[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-15] プラズマ圏におけるホイッスラーモード・ヒスの非線形成長過程

*大村 善治¹、殷 振興¹、謝 怡凱¹ (1.京都大学生存圏研究所)

キーワード：ホイッスラーモード波、非線形波動粒子相互作用、プラズマ圏、粒子シミュレーション

Whistler-mode hiss emissions play important roles in scattering energetic electrons in the plasmasphere. We study generation processes of hiss emissions, assumig that waves grow from thermal fluctuations receiving energy from energetic electrons through pitch angle diffusion to lower pitch angels. The wave growth due to the diffusion is well described by the linear growth rates. As the wave grows to an amplitude large enough to cause nonlinear trapping of resonant electrons, the nonlinear wave growth occurs due to formation of nonlinear resonant currents which induces frequency variation and wave growth simultaneously. Depending on the shape of the velocity distribution function near the resonance velocity, either electron holes or hills are formed which result in rising or falling tone emissions. The frequency variation and the spatial gradient of the background magnetic field play important roles in the nonlinear wave growth [1]. We have investigated the transition of the linear growth process to the nonlinear growth process by performing full-particle simulations with different levels of thermal fluctuations and different gradients of the background parabolic magnetic field [2]. We find coherent waves with frequency variation appear out from the waves growing due to the linear growth rate. Coherent waves grow through propagation in an optimum parabolic configuration of the background magnetic field. Even in a system with uniform background magnetic field and without positive linear growth rates, we find that small coherent wave packets are formed from thermal fluctuation because of nonlinear wave growth due to frequency variation. Coherent waves can exist simultaneously with nonlinear trapping potentials at different velocities in the velocity phase space, which correspond to different frequencies. Multiple nonlinear wave growth processes of coherent waves at different frequencies result in generation of hiss emissions as we find in the plasmasphere.

References:
[1] Omura, Y., Nonlinear wave growth theory of whistler-mode chorus and hiss emissions in the magnetosphere. Earth Planets Space 73:95, 2021.
[2] Yin, Z., “Simulation study on whistler-mode hiss emissions in the magnetosphere”, Master thesis, Department of Electrical Engineering, Kyoto University, 2025.