The Porcupine sounding rocket experiment included a spinning daughter payload equipped with a xenon plasma gun that emitted an ion beam nearly transverse to the magnetic field. Wave instruments on the main payload detected electrostatic waves at hydrogen gyroharmonics and at the lower hybrid frequency within the beam. We have performed a series of computer simulations to investigate the behavior of a system including a xenon beam, modeled from the rocket experiment, and a background ionosphere plasma of oxygen plus a small concentration of hydrogen. In these one-dimensional simulations, a spatially homogeneous xenon beam is injected perpendicular to the magnetic field and at various angles with respect to the wave propagation vectors. Waves are excited over a range of these angles with a maximum growth near 00, when the xenon beam is in the wave propagation direction. Despite the small hydrogen concentration (~1%), the wave spectral analysis shows narrow peaks near the hydrogen gyrofrequenies at low beam densities, which disappear at high beam densities when the spetrum is dominated by the lower hybrid peak. A similar transition in the spectrum was seen on the rocket flight due to beam spreading with increasing distance from the xenon gun. Theoretical interpretation is given in terms of a transition from a resonant instability at low beam densities to a nonresonant instability at high beam densities.