Discrete Rising Tone Elements of Whistler-Mode Waves in the Vicinity of the Moon: ARTEMIS Observations

We report on discrete rising-tone elements of whistler-mode waves observed by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) in the vicinity of the Moon. The two-probe ARTEMIS observations suggest that a free energy source for the wave generation is provided by electron anisotropy resulting from lunar surface absorption and magnetic reflection. High time resolution dynamic spectra reveal that the waves consist of multiple rising tone elements, exhibiting striking similarities to the well-known whistler-mode chorus in planetary magnetospheres. The observed frequency sweep rates are generally consistent with those predicted by the nonlinear growth theory of chorus emissions by Omura et al. (2008, ). These results imply that whistler-mode waves can grow nonlinearly into chorus-like emissions even around airless bodies without magnetospheres and that a well-defined dipole field is not a prerequisite for the chorus generation. Plain Language Summary Whistler-mode chorus emission is a type of electromagnetic waves which sound like chirping of birds when converted into audio, because of the feature that its frequency rises or falls repeatedly in a second. Chorus is known to occur in planetary magnetospheres, and is important because it plays a role in formation of the hazardous radiation belts in the Earth's magnetosphere. Is it possible that chorus waves occur, for example, around the Moon? Although such an event has not been reported around airless bodies without magnetospheres like the Moon, whistler-mode waves, which can grow into chorus under certain conditions, are known to occur near the Moon. As there exist "seeds" of chorus, we decided to survey data obtained by spacecraft orbiting around the Moon, and we did find waves with chirping like chorus. In this paper, we investigate these chorus-like events in detail, and demonstrate that they are Moon-related waves, and that their chirping can be explained by the growth theory of chorus. These results provide a new insight into the lunar electromagnetic environment, which is getting important given the ongoing and planned exploration of the Moon, and also enable us to test the chorus theories with the exotic lunar conditions.