Characterizing the near Lunar Plasma Environment

of Science Topic: The ambient plasma and solar UV incident on the lunar surface cause it to become electrically charged [1,2]. On the dayside, solar UV photocharging typically dominates, so the surface charges positive [3,4]; while on the nightside the plasma electrons usually dominate and thus the surface charges negative [5]. The resulting surface electric potential is confined to a near-surface sheath region. The height of this sheath region, which is controlled largely by plasma density and temperature, determines the electric field strength [6]. This interaction is complicated by variations in: solar UV intensity, the ambient plasma, formation of dusty plasmas, surface composition and topology, magnetic anomalies and the lunar wake. Value of Science Topic: Surface charging is believed to drive the transport of micron-scale dust, a recognized potential hazard [7]. The differential charging of objects on the surface could lead to unanticipated electrical discharges. Understanding this environment will be of benefit to on manned and robotic surface exploration activities and various other scientific observations (e.g., lunar-based astronomy). Surface charging is a fundamental universal process affecting all airless regolith-covered bodies. Crossdisciplinary impacts: Astronomy and Astrophysics, Environmental Characterization and Operational Environmental Monitoring. Description of Science Topic: The surface of the Moon, like any object in a plasma, charges to an electric potential that minimizes the total incident current [8]. The charging currents come from four main sources: photoemission of electrons, plasma electrons, plasma ions and secondary electrons (arising from surface ionization by plasma electrons). The lunar dayside typically charges a few volts positive with a “photoelectron sheath” extending to ~1 m [3,9]. On the nightside, the lunar surface usually charges ~50-100 volts negative in the solar wind with a “Debye sheath” extending up to ~1 km [1,9,10]. This basic picture comes from Apollo-era and Lunar Prospector observations together with analytical theory. Lunar surface charging in the solar wind is complicated by variations in the solar spectrum, ambient plasma environment, surface composition [4] and topology, magnetic anomalies [11], and the formation of