Plasma and magnetic field data observed during two ISEE crossings of the plasma sheet boundary layer are studied in conjunction with a linear stability theory analysis. The plasma distribution functions exhibit three distinct potential sources of free energy: ion/ion relative drifts, electron/ion relative drifts corresponding to a nonzero field-aligned current, and large ion temperature anisotropies. These observed distributions are used as source functions in a linear Vlasov theory of electromagnetic instabilities at propagation parallel or antiparallel to the average magnetic field. The theory shows that the field-aligned current gives rise to the low frequency kink-like instability, and anisotropies of the flowing ion components give rise to two distinct branches of the ion cyclotron anisotropy instability with spacecraft frame frequencies both below and above the proton cyclotron frequency &OHgr;p. Analysis of the magnetic field data demonstrates that the appearance of two fast, counterstreaming, anisotropic ion components corresponds to a significant increase in the magnetic fluctuation energy density both below and above &OHgr;p, in full agreement with the theoretical predictions. ¿ American Geophysical Union 1989