Abstract
Abstract The first successful observations of resonant scattering emission from the lunar sodium exosphere were made from the lunar orbiter SELENE (Kaguya) using TVIS instruments during the period 17–19 December, 2008. The emission intensity of the NaD-line decreased by 12±6%, with an average value of 5.4 kR (kilorayleighs) in this period, which was preceded, by 1 day, by enhancement of the solar proton flux associated with a corotating interaction region. The results suggest that solar wind particles foster the diffusion of sodium atoms or ions in the lunar regolith up to the surface and that the time scale of the diffusion is a few tens of hours. The declining activity of the Geminid meteor shower is also one possible explanation for the decreasing sodium exosphere.
Highlights
In-situ measurements carried out during the Apollo missions revealed that the Moon has a surface boundary exosphere (Hoffman et al, 1973; Hodges, 1973, 1975)
The results suggest that solar wind particles foster the diffusion of sodium atoms or ions in the lunar regolith up to the surface and that the time scale of the diffusion is a few tens of hours
A spherically symmetric distribution of Na atoms expressed as below was used in the simulation, n(zgp) = n0 exp −zgp/H ∗, (1)
Summary
In-situ measurements carried out during the Apollo missions revealed that the Moon has a surface boundary exosphere (Hoffman et al, 1973; Hodges, 1973, 1975). Optical remote sensing techniques enable us to investigate the source mechanism of Na atoms in terms of: (1) thermal desorption, (2) micrometeoroid impacts, (3) photodesorption by solar illumination, and (4) sputtering by solar-wind particles, including chemical reactions (see the review by Stern, 1999). These source mechanisms, which give a wide variety of release velocities and ejection rates acting in different surface regions, produce the characteristic distribution and dynamics of the lunar exosphere. Several observational studies have been carried out to test this effect (Potter and Morgan, 1994; Mendillo et al, 1999; Potter et al, 2000; Wilson et al, 2006), continuous observation from the ground is difficult due to the strong reflection of sunlight during the full moon period
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