Whistler waves observed near lunar crustal magnetic sources

We present Lunar Prospector Magnetometer ( LP MAG) observations of monochromatic circularly polarized low frequency waves at frequencies of 0.4 - 4 Hz near the Moon. We observe such waves on about 6.6% of LP orbits in the solar wind, outside of the lunar wake. About a third of them occur on orbits when we also observe external magnetic enhancements ("limb shocks''), and most are clearly associated with lunar crustal magnetic sources, indicating an origin related to solar wind interaction with lunar magnetic fields. The inferred propagation vectors and polarizations of these waves are consistent with the expected characteristics of whistler-mode waves. These observations could indicate either upstream whistler waves produced at shock surfaces above lunar crustal magnetic sources, or phase-standing whistler wakes formed by direct solar wind interaction with lunar crustal magnetic fields.