A New Long-Term Study of the Moon’s Extended Tail of Sodium Atoms with Implications for Sputtering Source Rates from the Lunar Regolith

Abstract:

The lunar surface is constantly bombarded by the solar wind, photons, and meteoroids. All of these sources can liberate atoms from the regolith. Some of these atoms have energies that allow them to escape the Moon. The most readily observable atom liberated from the Moon is Sodium (Na). These ejected Na atoms are subsequently accelerated by solar photon pressure to form a long comet-like tail opposite to the sun. Once a month, near new moon, this stream of Na atoms encounters the Earth where they are deflected by the Earth's gravity and "focused" into a beam of significantly enhanced Na density. This beam, when viewed from the night side of the Earth, appears as a small (~3^o diameter) diffuse spot with a peak brightness ~80 Rayleighs.

The Center for Space Physics at Boston University operates 12 All-Sky Imagers (ASI) around the globe. At seven of these sites, the ASIs are equipped with filters to monitor the mesospheric Na brightness. The Sodium Moon Spot (SMS) appears routinely in the images recorded on days near new moon. Thirteen years (2006-2019) of data from the ASI at the El Leoncito Observatory (Argentina) have been analyzed. We document changes in the SMS shape and brightness on a daily basis. A rigorous set of image processing protocols were developed to specify SMS brightness values. New geometry based relationships have been discovered that affect the SMS brightness. The SMS brightness is affected by the distance from the Earth to the Moon, as well as the ecliptic latitude of the Moon. After removing these geometry affects, the data were analyzed for long term and seasonal patterns that could be attributed to changes in likely source mechanisms. We do not find a correlation between the SMS brightness over a long (solar-cycle) time frame, but do detect a weak annual pattern. The former rules-out changing solar wind conditions as the primary cause of the SMS brightness variability. The annual peak of the SMS brightness occurs in the April-May time frame, with minimum brightness in December-January, suggesting a possible correlation with sporadic meteor rates observed at Earth. The new insights gained from this long term study put new limits on the potential sources of the Na atoms coming from the Moon.