Amplitude dependence of the generation process of whistler-mode chorus emissions

We show results of the self-consistent simulations reproducing the chorus generation process in the magnetosphere. We have performed simulations with different initial number densities of energetic electrons and have shown that the frequency sweep-rates of reproduced chorus vary depending on the variation of the wave amplitude of each chorus element. The simulation results reveal that the characteristic frequency variation of chorus elements showing rising tones has been formed at the region very close to the magnetic equator (within 10 c Ω_{e0}; c is the speed of light and Ω_{e0} is the electron gyro-frequency at the magnetic equator) while the wave amplitude of elements have been significantly intensified through their propagation away from the equator ( 100 c Ω_{e0}). The spatial scale of the region where the explosive wave growth has been observed is varied in each simulation run, corresponding to the difference of the wave amplitude of reproduced chorus elements. Based on the nonlinear wave growth theory, we have clarified that the theoretically estimated frequency sweep-rates are consistent with the simulation results. We have also analyzed the amplitude threshold in generating coherent chorus elements by evaluating the wave magnetic field amplitude during the time interval at which a first coherent chorus element emerges. We find that the estimated threshold values of wave magnetic field amplitude are consistent with the simulation results.