Jody K. Wilson

Atoms in the thin lunar exosphere are liberated from the Moon's regolith by some combination of sunlight, plasma, and meteorite impact. We have observed exospheric sodium, a useful tracer species, on five nights of full Moon in order to test the effect of shielding the lunar surface from the solar wind plasma by the Earth's magnetosphere. These observations, conducted under the dark sky conditions of lunar eclipses, have turned out to be tests of the differential effects of energetic particle populations that strike the Moon's surface when it is in the magnetotail. We find that the brightness of the lunar sodium exosphere at full Moon is correlated with the Moon's passage through the Earth's magnetotail plasma sheet. This suggests that omnipresent exospheric sources (sunlight or micrometeors) are augmented by variable plasma impact sources in the solar wind and Earth's magnetotail.

A new wide‐angle coronagraphic‐type imaging system used for the lunar eclipse of 16 July 2000 resulted in detections of the lunar sodium exosphere out to ∼20 lunar radii, approximately twice the size recorded with narrower fields of view during previous eclipses. These measurements and subsequent modeling provide a unique constraint on the fastest atoms ejected from the lunar surface that form the lunar exosphere, indicative of the most energetic space weathering processes acting on the lunar surface. At most, only a small fraction of the atoms are ejected from the surface with speeds faster than escape speed of 2.4 km s⁻¹, meaning solar photon radiation pressure largely contributes to the escape of sodium atoms which form the comet‐like tail. The total rate of sodium ejection from the surface for speeds >2.0 km s⁻¹ is comparable to estimates from previous lunar eclipse observations and earlier images of the lunar sodium tail.