Unusual Loss of Van Allen Belt Relativistic Electrons by Extremely Low-Frequency Chorus

Chorus waves with the frequency usually between 0.1 and 0.8 equatorial electron gyrofrequencyf(ce)can accelerate energetic electrons to relativistic energies (several mega-electron volts, MeV). Here, we report a unique event that both regular chorus and unusual chorus with extremely low frequency (ELF) below 0.1 f(ce)occurred during the 22 December 2014 geomagnetic storm. Relativistic electron fluxes decreased with decreasing wave frequency and had minima exactly corresponding to the presence of ELF chorus. Performing a two-dimensional simulation, we find that ELF chorus can result in unusual loss of relativistic electrons while regular chorus contributes to the acceleration, leading to the frequency-modulated fluxes of such high-energy electrons. The current results provide a new scenario that ELF chorus waves can potentially mitigate space weather by inhibiting relativistic electron flux enhancements.