Solar Wind Interaction with Lunar Crustal Magnetic Fields

The Earth's Moon lacks a global magnetic field, and to first order behaves as a solid obstacle to the solar wind, resulting in a plasma void behind the Moon. Interesting plasma physics occurs as solar wind plasma refills the wake cavity, leading to a potential drop across the wake boundary and a system of diamagnetic currents on the boundary surface; however, none of these processes affect the environment upstream of the Moon. The presence of remanent crustal magnetism on the lunar surface significantly affects this picture. Magnetic perturbations were detected near the lunar limb by Explorer 35 and the Apollo subsatellites, suggesting plasma compression or a "limb shock". A number of mechanisms for producing a source of backpressure capable of affecting the solar wind were initially proposed, including local increases in surface conductivity and photoelectrons. Eventually an association with crustal sources was convincingly demonstrated, though. We now present observations of over 1200 lunar limb shocks from 3813 Lunar Prospector orbits. We observe magnetic enhancements by factors as large as 2-3 over solar wind fields, far too large to be explained by uncompressed crustal fields at spacecraft altitude. Some of these magnetic enhancements are located far enough upstream from the pressure balance obstacle created by the crustal fields to require the presence of a shock. The presence of electron energization and whistler wave activity also supports this conclusion. Other enhancements, located near the limbs, could be explained by shocks or merely plasma compression. We characterize lunar limb shock occurrence as a function of surface geographic location and crustal magnetism, solar zenith angle, and upstream solar wind conditions determined by WIND and propagated to lunar orbit. The likelihood of observing a limb shock clearly depends on all of these variables, suggesting that the lunar environment is such that a fluid shock-like interaction can only take place for certain solar wind conditions and over certain magnetic anomalies.