Observations and parametric study on the role of plasma density on extremely low-frequency chorus wave generation

Extremely low-frequency (ELF) chorus waves with frequencies below 0.1 f ce can cause the scattering losses of relativistic electrons. Previous studies have suggested that ELF chorus waves are excited by anisotropic electrons with energies up to a few hundred keV. Here, we report an interesting event observed using the Van Allen Probes on November 1, 2012, where distinct ELF chorus waves occurred exactly corresponding to the enhancement of the plasma density. Using the correlated data of particles and plasma environments, the continuous calculation of growth rates along the satellite trajectory was conducted. The linear analysis results display a similar pattern to the observation results. To further identify the impact of the plasma density on the frequency control of ELF chorus waves, the growth rates were then calculated by only changing the value of the plasma density. The calculation results show that the growth rates move to lower frequencies with plasma density enhancement during the event. These findings indicate that the increased plasma density has a considerable impact on the wave frequency. The current study can enrich our understanding of the possible role of plasma density on the generation of ELF chorus waves.