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.