Unrestricted Solar Energetic Particle Access to the Moon While Within the Terrestrial Magnetotail

This study presents observations of Solar Energetic Particle (SEP) protons that have penetrated Earth's magnetotail to reach the lunar environment. We apply data from Wind as an upstream monitor and compare to observations from THEMIS-ARTEMIS within the tail to show clear signatures of SEPs at the Moon during two events. Combining modeling and data analysis, we show that SEPs above energies of similar to 25 keV gain access to the Moon's position deep within the magnetotail through field lines that are open on one end to the solar wind. These results contradict previous studies that have suggested that the magnetotail is effective in shielding the Moon from SEPs with energies up to 1 GeV. Instead, we highlight that Earth's magnetosphere provides poor protection to the Moon from SEPs, which irradiate the lunar surface even within the tail. Our results have important implications regarding the safety of astronauts during upcoming lunar missions. Plain Language Summary During two thirds of its orbit, the Moon is located outside of Earth's magnetosphere. At these locations, the lunar surface is exposed to a variety of charged particles including (low-energy) solar wind plasma and (high-energy) Solar Energetic Particles (SEPs), the latter of which are responsible for weathering and chemically altering the lunar surface. For the remaining one third of its orbit, the Moon is located within Earth's magnetosphere. Previous studies have suggested that the ambient magnetic field within this cavity shields the Moon from the highly energetic SEPs. However, we report clear observations by NASA's THEMIS-ARTEMIS mission of these potentially hazardous charged particles when the Moon was embedded deep within Earth's magnetosphere. We also apply computer simulations to illustrate that Earth's magnetic tail is porous, allowing SEPs to efficiently leak into the terrestrial magnetosphere. Our findings are therefore highly relevant for the safety of astronauts during the upcoming missions to explore the lunar environment.