Diagnostic Simulations of the Lunar Sodium Tail and Bright Spot

Numerical simulations of the lunar sodium tail and its bright 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 the New Moon. Compared with the high-resolution spectrum of the lunar tail by Mierkiewicz et al. (2006), we simulated velocity distributions of the sodium tail using 3 physical sodium sources and variable solar radiation pressure effects: photon stimulated desorption (PSD), solar-wind ion sputtering (SIS), and meteor impact vaporization (MIV) including the effect of the anisotropy of the dust distribution reported by the LADEE observations. Based on the two-component exosphere model (i.e., dayside and isotropic sources) with the 3-major sodium sources, we simulated velocity distributions of the lunar tail and bright spot considering the different source ratios and temperatures. We found the upper-limits of Maxwellian temperatures of 1800 K for the PSD and 2500 K for the MIV, which can account for the previously observed peak velocity. We confirm that a relatively short ionization time of 36 hours is consistent with the observed velocity distributions. Based on the previous intensity observations of the bright spot, the total production rate of the lunar sodium is estimated to vary between (0.4–1.3)×10²²/sec. We introduce an interesting event at the lunar sodium tail, the Ring effect, which can occur during the total solar eclipse due to the umbra and penumbra effects of the Earth.