On the Role of Dust in the Lunar Exo-Ionosphere

There is evidence to suggest that electron concentrations above the dayside lunar surface can be significantly higher than expected from either the photo-ionization of exospheric neutrals or any other well-known processes. The Soviet Luna 19 and 22 spacecraft performed dual-frequency radio occultation experiments between the Moon and the Earth, and from the shift between the two frequencies it was possible to determine variations in electron concentration along the line-of-sight. Such measurements were used to estimate electron concentration profiles above the lunar limb, which surprisingly indicated a peak electron concentration of ~500-1000 cm-3 and scale heights of ~10-30 km. Recent analyses of radio measurements from the Japanese Kaguya (SELENE) mission appear broadly consistent with earlier findings, but with a better appreciation for the uncertainties associated with this difficult experiment. It has been suggested that electrically charged lunar exospheric dust could contribute to these enhanced electron concentrations. Here we describe how it is possible to estimate the electron concentrations produced by charged dust. This is then used in conjunction with exospheric dust distribution models to estimate the electron concentrations expected based on Apollo 15 observations of lunar horizon glow. Conversely, the electron concentrations inferred from Luna 19 data are used to estimate dust concentrations above the limb. Radio occultation experiments appear to provide another valuable perspective on the lunar environment that will be complemented by measurements from future lunar missions, such as LRO, LCROSS, and LADEE.