Electromagnetic Wave Power Observed Near the Moon during Terrestrial Bow Shock Crossings and Its Importance for Subsurface Sounding

Electromagnetic (EM) sounding is a geophysical method that is used to detect the subsurface properties of both the near-surface and deep interior of planetary bodies. In the magnetotelluric (MT) method, the orthogonal components of the horizontal electric and magnetic fields on the surface are used to discern the conductivity structure at depths of tens of meters to hundreds of kilometers. On the terrestrial planets, electromagnetic discharges (lightning) or magnetic field variations due to interactions with the solar wind can provide MT source signals. On airless bodies such as the Moon, solar wind turbulence and other plasma waves can create source signals. The Moon crosses the terrestrial bow shock twice during its orbit around Earth. During this transition, there are rapid, large amplitude variations in the ambient magnetic field as well as significant changes in the plasma environment. These abrupt changes in magnetic field strength produce strong source signals over a broad range of frequencies. Using Lunar Prospector data and new ARTEMIS data, we analyze the EM signal strength in the vicinity of the Moon during terrestrial bow shock crossings from frequencies of a few Hz down to several mHz. These frequencies can probe the internal structure of the Moon from the near surface down to a depth of over 1000 km. These source signal measurements can be used as a step toward understanding the depth and resolution that could be obtained from simple detectors on the surface of the Moon made by future missions.