Generation of Multiband Chorus in the Earth's Magnetosphere

Multiband chorus waves, where the frequency of upper band chorus is about twice that of the lower band chorus, have recently been reported based on THMEIS observations. The generation of multiband chorus waves is attributed to the mechanism of lower band cascade, where upper band chorus is excited via the nonlinear coupling process between lower band chorus and the associated density mode with the frequency equal to that of lower band chorus. In this letter, with a one-dimensional (1-D) particle-in-cell (PIC) simulation model, we have successfully reproduced multiband chorus waves. During the simulation, the significant density fluctuation is driven by the fluctuating electric field along the wave vector of the pump wave (lower band chorus), which can be directly observed in this self-consistent plasma system. After quantitatively analyzing resonant conditions among wave numbers, we confirm that the second harmonic of the pump whistler-mode wave (upper band chorus) is generated due to the coupling between the pump wave and the density fluctuation along its wave vector. The third harmonic may also be excited through the coupling between the second harmonic and the density fluctuation. Our simulation results not only provide a theoretical support to lower band cascade, but also propose a new pattern of evolution for whistler-mode waves in the Earth's magnetosphere.