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Diagnostic Simulations of the Lunar Sodium Tail and Bright Spot

Numerical simulations of the lunar sodium tail and its bright sodium spot are presented. As an attempt to do reverse engineering to understand the lunar sodium exosphere, we simulated velocity distributions of the sodium tail and its bright spot that emerges in the night sky around new Moon phases. In this work, three physical sodium sources (i.e., photon‐stimulated desorption [PSD], solar‐wind ion sputtering [SIS], meteor impact vaporization [MIV]), the dust anisotropy effect reported by Lunar Atmosphere and Dust Environment Explorer (LADEE), three gravitational sources (i.e., the Moon, the Earth, and the Sun), and variable solar radiation pressure effects are considered. We simulated velocity distributions of the lunar bright spot considering different source contribution ratios and their temperatures in order to find the best initial parameters which can account for the important observed recessional velocity of 12.4 km/s by Mierkiewicz et al. (2006, https://doi.org/10.1029/2006GL027650). We found that the upper limits of Maxwellian temperatures of 1,800 K for the PSD and 2,500 K for the MIV and that the best contribution ratio of the three major sodium sources (i.e., PSD: SIS: MIV) is found at the case of 2: 1: 3 (i.e., MIV dominant). Based on the previous brightness observations of the bright spot, the total production rate of the lunar sodium is estimated to usually vary between 0.4 and 1.3 × 1022/s. We introduce an interesting event in the lunar sodium tail, the Sodium Ring phenomena, which can occur transiently during the total solar eclipse due to the umbra and penumbra effects of the Earth.