Simultaneous Oscillations of Low Energy Plasma and Magnetic Field Variations in the Lunar Wake

We have studied well-defined narrowband ultra-low frequency (ULF) waves observed in the lunar wake when the Moon was in the solar wind on August 24, 2008. The observed ULF waves oscillated at a frequency of 24 mHz, which is below the local proton cyclotron frequency (~38 mHz), with a strong wave power and at ~47 mHz with a weak wave power. The separated frequency peaks could be attributed to fundamental and second harmonics of the waves. We found that there was an entry of the solar wind protons into the lunar wake when the ULF wave was observed in the lunar wake. Such entry of the solar wind protons could be an energy source for the wave activity in the lunar wake. During the period of the ULF wave activity, the ion and electron flux oscillations were simultaneously detected in various energy channels. We found the energy dependence of the frequencies of the proton flux oscillations. Such a energy-dependent frequency is thought to be a consequence of the wave-particle resonant interaction. Unlike the protons, the electron flux exhibits spectral enhancements in the same frequency band of ULF waves without energy dependence. In this study, we investigate the wave properties and relationship between the magnetic field and plasma flux oscillations using Kaguya magnetic field and plasma particle data. We also discuss the source mechanisms of ULF waves in the lunar wake.