Narrowband Ion Cyclotron Waves at the Moon in the Terrestrial Magnetotail

Abstract:

Recent studies of the observations from the Apollo lunar surface magnetometers and the ARTEMIS spacecraft in the lunar orbit have found a class of narrowband ion cyclotron waves at the Moon. With frequencies of the order of 0.1 Hz, these narrowband waves are present only when the Moon is in the terrestrial magnetotail. The peak frequency is at or below the proton gyrofrequency, and the polarization is predominantly left-handed. These waves can propagate at large angles to the background magnetic field, suggesting that they are generated near the Moon and can reach the lunar surface before they are severely damped. We expect that these narrowband ion cyclotron waves result from the anisotropies of ion temperature in the vicinity of the Moon, such as those associated with pickup ions originating from the lunar exosphere or the absorption of ions at the Moon. The particle observations by the ARTEMIS spacecraft during a narrowband wave event indicated that most ions, especially those with lower velocities, that flowed to the Moon were absorbed, providing evidence of temperature anisotropy that could lead to ion cyclotron instability. Simultaneous observations at the Apollo 15 and 16 sites revealed small yet persistent differences in wave amplitude and phase, suggesting that the wave signals could be modified by the mini-magnetosphere above strong crustal magnetic field.