The Low-Altitude Plasma Instrument (LAPI) provided high-resolution velocity space measurements of positive ions and electrons from 5 eV to 32 keV, and a monitor of electrons with energies above 35 keV. Pitch angle measurements covered the full 180 deg range. Data from this investigation and supporting measurements were used to study (1) the identification and intensities of Birkeland currents, (2) auroral particle source regions and acceleration mechanisms, (3) the existence and role of E parallel to B, (4) sources and effects of polar cap particle fluxes, (5) the transport of plasma within and through the magnetospheric cusp, (6) dynamic configurations of high-latitude flux tubes, (7) loss-cone effects of wave-particle interactions, (8) hot-cold plasma interactions, (9) ionospheric effects of particle precipitation, and (10) plasma convection at high altitudes. The instrument contained an array of 15 parabolic electrostatic analyzers of the ISIS 2 type, each with an electron channel and an ion channel, in order to obtain detailed pitch-angle distributions as a function of energy. Two Geiger-Mueller counters were mounted on the scan platform. The basic mode of operation provided a 32-point energy spectrum in the range 5 eV to 32 kev every second. The voltages on the electrostatic analyzers were programmable to allow for greater space/time resolution over limited portions of the energy and angular distributions. The instrument was mounted on a one-axis scan platform controlled by a magnetometer, whose purpose was to maintain the detector array, which spanned 180 deg, at a nearly constant angle to the magnetic field. Additional details are found in J. D. Winningham et al., Space Sci. Instrum., v. 5, n. 4, p. 465, 1981. From March 16, 1982 to April 4, 1982 the instrument was turned off for corrective action.