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 fce ).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～1 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.
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