With a new, orbit-averaged hybrid computer simulation code, we study a cold, fast low-density ion beam which propagates along the ambient magnetic field as it interacts with a much denser fluid background. We examine the character of the interactions as we vary the ion beam density relative to the background density over the range 1¿10-5 to 3¿10-3. The low beam density simulations may not be directly observable upstream of the Earth's bow shock, but they are included to help develop an understanding of the results seen in the simulations with high-beam density. However, our highest density simulation falls within the range of solar wind data. All the simulations, regardless of the relative beam density, show three distinct phases: (1) an early or ''linear'' phase; (2) an intermediate or ''trapping'' phase; and (3) a late or ''decorrelation'' phase. In the early phase, the beam excites a nearly monochromatic Alfv¿n wave whose amplitude grows exponentially at a rate given by linear perturbation theory. The wave amplitude saturates when the linear growth rate is of the order of the trapping frequency.

In the intermediate phase, the wave traps the beam, and the beam ions and the wave interchange energy at the trapped particle oscillation frequency. Views of particle phase space show that the trapped beam ions are also bunched in gyrophase. In the final phase, the beam ions decorrelate and fill the available phase space. When the Alfv¿n wave field is nearly monochromatic, as it is throughout each of the lower-density beam simulations, the beam ion phase space orbits are constrained by a constant of the motion related to the helical symmetry of the entire system. The particle orbits are parabolas, rather than the circles of constant energy predicted by quasi-linear theory. For these simulations, the final particle distribution superficially appears to be a drifting Maxwellian with a substantial thermal width, but in fact the distribution retains memory of its beamlike origin: the particles occupy a finite region of phase space bounded by two parabolas in (v⊥,v∥) space due to the constant of motion.

In the highest density simulation, there appears to be sufficient parallel pressure to drive a conventional, linearly polarized firehose mode locally unstable. The presence of this addition large-amplitude mode destroys the helical symmetry, and the particles scatter freely to form a hollow distribution in v⊥,v∥) phase space. ¿ American Geophysical Union 1989