FAST SATELLITE-OBSERVATIONS OF ELECTRIC-FIELD STRUCTURES IN THE AURORAL-ZONE

Electric field and energetic particle observations by the Fast Auroral Snapshot (FAST) satellite provide convincing evidence of particle acc eleration by quasi-static, magnetic-field-aligned (parallel) electric fields in both the upward and downward current regions of the auroral zone. We demonstrate this by comparing the inferred parallel potential s of electrostatic shocks with particle energies. We also report nonli near electric field structures which may play a role in supporting par allel electric fields. These structures include large-amplitude ion cy clotron waves in the upward current region, and intense, spiky electri c fields in the downward current region. The observed structures had s ubstantial parallel components and correlative electron flux modulatio ns. Observations of parallel electric fields in two distinct plasmas s uggest that parallel electric fields may be a fundamental particle acc eleration mechanism in astrophysical plasmas.