A hemispherical electrostatic analyzer was used to extend descriptions of the particle (electron and positive ion) populations in the solar wind, magnetosheath, and magnetotail. Energy spectral analysis was accomplished by charging the plates to known voltage levels and allowing them to discharge with known RC time constants. The analyzer had four commandable modes. The first mode was designed for the measurement of solar wind protons and alpha particles. During eight spacecraft revolutions, 32-level energy spectra were obtained in eight angular ranges centered on the sun. The energy levels extended from 100 eV to 8 keV. The second mode was designed for the measurement of solar wind heavy ions. This cycle was the same as the first except that the energy per charge levels were limited to 900 eV to 8 keV, and the efficiency of counting heavy ions was increased relative to protons and alpha particles. The third mode was designed for the measurement of solar wind and magnetosheath electrons and magnetosheath positive ions. This was a combination cycle in which electron and positive ion spectral sweeps were alternated. During a cycle of nine spacecraft revolutions, eight electron spectra and eight positive ion spectra were obtained. The combined data for electrons in this mode consisted of 16-level energy spectra taken in 32 evenly spaced angular ranges. The spectra extended from 4 to 1000 eV. The data for positive ions consisted of 32-level spectra taken in the same 32 angular ranges. The energy per charge spectra extended from 100 eV to 8 keV. The fourth mode was designed for magnetotail electrons and positive ions. Electrons and positive ions were studied with 16-level spectra in 32 evenly-spaced angular ranges for both electrons and positive ions. The energy per charge ranges were 6 eV to 24 keV for electrons and 45 eV to 34 keV for positive ions. For further details see W. C. Feldman et al., J. Geophys. Res., v. 80, p. 4181, 1975.