FAST OBSERVATIONS IN THE DOWNWARD AURORAL CURRENT REGION - ENERGETIC UPGOING ELECTRON-BEAMS, PARALLEL POTENTIAL DROPS, AND ION HEATING

Observations of plasma particles and fields by the FAST satellite find evidence of acceleration of intense upgoing electron beams by quasi-s tatic parallel electric fields. The beam characteristics include a bro ad energy spectrum with peak energies between 100 eV and 5 keV, perpen dicular temperatures less than 1 eV, and fluxes greater than 10(9)/cm( 2)sec. Diverging electrostatic shocks associated with the beams have i ntegrated potentials that match the beam energy. These beams are found in regions of downward Birkeland current and account for the total fi eld-aligned current when they are present. The most energetic ion coni cs in the auroral zone are found coincident with these beams, in agree ment with the model for ''trapped'' conics. The measured particle dens ities of the electron beams and associated ion conics are approximatel y equal and typically range from 1 to 10 cm(-3), with no evidence for additional cold density. The beams are seen frequently at altitudes be tween 2000 and 4000 km in the winter auroral zone. Their probability o f occurrence has a strong dependence on season and altitude and is sim ilar to that for upgoing ion beams in the adjacent upward current regi ons. This similarity suggests that the density and scale height of ion ospheric ions play an important role in the formation of both types of beams.