The generatin of extremely low-frequency chorus waves associated with plasma density enhancement

Chorus waves with extremely low frequency (ELF) below 0.1 f_ce are proposed to be a potential mechanism of scattering losses of relativistic electrons in the radiation belt. However, the generation of ELF chorus is still an open question. Here we report three interesting events that the occurrence of ELF chorus waves shows evident correlation with the increase of background plasma density while the disturbance of ambient magnetic field is negligible. We calculate the growth rate of chorus waves by using the correlated data of waves and particles form the Van Allen Probes. The calculated growth rates agree well with the wave along the satellite orbit. The current study suggests that the plasma density may play an important role on controlling the wave frequency during the chorus generation process.