Ion Cyclotron Waves on Lunar Surface: Apollo Observations and Implications for Future Lunar Missions

Our previous study of the restored Apollo Lunar Surface Magnetometer (LSM) data discovered that narrowband ion cyclotron waves were often observed at the Apollo 15 and 16 landing sites when the Moon was in the Earth’s magnetotail (Chi et al., 2013). Two mechanisms have been proposed to explain the excitation of ion cyclotron waves at the Moon: the absorption of ions at the lunar surface and the pickup ions from the lunar exosphere. Either process can lead to an ion velocity distribution unstable to ion cyclotron instability, but it is of particular interest to investigate which ion cyclotron waves are associated with the latter mechanism so that the observations of them can provide hints to the type and the number of pickup ions escaped from the lunar exosphere. More recently, Nakagawa et al. (2018) examined the Kaguya data and found similar ion cyclotron waves in the Earth’s magnetotail but at a very low occurrence rate.

In this study, we perform statistical analysis on the full set of the restored LSM data, including those from the Apollo 12, 15, and 16 missions between 1969 and 1975, that were only partially available to our previous study. We find that the ion cyclotron waves were observed by Apollo 15 LSM approximately 5% of the time, which is about six times more frequently than that found in Kaguya observations. A slightly lower occurrence rate of ion cyclotron waves is found in the Apollo 16 LSM data because of the strong local crustal magnetic field at the Apollo 16 site and the conservation of the Poynting flux. Future joint measurements by lunar landers and orbiters can enable a true comparison of the ion cyclotron waves on the lunar surface and at different altitudes of the exosphere.