The electric potential of the lunar surface

The sunlit lunar surface potential has been determined experimentally using 2 months of data provided by the Apollo Lunar Surface Experiment Package (ALSEP) Suprathermal Ion Detector Experiment (SIDE) deployed at the Apollo 14 and 15 sites. The magnitude of the lunar surface potential was found to vary as the moon passes through the different plasma environments, such as the solar wind, magnetosheath and the earth's geomagnetic tail, in its orbit around the earth. In the dayside solar wind and/or the magnetosheath the potential is approximately +1 volts for the solar zenith angles less than 4°. At higher zenith angles, about 5°, an asymmetry exists where by the potential rises to +18 volts in the dawn side, and drops to +17 volts in the dusk side. In the geomagnetic tail, the data indicate potentials of +1 volts or greater are common in some regions in the tail. This result probably applies to the higher plasma density regions of the plasma sheet and boundary layer. Computation of the dayside surface potential using recently measured values of the magnetosheath parameters and photoelectron flux yield approximately +11 volts potential, which agrees with our observational results. As a result of the positive potential on the dayside of the moon, an electron plasma sheet is formed adjacent to the surface to shield the net positive charge on the lunar surface. The electric field is calculated to be approximately > 1 V/m, directed outward