On the occurrence of magnetic enhancements caused by solar wind interaction with lunar crustal fields

We use Lunar Prospector data to identify 990 magnetic enhancements, previously termed "limb shocks'' or "limb compressions'', external to the lunar wake. We find that they are clearly associated with lunar crustal sources, and sometimes occur far upstream from the limb at altitudes of similar to 100 km. This is inconsistent with compressional disturbances convecting downstream with the solar wind, and implies that crustal fields are sometimes strong and coherent enough to produce a fluid-like interaction where compressional waves steepen to form a shock extending upstream from their source. The likelihood of observing enhancements partly depends on upstream solar wind conditions, with low proton gyroradius and low beta particularly favored. Low Mach numbers, implying a larger shock standoff distance, are also favored for observations which suggest shock-like behavior.