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

Abstract

AbstractWe 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, https://doi.org/10.1029/2007JA012622). 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.