2023 ARTEMIS SCIENCE NUGGETS
Solar Energetic Proton Access to the Moon While Located Within the Terrestrial Magnetotail
by Lucas Liuzzo
Space Sciences Laboratory, University of California, Berkeley
Introduction
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 play a role in 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 magnetospheric cavity shields the Moon from the highly energetic SEPs. However, we report clear observations of these potentially hazardous charged particles even when the Moon was embedded deep within Earth's magnetosphere, using data from NASA's THEMIS-ARTEMIS (Time History of Events and Macroscale Interactions during Substorms-Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) mission. 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.
Results
SEP observations within the tail
The ARTEMIS probes transit the terrestrial magnetotail over a week-long period every ~28 days. Figure 1 displays observations from NASA’s Wind and ARTEMIS spacecraft on 23 to 24 June 2013, during which a high-energy, proton SEP event (generated by an interplanetary coronal mass ejection passing nearby Earth) was detected. While the Wind spacecraft was located far upstream of the planet (and therefore saw a “pristine” SEP environment that is unaffected by Earth), the ARTEMIS probes were located deep within the terrestrial magnetotail. Panels 1d and 1f illustrate that a nearly identical enhancement was observed by the ARTEMIS SSTs at energies 100 keV ≲ E ≲ 1 MeV as was observed by Wind (panel 1c) at this time. The similarity between the Wind and ARTEMIS SST observations suggests that Earth's magnetotail is unable to effectively shield SEP ions from reaching the lunar surface.
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| Figure 1: Position of, and observations from, the ARTEMIS and Wind spacecraft during the 23-24 June 2013 SEP event. At high energies (>100 keV), Wind and ARTEMIS observed nearly identical signatures of Solar Energetic Particles, despite ARTEMIS’ location deep within Earth’s magnetosphere. |
Modeling SEP access to the magnetotail
To better understand SEP dynamics and to shed light on their access to the Moon, we traced test particle trajectories as they travel through the terrestrial magnetosphere. To constrain the magnetospheric fields, we applied the Open Geospace General Circulation Model (OpenGGCM) which is publicly available through NASA’s Community Coordinated Modeling Center (CCMC). Figure 2 illustrates how these SEPs gain access to the Moon: energetic protons travel at least 300 Earth radii downstream before they “leak into” the terrestrial magnetotail along magnetic field lines connected to the solar win. After they enter the magnetosphere, they turn around and rapidly approach the Moon. Some of these passing particles impact the lunar surface, while some of them travel to the Earth where experience the strong magnetic fields associated with the terrestrial magnetic dipole, are reflected, and impact the other side of the Moon, thereby globally bombarding the lunar surface.
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| Figure 2: Access of Solar Energetic Particles to the Moon. These high-energy particles traveling in the solar wind first pass Earth and travel far downstream of the planet before they are funneled into the terrestrial magnetosphere, turn around, and precipitate onto the Moon. |
Conclusion
This study has presented THEMIS-ARTEMIS observations obtained while the probes were embedded deep within Earth's magnetotail. We have provided evidence of Solar energetic protons detected by the probes that were nearly unchanged when compared to their detection upstream by Wind. This indicates the direct access of SEPs to the lunar orbit, even when the Moon is within the magnetotail. The open nature of the magnetic field lines suggests that the magnetosphere is ineffective in shielding particles above keV energies from reaching the Moon, contradicting previous studies that have suggested that the Moon is protected from these particles when in the tail.
Notably, shielding future astronauts and equipment on the lunar surface from energetic particle radiation is a key consideration of the upcoming missions to the Moon, including the crewed Artemis missions to explore the south pole. Although penetrating particles at energies greater than ∼1 MeV are responsible for the most severe damage to astronauts and electronics, our finding that the magnetotail is open to SEPs indicates that the more damaging, higher-energy particles likely also have nearly unrestricted access to the lunar surface. Hence, SEPs present a clear potential hazard to future exploration of the lunar surface, even for times when the Moon is within the terrestrial magnetotail.
References
Liuzzo, L., Poppe, A. R., Lee, C. O., Xu, S., and Angelopoulos, V. (2023). Unrestricted solar energetic particle access to the Moon while within the terrestrial magnetotail. Geophysical Research Letters, 50, e2023GL103990, doi:10.1029/2023GL103990.Biographical Notes
Lucas Liuzzo is an Assistant Research Scientist at the Space Sciences Laboratory located at the University of California, Berkeley. His research focuses on the interactions of moons throughout the solar system with the plasmas impinging onto them.
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