Statistical Analysis of Lunar 1 Hz Waves Using ARTEMIS Observations

Like 1 Hz waves occurring in the upstream of various celestial bodies in the solar system, 1 Hz narrowband whistler-mode waves are often observed around the Moon. However, the wave properties have not been thoroughly investigated, which makes it difficult to proclaim the generation mechanism of the waves. Using 5.5-year wave data from ARTEMIS, we perform a detailed investigation of 1 Hz waves in the near lunar space. The amplitude of lunar 1 Hz waves is generally 0.05-0.1 nT. In the GSE coordinates, the waves show no significant regional differentiation pattern but an absence inside the magnetosphere. Correspondingly, in the SSE coordinates, they can occur extensively at ~1.1-12 RL, while few events observed in the lunar wake due to a lack of interaction with the solar wind. Furthermore, the wave distributions exhibit modest day-night and dawn-dusk asymmetries, but less apparent north-south asymmetry. Compared with nightside, more intense waves with lower peak wave frequency are present on the dayside. The preferential distribution of 1 Hz waves exhibits a moderate correlation with strong magnetic anomalies. The waves propagate primarily at wave normal angles < 60° with an ellipticity of [-0.8, -0.3]. For stronger wave amplitudes and lower latitudes, 1 Hz waves generally have smaller wave normal angles and become more left-hand circularly polarized. Owing to the unique interaction between the Moon and solar wind, our statistical results might provide new insights into the generation mechanism(s) of 1 Hz waves in planetary plasma environments and promote the understanding of lunar plasma dynamics.