Characterizing lunar 1 Hz whistler waves across a solar cycle using ARTEMIS observations

Whistler-mode waves are ubiquitous in space environments and constitute a key mechanism for energy transfer and transformation. The near-1 Hz narrowband whistler-mode waves are commonly observed in lunar space. However, the generation mechanism of narrowband 1 Hz whistler-mode waves in the lunar environment, where no global magnetosphere or permanent bow shock exists, remains an open question. This study examines 1 Hz waves in the lunar environment by analyzing 12 years (2012–2023) of ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun) mission data (across an entire solar cycle). The spatial distribution, spectral characteristics, and polarization properties of these waves are investigated alongside their dependence on upstream solar wind parameters and lunar magnetic anomalies. The results reveal that 1 Hz waves are predominantly observed in the solar wind near the Moon, with clear dawn–dusk and north–south asymmetries. Wave amplitudes range from 0.03 to 1 nT, and approximately 90% of the events demonstrate no direct magnetic connectivity to the Moon. Importantly, wave amplitude shows a positive correlation with the solar wind dynamic pressure (P_dyn) and the total interplanetary magnetic field (B_total) and an inverse correlation with the Alfvén Mach number (M_A), underscoring the influence of upstream conditions on wave properties. Our findings reveal that the majority of waves occur on unconnected field lines, indicating a more complex generation and propagation scenario than previously assumed. Furthermore, wave properties are quantitatively shown to be strongly modulated by upstream solar wind conditions. These results provide critical statistical constraints for future studies of wave generation in the unique plasma environment of an unmagnetized body.