Evidence for detection of energetic neutral atoms by LADEE

The integrated current of the Lunar Dust Experiment (LDEX) instrument aboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) includes sources of current in addition to small (<0.3μm) dust grains. LDEX current correlates strongly with solar wind density and flux, though solar wind ions should be excluded from the signal. Energetic neutral atoms (ENAs), produced by solar wind that is neutralized and reflected from the moon's surface, might contribute to LDEX current. To investigate this possibility, we developed a geometrical model for ENA detection. We find high correlation between predicted ENA flux and LDEX current. Based on the inverse dependence of ENA detection sensitivity on solar wind speed, we exclude sputtering from the spherical surface inside the instrument as the mechanism for ENA detection. Additionally, we observe a drop in LDEX current that is consistent with a drop in the predicted ENA flux associated with a lunar magnetic anomaly (LMA) centered at -178°E and 6°N selenographic longitude and latitude, respectively. This provides additional confirmation that ENAs are the main contributor to the LDEX current rather than ions, because the ENA flux decreases in LMA regions and reflected hydrogen ion flux increases in LMA regions.