ARTEMIS Observations of Solar Wind Proton Scattering off the Lunar Surface

Area of Science:

• Space Physics
• Lunar Science
• Plasma Physics

Background:

• The Moon lacks a substantial atmosphere or magnetic field, making its surface directly exposed to solar wind.
• Understanding solar wind-surface interactions is crucial for lunar exploration and planetary science.

Purpose of the Study:

• To analyze the scattering of solar wind protons off the lunar surface.
• To develop an improved model for scattered proton energy spectra.
• To investigate the charge state ratio of scattered particles and its dependence on solar wind conditions.

Main Methods:

• Utilizing six years of ion observations from the ARTEMIS satellites at the Moon.
• Analyzing scattered proton energy spectra and directional distributions.
• Comparing observed proton scattering with existing models for energetic neutral atoms.

Main Results:

• Scattered proton distributions resemble those of energetic neutral atoms, peaking backward (Sunward).
• A revised model for scattered proton energy spectra is presented.
• The positive to neutral charge state ratio increases with exit speed but decreases with impact speed.

Conclusions:

• Solar wind proton scattering follows predictable patterns influenced by incidence angle and speed.
• Scattering efficiency decreases significantly with increasing solar wind speed (from ~0.5% to ~0.3%).
• These findings refine models of lunar surface plasma interactions and particle sputtering.