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Observation of Oblique Lower Band Chorus Generated by Nonlinear Three‐Wave Interaction

Oblique whistler mode waves have been suggested to play an important role in radiation belt electron dynamics. Recently, X. Fu et al. (2017, https://doi.org/10.1002/2017GL074411) proposed that highly oblique lower band whistler waves could be generated by nonlinear three‐wave resonance. Here we present the first observational evidence of such process, using Van Allen Probes data, where an oblique lower band chorus wave is generated by two quasi‐parallel waves through nonlinear three‐wave interaction. The wave resonance condition is satisfied even in the presence of frequency chirping of one of the pump waves. Different from the simulation results of X. Fu et al. (2017), simultaneous particle data do not show a plateau in the electron distribution, which could be due to the very weak intensity of the generated waves. These results should help to better understand the generation of oblique waves in the inner magnetosphere and their relative roles in energetic electron dynamics.