The ISEE 1 and ISEE 2 observations of the lobe/mantle and of the plasma sheet region, at distances between 10 and 20 Re downtail during 1978 and 1979, have been examined. Cold, tailward flowing ionospheric O+ streams (at energies between 50 eV/q and 5 keV/q), observed during geomagnetically disturbed periods, have been statistically analyzed. At the crossing between the lobe/mantle region and the plasma sheet the characteristics of the streams change significantly. These changes suggest the action of energization and isotropization processes which accelerate the ionospheric ions and make them a part of the plasma sheet plasma. The region where these processes are observed is assumed here to be the plasma sheet boundary layer. It is shown that the stream flow pattern in the lobe/mantle region is in good agreement with the tail lobe ion spectrometer model for the thermal speed distribution as well as for the flow velocity and density distributions. This agreement also holds in a qualitative sense in the plasma sheet boundary layer, assuming the appropriate electric field configuration; the E¿B drift direction along the YGSE axis appears to reverse with respect to what is observed in the lobe/mantle region, in agreement with the assumed reversal of the Z component of the dc electric field at the plasma sheet boundary. ¿ American Geophysical Union 1990