Boundary between the near-Earth (less than 10 R(sub E)) plasma sheet and outer plasma sheet: Equatorial observations at 9-15 R(sub E) by Geotail

Abstract

Equatorial observations of a boundary between the near-Earth (less than 10 R(sub E)) plasma sheet and outer plasma sheet in the magnetotail are reported in the present paper. Analyzing energy-time spectrograms of low energy particles obtained by the Geotail spacecraft, it is found that there often exists a boundary between the near-Earth plasma sheet and the tail plasma sheet. At this boundary, fluctuating particle spectra in the magnetotail change to stable spectra in the near-Earth region and energy flux of particles increases rapidly. Repeated observations of these signatures suggest that there often exists a spatial boundary between the two regions. In the present paper, this boundary is called 'near-Earth plasma sheet boundary (NEPS boundary)' and it is examined in detail. It is shown that the NEPS boundary is located usually at distances of around 9 - 15 R(sub E) on the night side near the magnetospheric equator. Also discussed is a new method of mapping, in which the NEPS boundary is used as a tracer to map auroral regions to the magnetosphere without using field-line models.