Solar Energetic Electron Access to the Moon Within the Terrestrial Magnetotail and Shadowing by the Lunar Surface

We present measurements of 30-700 keV Solar Energetic Electrons (SEEs) near the Moon when within the terrestrial magnetotail by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun spacecraft. Despite their detection deep within the tail, the incident flux and spectral shape of these electrons are nearly identical to measurements taken upstream of Earth in the solar wind by the Wind spacecraft; however, their pitch angle distribution is isotropized compared to the more field-aligned distribution upstream. We illustrate that SEEs initially traveling Earthward precipitate onto the lunar far-side, generating extended shadows in the cis-lunar electron distribution. By modeling the dynamics of these electrons, we show that their precipitation patterns on the lunar near-side are comparatively reduced. The non-uniform precipitation and accessibility of potentially hazardous electrons to the Moon's surface are highly relevant in the context of astronaut safety during the planned exploration of the lunar environment. The Moon is located within the tail of Earth's magnetosphere during one-third of its orbit. Although the strong terrestrial magnetic field prevents high-energy particles from reaching Earth's surface, the Moon does not receive the same protection when it is within the terrestrial magnetotail. Instead, we show that the high-energy electron flux near the Moon is unchanged during intense solar energetic electron events compared to measurements taken far upstream of Earth. However, the precipitation of these particles onto the lunar surface is non-uniform. Since these electrons gain access to the magnetosphere from down-tail of the Moon, they preferentially bombard the lunar far-side surface. This creates a shadow in the electrons on the nearside that extends far beyond the Moon toward Earth. Hence, despite the high flux of these particles that are potentially hazardous to future activities on the lunar surface, there exist regions across the lunar near-side where the relative flux of these electrons is reduced relative to the upstream value when the Moon is within the magnetotail. These findings provide context for the fundamental scientific understanding of high-energy solar electrons and their access to the lunar surface. High-energy solar energetic electrons (SEEs) have direct access to the lunar environment when in the terrestrial magnetotail Precipitation onto the lunar nightside carves-out electrons from the ambient distribution, generating extended shadows far from the Moon When in the tail, the lunar surface is non-uniformly bombarded by Earthward-traveling SEEs, with reduced access to the dayside hemisphere