Particle and field data obtained by eight ISEE spacecraft experiments are used to define more precisely the characteristics of the high-latitude boundary region of the plasma sheet. We find that there is a region immediately adjacent to the high-latitude plasma sheet boundary whose particle and field characteristics are distinctly different from its neighbors, the lobe and the plasma sheet. This region supports intense ion flows, field-aligned currents, large amplitude electric fields, and enhanced broad band electrostatic noise. A detailed analysis of events detected on April 19, 1978, shows that the plasma distributions in the region are unstable. For instance, bidirectional field-aligned electron distributions are observed at nearly all energies (a few electron volts to a few hundred keV). Both the differential energy spectra and the reduced distribution function F(υ∥) = 2π∫υ⊥f(υ∥, υ⊥) dυ⊥ show significant peaks at 100-400 eV. These peaks comprise electrons coming from the earthward direction. Ions below a few keV are convected in the direction perpendicular to B, whereas the higher energy ions (>30 keV) are field-aligned and travelling mainly toward the earth. The electric field in the region is intense, spiky, and possibly time varying. The magnetic By component increases dramatically from a few nanoteslas to more than 20 nT, indicating presence of a field-aligned current. These features are in contrast with the plasma characteristics outside the region. In the adjacent regions (closer to the lobe and the inner plasma sheet regions), the electron distributions are nearly isotropic. Ion flow is virtually at a standstill. The high-energy ions (>30 keV) are anisotropic and field-aligned but bidirectional.