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We utilize measurements of electron plasma frequency oscillations made by the two-probe Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun mission to investigate the charged particle density in the lunar environment as the Moon passes through the Earth's geomagnetic tail. We find that the Moon possesses a tenuous ionosphere with an average density of similar to 0.1-0.3cm(-3), present at least 50% of the time in the geomagnetic tail, primarily confined to within a few thousand kilometers of the dayside of the Moon. The day-night asymmetry and dawn-dusk symmetry of the observed plasma suggests that photoionization of a neutral exosphere with dawn-dusk symmetry produces the majority of the lunar-derived plasma. The lunar plasma density commonly exceeds the ambient plasma density in the tail, allowing the presence of the lunar ionosphere to appreciably perturb the local plasma environment.

Plain Language Summary Though usually considered an airless body, the Moon actually possesses a very tenuous atmosphere. Ionization of this atmosphere, primarily by sunlight, creates a lunar ionosphere roughly one million times more tenuous than that of the Earth. At most times, this ionosphere does not noticeably affect the surrounding environment. However, when the Moon passes through the near vacuum of the geomagnetic tail of the Earth each lunar month around full Moon, its presence becomes more important. In this unique environment, the lunar-derived charged particle density becomes comparable to the ambient value, and the presence of the lunar ionosphere can appreciably affect its surroundings. We utilize measurements from lunar orbit to investigate the density, structure, and dynamics of the faint lunar ionosphere and its interaction with the local environment of the geomagnetic tail.