The surface of the Moon is directly exposed to the surrounding space plasma and energetic particle (>10 keV) populations, as well as solar ultraviolet and soft X-ray radiation on the dayside, which result in it becoming electrically charged. Under certain conditions it is possible that the resulting near-surface electric fields could have a significant influence on the transport of charged lunar dust (< ~10 microns in radius). These processes are anticipated to be most extreme at various intervals during the passage of a coronal mass ejection (CME). Such an event is studied here using solar wind observations from April/May 1998 as part of the Solar Storm-Lunar Atmosphere Modeling (SSLAM) Lunar Extreme Workshop (LEW) to investigate the entire lunar surface-exosphere-space plasma system during a space weather event at the Moon. This workshop was organized by the NASA Lunar Science Institute (NLSI) Dynamic Response of the Environment At the Moon (DREAM) team.
In this study, surface charging models are used to predict surface potentials and electric fields during the April/May 1998 CME event, as well as assess the importance of the different current sources and the implications for electrostatic dust transport. As expected, at the subsolar point it is found that surface charging is dominated by photoemission currents, although secondary electron emission due to plasma electrons can often have a noticeable influence. Meanwhile at the terminator, surface charging is dominated by plasma electrons and the associated secondary electron emission. Occasionally the secondary electron emission is predicted to be sufficiently intense to be able to charge the shadowed surface positive. The location of the transition from positive to negative surface charging on the lunar dayside, also referred to as the Dead Zone, is at times predicted to be significantly affected by secondary electron emission, which could result in enhanced horizontal electric fields close to the terminator and other sunlit-shadow boundaries. The dynamic surface charging environment predicted at the Moon during the passage of the April/May 1998 CME resulted in the anticipated influence of electrostatic forces on the transport of charged dust being highly variable and time-dependent.
Perhaps most significantly, it is found that the lunar surface potential at the terminator could be rapidly driven many kilovolts negative during brief intervals when the current due to energetic electrons with energies >10 keV exceeds that due to thermal plasma ion motion. Therefore, monitoring the ratio of energetic electron to thermal plasma ion fluxes upstream of the Moon could provide a simple yet effective predictive indicator for extreme lunar surface charging conditions near the terminator and on the nightside.