A numerical study of chorus generation and the related variation of wave intensity using the DAWN code

Chorus waves play an important role in energetic electron dynamics in the inner magnetosphere. In this work, we present a new hybrid code, DAWN, to simulate the generation of chorus waves. The DAWN code is unique in that it models cold electrons using linearized fluid equations and hot electrons using particle‐in‐cell techniques. The simplified fluid equations can be solved with robust and simple algorithms. We demonstrate that discrete chorus elements can be generated using the code. Waveforms of the generated element show amplitude modulation or “subpackets,” and the frequency sweep rate of the generated element is found to be consistent with that of observed chorus waves. Using the DAWN code, we then investigate the variation of wave intensity ( ∝ B w 2 ) with respect to linear growth rates on the equatorial plane. Previous observations showed that the change in linear growth rates of whistler waves modulated by external processes such as density modulations is usually small ( O ( 1 0 − 1 ) ), while the variation of the wave intensity is large ( O ( 1 0 1 − 1 0 2 ) ). Using a chosen set of background plasma parameters, we demonstrate that a small change ( O ( 1 0 − 1 ) ) in linear growth rates can lead to significant variation ( O ( 1 0 1 ) ) of wave intensity only in the transition from the broadband whistler wave generation regime to the chorus wave generation regime. Our results demonstrate the importance of including nonlinear dynamics of chorus generation in understanding the whistler wave intensity modulation process in the inner magnetosphere.