Radio Occultation Observations of the Lunar Ionosphere Variations Over GRAIL Mission Period

The Signal of Opportunity using NASA’s Gravity Recovery and Interior Laboratory (GRAIL) radio signals to remotely sense the Lunar ionosphere has been successfully demonstrated. The GRAIL mission consisted of an identical pair of spacecraft approximately 100 km apart in a circular polar orbit around the Moon; during the science mission period, the GRAIL’s X-band Radio Science beacon (RSB) data provide applicability for the radio occultation of the lunar electron density profiles with the uncertainty of frequency residual measurement ~ 1 mHz corresponding to ~ 2 x 10⁸ m^-3 electron density uncertainties. We will present our observation updates of the Lunar ionosphere in terms of the near-surface electron profiles versus altitude retrieved from the RSB data to understand its spatial and temporal variations during the GRAIL science mission period. The nature of the lunar ionosphere is a long-standing mystery; GRAIL’s observations of the near-surface electron density profiles and its responses to solar winds and storms impact the near-surface plasma environment.