Observations of narrowband ion cyclotron waves on the surface of the Moon in the terrestrial magnetotail

By examining the data collected by the Apollo 15 and 16 Lunar Surface Magnetometers between April and July of 1972, we have found a class of narrowband waves observed at the surface of the Moon. With frequencies ranging from 0.04 to 0.17 Hz, these narrowband waves were present only when the Moon was in the terrestrial magnetotail, and the wave properties are consistent with those of ion cyclotron waves. We propose two scenarios where anisotropies of ion temperature can occur in the vicinity of the Moon to excite the ion cyclotron waves observed at the lunar surface. Simultaneous observations at the Apollo 15 and 16 sites revealed small but persistent differences in wave amplitude and phase, suggesting that the wave signals were modified by the mini-magnetosphere above the Apollo 16 site. It is also suggested that subsurface electrical conductivity may possibly be involved in the amplitude change and small phase difference between the two sites.