Discrete rising tone elements of whistler-mode waves in the vicinity of
the Moon: ARTEMIS observations
Abstract
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.