laminar ion sound structures in a
collisionless plasma. Reflection of a small fraction of the upstream ions
converts the well known ion acoustic soliton into a structure with a steep
potential gradient upstream and with downstream oscillations. The theory
provides a simple interpretation of results dating back more than forty years
but, more importantly, is shown to provide an explanation for recent
observations on laser produced plasmas relevant to inertial fusion and to ion
acceleration.
