Quasi-static, magnetic-field-aligned (parallel) potentials have been considered the primary source of charged particle acceleration in the aurora where precipitating electrons create a visible display. This finding has been controversial since, at one time, it was widely believed that parallel potentials could not be supported by a collisionless plasma. We present observations from the fast auroral snapshot (FAST) satellite which strongly support this acceleration mechanism and, moreover, show evidence of a second plasma regime region which supports quasi-static parallel potentials. The uncovering of parallel potentials in two plasma regimes suggests that they may be fundamental in astrophysical plasmas, supplementing the classical mechanisms of Fermi and betatron acceleration. We summarize the observations that demonstrate this acceleration mechanism. We also summarize evidence of Debye-scale plasma structures which are associated with these parallel potentials. These small-scale structures appear to be three-dimensional electron phase space holes, a new type of plasma structure.
Electron trapping and acceleration by drift electric fields in plasma two-stream instability