Global Survey of Whistler-Mode Waves in the Lunar Plasma Environment: Insights From 11 Years of ARTEMIS Observations

Abstract

Whistler-mode waves are among the most ubiquitously observed plasma waves in the vicinity of Earth's Moon, providing an important diagnostic tool for plasma processes. However, a complete understanding of key plasma parameters responsible for whistler-mode generation remains elusive. In this study, we conduct a comprehensive statistical survey using 11 years of in-situ measurements from the ARTEMIS mission to reveal the global distribution of whistler-mode wave amplitude and occurrence rate as the Moon traverses through the solar wind, magnetosheath, and magnetotail during a typical lunar orbit. Our findings reveal that the highest whistler-mode wave amplitudes are observed when the Moon crosses Earth's magnetosheath. A parametric study is conducted to explore the correlation between whistler-mode wave occurrence rate and plasma parameters such as electron temperature anisotropy and heat flux. These parameters are analyzed separately for low-energy (<100 eV) and high-energy (>100 eV) electrons. The results indicate that whistler-mode wave occurrence rates exhibit a stronger positive correlation with the temperature anisotropy of high-energy electrons compared to low-energy electrons. Moreover, the parallel heat flux (normalized by the free streaming heat flux) of high-energy electrons shows a moderate positive correlation with whistler-mode wave amplitude (normalized to the background magnetic field) in both the solar wind and magnetosheath regions. We further investigate the influence of the background magnetic field line connection to the lunar surface on whistler-mode waves. Overall, our analysis demonstrates that the temperature anisotropy of high-energy electrons has a higher positive correlation with magnetic field connection than that of low-energy electrons.