ULF waves observed near the Moon by Lunar Prospector: Occurrence rate and its IMF dependence

We have studied the statistical properties of ultra-low frequency (ULF) waves using Lunar Prospector (LP) magnetometer data obtained at ~100-km altitude from the lunar surface during ~9-month period from 20 February 1998 to 31 December 1998. To avoid wave activities originating from the Earth's bow shock, the LP magnetic field data were used when the Moon was in the solar wind (X_GSE > 0). The ULF waves at LP orbit are mostly identified in the frequency band of ~10-60 mHz. It is found that the occurrence rate of ULF waves was high in the region of the solar zenith angle less than 90° and above the strong magnetic anomalies on the dayside. We also observed a pronounced morning-afternoon asymmetry in the wave occurrence rate. The occurrence rate is much higher in the prenoon sector than in the postnoon sector when the interplanetary magnetic field (IMF) is directed along the Parker spiral. However, the high occurrence rate in the prenoon sector significantly decreases for the interval of an ortho-Parker spiral IMF orientation. These observations indicate that the ULF waves observed at LP are driven by solar wind interaction with lunar magnetic anomalies.