Three-dimensional simulations of the lunar sodium exosphere and its tail

Abstract

[1] The brightness distributions of the lunar sodium exosphere reported by Flynn and Mendillo (1993) and the receding velocities of the lunar sodium tail observed by Mierkiewicz et al. (2006) are reproduced by 3-D Monte Carlo simulations. We consider the effects of two spatially different sodium sources simultaneously: dayside source with the dependency of solar zenith angle and an isotropic source due to micrometeoroid impact. In the simulations, the following effects are taken into account: (1) the gravity of the Moon, the Earth, and the Sun with the orbital motion of the Moon; (2) photoionizations, solar radiation pressure, and returns to the lunar surface; and (3) the shadows of the Earth and Moon. The sodium brightness observed by Flynn and Mendillo (1993) is successfully modeled, from which the most probable source ratio between the isotropic and dayside sources is estimated to be 70%∼80%: 30%∼20%, respectively. The best coma model provides an initial velocity of 2.0 km/s with a narrow Doppler width of 0.2 km/s and a total production rate of 0.7 × 10 22 /s. On the basis of the best 3-D lunar coma model, we also simulate the receding velocity distribution of lunar sodium tail, and we find satisfactory models for high receding velocity and its wide dispersion observed by Mierkiewicz et al. (2006) considering the effect of variable solar radiation pressure and appropriate ionization times.

The Phobos neutral and ionized torus

“…The Na distribution for MIV neutrals is complex, reflecting the role that solar radiation pressure plays in distorting the individual neutral trajectories. The distorted Na neutral distribution is analogous to observations of solar radiation‐induced perturbations of Na clouds at Earth's Moon [e.g., Mendillo et al , ; Lee et al , ], comets [e.g., Cremonese et al , ], and the Jovian system [e.g., Smyth and Combi , ; Smyth , ; Burger and Johnson , ]. Finally, similar to the sputtered neutral distribution, Al neutrals form a small coma around the instantaneous position of Phobos itself.…”

Long‐Term Observations and Physical Processes in the Moon's Extended Sodium Tail

“…It was noted that changes in the lunar source rate or Na photoionization lifetime could also produce a similar spectrum. Lee et al (2011) also used a three-dimensional Monte Carlo model but incorporated two simultaneous Na source functions; a dayside source with a solar zenith angle dependency and an isotropic source resulting from micrometeoroid impacts. The authors reported "satisfactory" agreement between the modeled recessional velocity distribution and the E. Mierkiewicz et al (2006) observations and suggested that variations in solar radiation pressure and the Na ionization lifetime may be important factors in the modeled differences.…”

“…Lee et al. (2011) also used a three‐dimensional Monte Carlo model but incorporated two simultaneous Na source functions; a dayside source with a solar zenith angle dependency and an isotropic source resulting from micrometeoroid impacts. The authors reported “satisfactory” agreement between the modeled recessional velocity distribution and the E. Mierkiewicz et al.…”

Diagnostic Simulations of the Lunar Sodium Tail and Bright Spot

“…This Monte Carlo code can simulate both the lunar coma and the tail regions producing many data for their morphology, brightness, and velocities with an adopted ionization life time of sodium. In this work, we used 36 h for the ionization life time of lunar sodium (e.g., Lee et al, 2011;Line et al, 2012;Tenishev et al, 2013). When considering the variable solar radiation pressure effect on the sodium velocity, a relatively ionization time of 36 h seems to be reasonable for the lunar sodium.…”

“…In Section 3, the methods of modeling the lunar exosphere and toy‐modeling schemes for the tail simulations are introduced. Based on our two‐component model (i.e., dayside and isotropic sources; Lee et al., 2011) with the three major source processes (i.e., photon‐stimulated desorption [PSD], solar‐wind ion sputtering [SIS], and meteor impact vaporization [MIV]). We simulated a large number of different cases considering the various initial conditions of the three major sources of the lunar sodium coma models.…”