Lunar Terminator Enhancement (LTE) of Magnetic Field and Its Asymmetry

The magnetic field data from Lunar Prospector are investigated in a new coordinate system related to the solar wind convection electric field . The data shows that at the low altitude (<45 km) the field magnitude enhances within a belt in front of the lunar terminator . This enhancement exhibits a strong left-right asymmetry in this coordinate system. Near the equator, the field enhancement at the flank that the solar wind convection electric field points away from is much stronger than that at the opposite flank , and its location shifts towards the Sun (~60°). At the polar region, however, this asymmetry is not seen clearly. Field reductions in the expansion region of the lunar wake are seen at both flanks behind the lunar terminator. The same left-right asymmetry exists and the reduction behind the right terminator is much more remarkable than that behind the left terminator. At the high altitude (85~115 km), the magnetic field around the terminator also enhances. This enhancement is weaker than that at the low altitude and is basically symmetric about the Sun-Moon line. Particle simulations indicate that these terminator field enhancements are the pickup effect of the reflected solar wind particles and that they might be independent to the lunar crustal fields.