LADEE/LDEX observations of lunar pickup ion distribution and variability

We report fortuitous observations of low-energy lunar pickup ion fluxes near the Moon while in the solar wind by the Lunar Dust Experiment (LDEX) on board the Lunar Atmosphere and Dust Environment Explorer (LADEE). We describe the method of observation and the empirical calibration of the instrument for ion observations. LDEX observes several trends in the exospheric ion production rate, including a scale height of approximately 100 km, a positive, linear correlation with solar wind flux, and evidence of a slight enhancement near 7-8 h local time. We compare the LDEX observations to both LADEE Neutral Mass Spectrometer ion mode observations and theoretical models. The LDEX data are best fit by total exospheric ion production rates of ≈6 × 10³ m^-3 s^-1 with dominant contributions from Al⁺, CO⁺, and Ar⁺, although the LDEX data suggest that the aluminum neutral density and corresponding ion production rate are lower than predicted by recent models.