Boundary between the near-Earth (<10RE) plasma sheet and outer plasma sheet: Equatorial observations at 9-15 RE by Geotail

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Equatorial observations of a boundary between the near-Earth (<10R_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, we find 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, we call this boundary near-Earth plasma sheet boundary (NEPS boundary)" and examine it in detail. We show that the NEPS boundary is located usually at distances of around 9-15R_E on the night side near the magnetospheric equator. We also discuss 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.
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