Ion temperature measurements from the Akebono suprathermal mass spectrometer: Application to the polar wind

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We present a method for estimating the thermal ion drift velocity, ion temperature, and spacecraft potential in the polar ionosphere from data acquired with the suprathermal mass spectrometer (SMS) on the Akebono satellite. The method is based on fitting the spin angle distributions of the observed ion flux for a number of retarding potential analyzer (RPA) settings to a drifting Maxwellian distribution. A nonlinear fitting procedure is used to relate the observed fluxes to the plasma parameters. The spacecraft potential is taken into account by means of the thin‐sheath approximation. The analysis is applicable to the “thermal” (total energy few eV) polar wind ion population. A study of a number of representative passes at various altitudes, latitudes, and local times indicates that all major ions (H+, He+, and O+) have low temperatures, in the range of 0.05–0.35 eV, with little temperature dependence on altitude, longitude, or latitude. The velocity estimates confirm the previous analysis using the moment method: all ions have a significant upward velocity component (antiparallel to the magnetic field, in the northern hemisphere) which increases with altitude. The velocity estimates from data at the higher RPA settings are sometimes higher than those at low RPA settings. This indicates that the actual ion distributions are not Maxwellian, perhaps due to a higher‐energy tail component drifting at higher velocity. All ions are in general supersonic at high altitudes.