Influence of plasma ions on source rates for the lunar exosphere during passage through the Earth's magnetosphere

The total electron flux in the lunar exosphere, measured by the Electron Reflectometer onboard the Lunar Prospector during its passage through the Earth's magnetosphere (June 7–16, 1998), is strongly correlated with variations in the column abundance of equatorial sodium observed coincidentally with the McMath‐Pierce solar telescope at Kitt Peak. At the same time, the exospheric temperature increased from a minimum of 1200 K to a maximum of 3000 K at minimum phase angle. A comparison of different source mechanisms, using the measured electron flux as the proxy for the ion flux, provides strong evidence that ion impact enhances the photon‐stimulated desorption yield. At the Moon photon‐stimulated desorption is shown to be the dominant source of exospheric sodium outside the magnetosphere. Inside the magnetosphere, where the ion flux and the associated photon‐stimulated desorption yield decrease, the contributions from impact vaporization to the observed column abundance and temperature are found to be substantial.