Intense Low-frequency Chorus Waves Observed by Van Allen Probes: Fine Structures and Potential Effect on Radiation Belt Electrons

Abstract:

Whistler-mode chorus emission in the low-density plasmatrough contributes significantly to the radiation belt electron dynamics. Chorus was usually considered to occur in the frequency range 0.1-0.8 f_ce (with the equatorial electron gyrofrequency f_ce ). We here report an event of intense low-frequency chorus with nearly half of wave power distributed below 0.1 f_ce observed by the Van Allen Probes on 27 August 2014. This emission exhibited little discrete rising tones but mainly the hiss-like signatures, had the high ellipticity of $\sim1$ and propagated quasi-parallel to the magnetic field. Compared with the typical chorus, the low-frequency chorus can produce weaker (2 times at ~ MeV and even up to several orders of magnitude at ~0.1MeV) momentum diffusion of the near-equatorially trapped electrons, but much stronger (1-2 orders of magnitude) pitch-angle diffusion near the loss cone. The acceleration and particularly loss effect of such intense low-frequency chorus may need to be taken into account in future radiation belt models.