Substorm Effects on Chorus Emissions Modulated by ULF Waves

Abstract Although cross‐scale wave‐particle interactions play a critical role in energy transfer in the magnetosphere, the influence of geomagnetic activity on the physical mechanisms involved remains unclear. In this study, we investigate this question by analyzing Van Allen Probes‐A observations of an event occurring on 25 July 2016. Ultra‐low frequency (ULF) waves, chorus waves, and enhancements of electron fluxes with substorm‐induced energy‐dispersive characteristics were detected simultaneously during this event. Cross‐wavelet analysis suggests that the chorus intensity is modulated by ULF waves. The frequency of chorus elements decreases concurrently with substorm injections. We perform linear instability analysis to interpret the frequency drop. It is found that substorm‐injected electrons with energies of tens to ∼200 keV have reduced perpendicular anisotropy, thereby changing the energy range responsible for chorus growth and modifying the generation frequency. Our results enhance knowledge of cross‐scale wave‐particle interactions and their roles in auroral evolution during substorm processes.