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

Abstract

Upgoing, counterstreaming, and downgoing field‐aligned electron beams observed simultaneously with upward ion beams in the auroral acceleration region on Akebono (Exos‐D) satellite are studied statistically. These electron beams are narrowly collimated within the local loss cone, generally have lower energies per charge than the potential drop above the satellite, and are associated only with low‐energy (<2 keV) ion beams. Their characteristics indicate that electrons of ionospheric origin are accelerated upward below the parallel auroral acceleration region (the region of upward electrostatic field) and are injected into the peripheral region of the parallel potential structure along the magnetic field line. Upflowing electron beams entering the bottom of the auroral acceleration region evolve into upgoing, counterstreaming, and possibly downgoing electrons, depending on the potential structure of the acceleration region. When the parallel potential drop is large enough to reflect the upflowing electrons back immediately within a short 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 average. Their occurrence decreases with increasing altitude. Upgoing electron beams are observed when the potential drop above the satellite is almost absent. Their occurrence shows no significant dependence on altitude, suggesting the penetration up to the top of parallel potential structure. A fraction of the downgoing electron beams are possibly caused by upflowing electron beams drifting to the adjacent magnetic field line being reflected back by the potential drop above the satellite. Some characteristics of downgoing electron beams, however, are different from those of upgoing and counterstreaming electron beams and suggest an alternative path of entering the parallel acceleration region.