Empirical energy spectra of neutralized solar wind protons from the lunar regolith

Abstract

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.