Field-aligned electron beams observed simultaneously with upflowing ion beams in the auroral acceleration region

Upgoing, counterstreaming, and downgoing field-aligned electron beams obser ved simultaneously with upward ion beams in the auroral acceleration region on Akebono (Exos-D) satellite are studied statistically. These electron be ams are narrowly collimated within the local loss cone, generally have lowe r energies per charge than the potential drop above the satellite, and are associated only with low-energy (<2 keV) ion beams. Their characteristics i ndicate that electrons of ionospheric origin are accelerated upward below t he parallel auroral acceleration region (the region of upward electrostatic field) and are injected into the peripheral region of the parallel potenti al structure along the magnetic field line. Upflowing electron beams enteri ng the bottom of the auroral acceleration region evolve into upgoing, count erstreaming, and possibly downgoing electrons, depending on the potential s tructure of the acceleration region. When the parallel potential drop is la rge enough to reflect the upflowing electrons back immediately within a sho rt distance, then the electrons come down on the same field line and appear as counterstreaming electron beams. The upward and downward components of observed counterstreaming electron beams have the same energy and flux on a verage. Their occurrence decreases with increasing altitude. Upgoing electr on beams are observed when the potential drop above the satellite is almost absent. Their occurrence shows no significant dependence on altitude, sugg esting the penetration up to the top of parallel potential structure. A fra ction of the downgoing electron beams are possibly caused by upflowing elec tron beams drifting to the adjacent magnetic field line being reflected bac k by the potential drop above the satellite. Some characteristics of downgo ing electron beams, however, are different from those of upgoing and counte rstreaming electron beams and suggest an alternative path of entering the p arallel acceleration region.