ULF Waves Observed by Lunar Prospector

We have studied the spatial distribution of ultra-low frequency (ULF) in the vicinity of the Moon using Lunar Prospector (LP) magnetometer data obtained at similar to 100 km altitude during the period from February 20, 1998 to November 30, 1998. The lunar ULF waves are mostly identified over a broad frequency range of 10-60 mHz in the spacecraft frame. The occurrence rate of the ULF wave is high on the dayside above strong magnetic anomalies and depends on the interplanetary magnetic field orientation. We examine wave properties including the wave propagation direction and polarization. They are very similar to Kaguya observations, indicating that the ULF waves observed at LP and Kaguya are generated by the same mechanism. Although lunar ULF waves are commonly observed on the dayside above lunar magnetic anomalies, how they are established as a regular oscillation is not completely understood. In the present study, we suggest that the electromagnetic ion beam instability, driven by protons reflected from lunar magnetic anomalies, is the likely source of ULF waves observed at LP.