Ion distributions and flows near the neutral sheet

In this paper we present a survey of the three‐dimensional ion and electron distributions measured on AMPTE IRM near the neutral sheet in the near‐Earth magnetotail (R ≈ 10–19 R E ). The data are classified according to substorm phases. Individual case studies of velocity distributions reveal that the ions in the neutral sheet consist of a single population. Fast and slow flows both occurred during the two extremes of magnetospheric activity, the substorm expansion phase and quiet times. Ion beams are occasionally observed, and their shapes in velocity space are similar to those observed in the plasma sheet boundary layer. A ring distribution is seen only twice in 50 events. The occurrence rate of fast flows is positively correlated with the global magnetic field elevation during disturbed times and anticorrelated with that elevation during quiet times. The global magnetic field elevation angle is related to the magnetic field configuration in the magnetotail. Therefore our results indicate a dependence of the flow speed within the neutral sheet on the magnetotail configuration at the point of observation.