Observations of the lunar sodium exosphere

Abstract

Measurements of sodium resonance emission showed that the sodium exosphere of the Moon extends up to 1500 km. altitude above the equatorial limb. The line‐of‐sight sodium atom densities at various altitudes were derived from the measured sodium emission intensities, using a calibration based on brightness of the lunar limb. The data were fit to a simple two‐parameter model of the exosphere. Above the equatorial limb, this yielded apparent temperatures of 950–1150°K., and surface densities of sodium atoms of 29–46 atoms/cm³. Above the north pole, adjacent to the polar terminator, the temperature was much less, 450°K., and the surface density rose to 65 atoms/cm³. Sodium emission could not be detected above the equatorial terminator, indicating a decrease to levels below about 200 Rayleighs, difficult to distinguish from the sodium nightglow. Solar wind impact on the surface does not seem to be an important factor, because the sodium density changed very little when the Moon was shielded from the direct solar wind by the Earth's magnetotail. This suggests that solar photon radiation plays a major role in maintenance of high kinetic temperatures in the sodium exosphere.