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Oblique whistler mode waves have been suggested to play an important role in radiation belt electron dynamics. Recently, X. Fu et al. (2017, ) 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.

Plain Language Summary Oblique lower band whistler mode waves play an important role in the energetic electron dynamics in the magnetosphere. Recent research has also tried to understand how these highly oblique waves are generated. This paper presents the observations which directly confirm a recently proposed generation mechanism of oblique waves via nonlinear wave-wave interaction. In this theory, an oblique whistler wave, called the daughter wave, can obtain energy from the coupling with two parent whistler waves. Our observation shows that the generated oblique whistler waves also exhibit frequency chirping, caused by the frequency chirping of one of the parent waves. However, the observed waves are too weak to effectively scatter or energize electrons. Our study should be useful to further understand how the oblique waves are excited and their effects on energetic electron dynamics in the inner magnetosphere.