Microstructure of the auroral acceleration region as observed by FAST

The Fast Auroral Snapshot (FAST) explorer satellite was designed to investigate the microscale structure of the auroral acceleration region that was unresolved by previous satellites. This paper will present highlights from the first 2 years of the FAST mission and compare them with previous observations and auroral models. In particular, we find good agreement with the overall field-aligned current systems previously discovered; however, we present evidence that the downward currents are often carried by energetic upgoing electrons. These upgoing electrons are correlated with diverging electrostatic shocks, indicating that quasi-static parallel electric fields are responsible for their energization. Some of these field-aligned fluxes contain large-amplitude fast solitary waves which produce strong modulations of the electrons. Observations in inverted-V arcs show that the parallel acceleration region contains narrow (similar to 10 km) fingers of potential that extend along the magnetic field. On these narrow kilometer scales, ion beam energies are found to agree with the inferred potential determined from the perpendicular electric field or from the widening of the electron loss cone, implying acceleration is typically quasi-static on ion transit times. We also find evidence that both the ion and electron upgoing beams produced by the parallel electric fields have plateaued parallel distribution functions. Fast solitary waves are a prime candidate to stabilize the electron beams and may provide the resistance that allows the downward directed parallel electric fields to form in the highly conducting ionosphere. Intense ion cyclotron waves and ion solitary waves are often observed during ion beams, but the stabilizing waves have not been identified. In addition, intense electromagnetic ion cyclotron waves are also observed in inverted-V arcs, along with strongly modulated electron fluxes, indicating that turbulent acceleration is occurring in addition to simple acceleration by a static potential drop.