Nonlinear Wave Growth Analysis of Chorus Emissions Modulated by ULF Waves

We present Van Allen Probes observations of periodic chorus wave emissions in the troughs of compressional ultralow frequency (ULF) waves. During this event, the spectral gap of chorus waves gradually widens as the spacecraft moves from the equatorial source region towards higher latitudes. Moreover, chorus wave intensity increases and frequency range widens after a substorm injection. We show that the periodic occurrence of chorus waves is attributed to the modulation of threshold amplitude for nonlinear growth of chorus waves by the second spatial derivative of ULF compressional magnetic field. The widening gap can be interpreted in terms of the nonlinear damping mechanism. A good agreement is also found between the nonlinear wave growth theory and the observations regarding the influence of substorm injection on the chorus. These findings support the applicability of the nonlinear theory in describing the chorus wave generation and damping, together with their modulations by ULF waves.

Automated summary

This study presents observations of periodic chorus wave emissions in the troughs of compressional ultralow frequency (ULF) waves using Van Allen Probes. The spectral gap of chorus waves was found to widen as the spacecraft moved towards higher latitudes, and the intensity and frequency range of chorus waves increased after a substorm injection. The periodic occurrence of chorus waves was attributed to the modulation of threshold amplitude for nonlinear growth of chorus waves by the second spatial derivative of ULF compressional magnetic field. The findings also supported the applicability of the nonlinear theory in describing the chorus wave generation and damping, together with their modulations by ULF waves.