ISEE 1 and 2 particle observations of outer plasma sheet boundary

Observations of particle structures by a medium‐energy particle experiment on ISEE 1 and 2 spacecraft at ∼20‐ R E distance in the geomagnetic tail are presented. Comparison of our data with plasma data indicates the existence of a layer of energetic electrons and ions just outside the plasma sheet. The region outside the plasma sheet in the high‐latitude lobe is permeated with low‐intensity, low‐energy (∼1.5 keV) ions. These particle structures have velocities of a few kilometers per second to ≳60 km/s. Large‐scale motions are observed with onset and recovery of substorms. The dimensions of the particle structures are estimated to range from ∼200 to 10,000 km at 20 R E . The particle structures are dependent upon the energy as well as on the species of the particles.