LATITUDINAL STRUCTURES OF NIGHTSIDE FIELD-ALIGNED CURRENTS AND THEIR RELATIONSHIPS TO THE PLASMA SHEET REGIONS

Using the magnetic field and low-energy particle data from the Akebono (EXOS D) satellite, the latitudinal structures of field-aligned currents and auroral particles in the nightside auroral oval region have been studied in detail for 12 northern hemisphere passes in the 20-04 MLT sector and at altitudes of 5000-10,000 km. It is found that the nightside field-aligned current system consists of the latitudinally narrow (approximately 1-degrees) current system located at the poleward boundary of the auroral oval and the latitudinally wide current system which occupies the main portion of the oval. The former has been designated as ''the boundary current system'' and the latter as ''the central current system.'' Both current systems are suggested to be located in the closed field line region, since the polar rain is observed just poleward of the boundary current system. The boundary current system is suggested to be independent of the traditional region 1 current system associated with inverted V electron precipitation, since suprathermal electrons with field-aligned pitch angle distributions are found to be the main charge carrier of this current system. Another important finding is that energy-dispersed ion precipitation events, which are interpreted to be the result of the E x E drift of ions flowing earthward in the plasma sheet boundary layer, appear in this boundary current region. The central current system consists of the two regions, the inverted V electron precipitation region on the poleward side and the unstructured electron and ion precipitation region on the equatorward side. It is suggested therefore that this central current system corresponds to the traditional region 1 and region 2 current system in the nightside sector. It is further found that the diffuse band of plasma sheet kilovolt electrons is continuously observed form the polar cap boundary defined as the polar rain terminator to the equatorward boundary of the unstructured precipitation region through the inverted V electron precipitation region. Consequently, inverted V structures and diffuse plasma sheet electron precipitation always coexist. From these spatial relationships, it is strongly suggested that the boundary current system is mapped to the plasma sheet boundary layer, while the central current system is mapped to the main or central plasma sheet.