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Description
We investigate nonlinear processes in the generation of whistler-mode chorus emissions in the Earth's inner magnetosphere by a series of electron hybrid code simulations. We also study propagation properties of chorus emissions under the presence of duct structure in the magnetosphere. Chorus emissions are coherent whistler-mode waves with varying frequencies in the typical frequency range of 0.2 to 0.8 f_ce0, where f_ce0 is the electron gyrofrequency at the magnetic equator. Chorus emissions play crucial roles in the evolution of radiation belt electrons. The generation process of chorus has been explained by the nonlinear wave growth theory [e.g., Omura, 2021] and has been reproduced by self-consistent numerical experiments [e.g., Katoh and Omura, GRL 2007, JGR 2011, 2013, EPS 2016; Katoh et al., JGR 2018]. The combination of the magnetic mirror force and the Lorentz force by coherent electromagnetic waves induces the phase-space deformation of the distribution function due to the nonlinear trapping of energetic electrons. The phase-space deformation results in the formation of nonlinear resonant currents, generating coherent wave elements with rising/falling tones. Theoretical and simulation studies revealed the existence of the threshold wave amplitude required for the chorus generation. The threshold amplitude depends not only on the properties of energetic electrons but also on the background plasma environment. These results suggest the importance of cross-scale coupling in the magnetosphere. The periodic enhancement of chorus emissions often observed in the magnetosphere can be explained by the modulation of the threshold wave amplitude under the presence of low-frequency magnetohydrodynamic waves. The modulation of the background plasma environment also alters the propagation property of whistler-mode waves.
