A new particle population near the high‐latitude plasma sheet

We present observations of a new, intermediate‐energy particle population between the lobe and the plasma sheet. These were seen during two equatorial magnetosphere transits by the Wind spacecraft, when it was 15 RE distant from the Earth, south of the current sheet and near 0300 local time, and tended to overlap tail‐thinning episodes. The observed plasma bulk parameters exhibited a continuous transition from characteristics of the plasma sheet to those of the lobe, suggesting either a mixing region for particles from the two regions, or a transport layer where some processes condition the plasma as it moves from one region to the other. While temperature and density are typically shown to be anticorrelated throughout equatorial transits which span the plasma sheet and the magnetosheath, these two parameters were seen to be positively correlated during the intervals when the new population was observed. Particle distributions obtained by the 3‐D Plasma Experiment (3DP) electrostatic analyzers (0.08–27 keV ions; 0.02–28 keV electrons) during these periods often include a kidney‐shaped low‐energy ion component that appears like the conies commonly seen at low altitudes. These components indicate a likely transverse acceleration mechanism which can extend nearly out to the orbit of Wind.