Generation of harmonics associated with electromagnetic ion cyclotron waves

Abstract:

Electromagnetic ion cyclotron (EMIC) waves play a key role in regulating the ring current ion dynamics and precipitating thermal (eV) as well as relativistic (MeV) electrons. Recently, a statistically significant number of events observed by spacecraft have shown harmonics in association with EMIC waves, which potentially lower the minimum resonant energy of relativistic electrons. However, the excitation mechanism of these EMIC harmonics remains not fully understood. In this work, we demonstrate that the harmonics are caused by spatial bunching of the electrons that are in nonlinear Landau resonance with the fundamental EMIC wave mode. As electrons trapped by the fundamental EMIC wave rotate in the resonant island, higher and higher harmonics appear successively in time and eventually disappear due to phase mixing of trapped electrons. Both the lag time between successive harmonics and the energy spectrum of harmonics predicted by theory show good consistency with observations.