As the Moon orbits around the Earth, it directly interacts with ambient plasma since it has neither a global magnetic field nor a dense atmosphere. Ambient plasma conditions vary through Earth’s magnetotail to the solar wind, and lunar surface is charged to an electric potential that reduces the total electric current to the minimum level as any other object in plasma [1].
The Moon receives solar irradiation continuously from the Sun while the fluxes of the solar wind ions and electrons reach the lunar surface directly. Since the electron mass is significantly smaller than ions’, most objects in a surrounding plasma tend to be negatively charged in certain conditions; however, the solar irradiation provides photoemission of electrons from the dayside of the Moon due to UV and X-rays. Therefore, the dominance of the photoemission current generates positively charged surface potential.
…show more content…
In addition, positively charged dayside region had been predicted before the Apollo missions [3]. Data analysis from Apollo missions showed that the lunar surface was charged positively to approximately +10 V under the solar wind while near terminator and night side regions were charged to approximately -100 V [4]. According to Stubbs et al., these theoretical model predictions are roughly in agreement with observations [5].
Lunar horizon glow (LHG) was first spotted by onboard TV cameras during Surveyor missions in 1966 and 1968, and it was significantly brighter than the levels that can be supported by micrometeorite ejecta [6]. These observations were related to the lunar terminator, and it has been proposed that LHG was produced by levitated or lofted charged dust grains by electric field above the terminator