Solar Energetic Particle Access to the Lunar Environment While Embedded Within Earth's Magnetotail

Solar energetic particles pose significant hazards to space exploration and habitation. In the context of the Moon, understanding the access of these high-energy particles to the lunar environment is critical when studying the composition of the surface, quantifying energy deposition at depth, and when considering the planned exploration during crewed and robotic missions. Of particular interest are times when the Moon is embedded deep within the tail of Earth’s magnetosphere, which occurs for approximately one-third of each lunation. Although the strong terrestrial magnetic field prevents high-energy particles from reaching Earth’s surface, the Moon does not receive the same protection while located within the terrestrial magnetotail. Instead, we show that the high-energy ions and electrons readily penetrate the tail along field lines that are open to the solar wind far downstream of the Moon. By applying a combination of data analysis and modeling techniques, we highlight the lack of shielding from these particles that the magnetotail provides to the lunar environment. Even so, we find that specific regions on the lunar near-side surface are still protected from precipitation by high-energy electrons by the solid body of the Moon: despite the high flux of potentially hazardous particles that can reach the local environment while within the magnetotail, the flux onto the lunar near-side is reduced. Our findings provide context for understanding access of high-energy solar particles to the lunar surface and are relevant for the safety of astronauts during the upcoming missions to explore the lunar environment.