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The ISEE‐3 spacecraft has been transferred in 1982 into an earth orbit which brings the satellite close to the tailward Lagrangian point L 2 at ∼ 220 R E and thus allows exploration of the distant geomagnetic tail. We report in this letter initial analysis of energetic proton measurements > 30 keV from the Max‐Planck‐Institut/University of Maryland sensor system on ISEE‐3. We find that suprathermal protons are a persistent feature of the distant tail. Differential intensitites at 30 keV are essentially constant between the lunar distance and 220 R E and about one order of magnitude smaller than in the near earth (≲ 20 R E ) plasma sheet. Assuming that these protons are convected with the local plasma flow we are able to derive plasma velocities. During time periods where a comparison is possible, these velocities compare favourably well with the velocities derived from the Los Alamos National Lab. plasma analyzer onboard the same spacecraft. The appearance of the plasma sheet as evidenced by the suprathermal particles is rather bursty, anisotropies are large and predominantly tailward.
Scholer et al. (Thu,) studied this question.