Electron beam generated solitary structures in a nonuniform plasma system

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.