The instrument has detected superthermal, energetic or fast ions whose energy exceeds 36 eV in the spacecraft frame of reference. These ions were observed in early orbits during measurements of the neutral density near periapsis, have an erratic and unpredictable signature, and occur at too high an altitude to be due to the neutral atmosphere. When the altitude of periapsis increased above the point where sensible neutral density measurements could be made, the instrument was configured specifically to detect superthermal ions. In general, for orbit numbers 1 to 645, data were taken from the RPA mode. The gas background signal with the filament on is about a factor of 10 less in this mode than in non-RPA mode, resulting in a lower detection threshold. For orbit numbers above 923, the instrument was deliberately configured with the filament off and non-RPA mode data was used. For mass 16 the RPA voltage is about +3.8 volts.