Recent observations by the FAST satellite have identified numerous occurrences in the downward auroral current region of fast solitary structures with a characteristic size of a few local Debye lengths. We utilize a 2.5D electrostatic particle-in-cell code to investigate the generation of such structures by the interaction of narrow energetic electron beams with a gravitationally-bound exponential atmosphere. Cold, narrow electron beams are injected from both the high-altitude and low-altitude edges of the simulation domain. Both upward and downward-going beams experience a strong beam-plasma instability yielding net positive potential structures. These structures, seen to be holes in electron phase space, then accelerate or decelerate along the density gradient. For the up-going beams, the potential structures escape from the initial region of intense interaction, after which they propagate surprisingly far. The down-going structures accelerate but become degraded and break up due to the finer scales present in the increased density region. The gradient-dependent acceleration therefore selects upward-going structures as likely to be more robust in the auroral region. ¿ 2000 American Geophysical Union