A statistical study of ®eld-aligned electron beams associated with ion conics events

A statistical study of ®eld-aligned electron beams associated with up ̄owing ion conics is presented from Exos-D (Akebono) observations below 10 000 km. The electron beams are narrowly collimated along the ®eld line and generally have energies of several tens of eV. They are divided in the analysis into three types: up ̄owing, down ̄owing, and counterstreaming. All the types of electron beams are almost equivalent in their energy and pitch angle characteristics and their association rate with up ̄owing ion events. About 50% of ion conics are found to be coincident with ®eld-aligned electron beams. There is also a correlation in energy between the ®eld-aligned electrons and ion conics. These show that the association is not a mere coincidence but rather that the ®eld-aligned acceleration of electrons is related to the perpendicular energization of ions. The association rate of up ̄owing electrons is high on the nightside, while that of down ̄owing electrons is high on the dayside. The association rate of down ̄owing electrons is high at low altitudes, and the rates of the three types of electron beams become equivalent at high altitudes. Data indicate that the height of the electron acceleration region is lower on the nightside. It is suggested that the average height of the electron acceleration region is around the satellite apogee (ÿ10000 km), and the average thickness of the region is about thousands km.