[1] We present an empirical model of the energy spectra for hydrogen energetic neutral atoms (ENA) backscattered from the lunar surface based on Chandrayaan-1 Energetic Neutral Atom (CENA) observations. The observed energy spectra of the backscattered ENAs are well reproduced by Maxwell-Boltzmann distribution functions. The backscatter fraction is constant and independent of any solar wind parameters and the impinging solar wind angle. The calculated backscatter fraction is 0.19, and the 25% and 75% percentiles are 0.16 and 0.21. The empirical parameters of the Maxwell-Boltzman distribution derived from the CENA imager have no correlations with the upstream solar wind parameters, except for a good correlation between the solar wind velocity and the temperature of the backscattered ENAs. These results suggest that the reflected ENAs have experienced several collisions during the interaction with the loose lunar grains, and are then released into space. The mathematical model of the energy spectra of the backscattered ENAs is expressed by a function of the solar wind flux and velocity, which can be used for future investigations of regolith-solar wind interaction.

17We report on measurements of extremely high reflection rates of solar wind 18 particles from regolith-covered lunar surfaces. Measurements by the Sub-keV 19 Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 20 spacecraft in orbit around the Moon show that up to 20% of the impinging solar 21 wind protons are reflected from the lunar surface back to space as neutral 22 hydrogen atoms. This finding, generally applicable to regolith covered 23 -2 -atmosphereless bodies, invalidates the widely-accepted assumption that regolith 24 almost completely absorbs the impinging solar wind. 25 26 27 129 Sciences, 114 (No.6), 749-760 (2005) 130 Clark, B. E., B. Hapke, C. Pieters, D. Britt, Asteroid Space Weathering and Regolith 131 Evolution, Asteriods III, edts.

[1] The Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft has produced for the first time an image of a lunar magnetic anomaly in backscattered hydrogen atoms. The image shows that a partial void of the solar wind, a mini-magnetosphere, is formed above the strong magnetic anomaly near the Crisium antipode. The mini-magnetosphere is 360 km across at the surface and is surrounded by a 300-km-thick region of enhanced plasma flux that results from the solar wind flowing around the mini-magnetosphere. The minimagnetosphere is visible only in hydrogen atoms with energy exceeding 150 eV. Fluxes with energies below 100 eV do not show corresponding spatial variations. While the high-energy atoms result from the backscattering process, the origin of the low-energy component is puzzling. These observations reveal a new class of objects, minimagnetospheres, and demonstrate a new observational technique to study airless bodies, imaging in backscattered neutral atoms. Citation: Wieser, M., S. Barabash, Y. Futaana, M. Holmström, A. Bhardwaj, R. Sridharan, M. B. Dhanya, A. Schaufelberger, P. Wurz, and K. Asamura (2010), First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms, Geophys. Res. Lett., 37, L05103,

The Sub‐keV Atom Reflecting Analyzer instrument on board the lunar orbiter Chandrayaan‐1 provided a large number of measurements of lunar energetic neutral atoms (ENAs). These ENAs were formerly solar wind ions, which were neutralized and backscattered from the lunar surface. The angles under which the ENAs are scattered strongly depend on the solar wind ions' incidence angle, which corresponds to the solar zenith angle (SZA). Our large dataset provides us with a complete coverage of the SZA and almost complete coverage of the scattering angles. When combining all available measurements, four distinct features are discernible with SZA increase: amplitude decrease, less azimuthal uniformity, bigger ratio of sunward versus anti‐sunward flux and shallower scattering. We analyzed more than 290′000 measurements and derived a mathematical description of the features and their dependencies on the SZA.

[1] Significant proton fluxes were detected in the near-wake region of the Moon by an ion mass spectrometer on board Chandrayaan-1. The energy of these nightside protons is slightly higher than the energy of the solar wind protons. The protons are detected close to the lunar equatorial plane at a 140°solar zenith angle, that is, ∼50°behind the terminator at a height of 100 km. The protons come from just above the local horizon and move along the magnetic field in the solar wind reference frame. We compare the observed proton flux with the predictions from analytical models of an electrostatic plasma expansion into a vacuum. The observed velocity is higher by a factor of 2 to 3 than the velocity predicted by analytical models. The simple analytical models cannot explain the observed ion dynamics along the magnetic field in the vicinity of the Moon.