An implicit-particle simulation of the collisionless parallel shock created at the interface between an injected beam and a stationary plasma is performed in one-dimensional geometry. The solar wind plasma, which consists of ions and electrons, is injected into a stationary dense plasma that corresponds to the planetary ionosphere. Electromagnetic waves with right-hand circular polarization that propagate upstream (R- waves) are generated at the interface of the two plasmas, which decelerate the solar wind to form a shock. The shock transition region is not monotonic but consists of two distinct regions, a pedestal and a shock ramp. The transition region, which contains the ionopause, is a few thousand electron skin depths long. The parallel shock varies in time and periodically collapses and re-forms. The right-hand circularly polarized electromagnetic waves that propagate downstream (R+ waves) are excited at the shock ramp. Nonlinear wave-particle interaction between the solar wind and the R+ waves causes wave condensation and density modulation. These R+ waves may be sweeping away the downstream plasma to suppress its thermal diffusion across the shock. The electrons at the shock ramp exhibit a flat-topped velocity distribution along the magnetic field owing to the ion acoustic-like electrostatic waves. ¿ American Geophysical Union 1996