One-dimensional hybrid (particle ion, massless fluid electron) simulations of quasi-parallel shocks and two stream interactions are carried out in order to study the mechanism by which large-amplitude electromagnetic waves are generated at the shock and maintained in a quasi-steady manner. It is concluded that a resonant interaction at the interface beween the incoming ions and the heated downstream ions (''interface instability'') is the most likely source of the waves that ultimately comprise the quasi-parallel shock. The effect of the generation of these waves at the shock transition results locally in a nonsteady shock ramp, which propagates downstream with respect to the average shock position and is then replaced by a new steepened ramp at the original front position. This process is complicated by several competing effects. First, backstreaming ions excite the beam cyclotron resonant instability at wavelengths longer than those generated at the shock interface which gives rise to compressive upstream perturbations that are then carried into the shock and interact with the shock-generated waves.

Second, the re-formation process sometimes leads to the appearance of cold dense ion beams just upstream from the shock which gyrate in the upstream magnetic field to produce local density enhancements that have some effect on where the new shock front forms. Third, whistlers at wavelengths shorter than the waves associated with the re-formation process are also present that can scatter the backstreaming ions into a hotter, more diffuse population. In order to separate these various processes, numerical experiments involving quasi-parallel shocks and the interaction of two plasma streams have been carried out for various upstream parameters. In these studies the upstream conditions can be controlled to some extent by eliminating the backstreaming ions and suppressing the shorter wavelength modes, and in the case of the two stream interactions the upstream and downstream plasmas can also be distinguished. The interface instability that results from the interaction of the incident ion stream and a less dense population of heated downstream ions generates intermediate scale (kc/&ohgr;i~1) magnetosonic waves with group velocities pointing upstream, but phase fronts carried downstream, consistent with the waves observed in the shock simulations. A linear analysis indicates the wave numbers of the most unstable modes decrease with increasing upstream &thgr;Bn and Mach number, in agreement with the simulations. Areas where further work is needed are also discussed. ¿ American Geophysical Union 1990