Generation of Highly Oblique Lower-band Chorus via Nonlinear Three-wave Resonance

TitleGeneration of Highly Oblique Lower-band Chorus via Nonlinear Three-wave Resonance
Publication TypeJournal Article
Year of Publication2017
AuthorsFu, X, S. Gary, P, Reeves, GD, Winske, D, Woodroffe, JR
JournalGeophysical Research Letters
Date Published09/2017
Keywordsoblique whistler; PIC simulation; Ray Tracing; three-wave resonance; Van Allen Probes
AbstractChorus in the inner magnetosphere has been observed frequently at geomagnetically active times, typically exhibiting a two-band structure with a quasi-parallel lower-band and an upper-band with a broad range of wave normal angles. But recent observations by Van Allen Probes confirm another type of lower-band chorus, which has a large wave normal angle close to the resonance cone angle. It has been proposed that these waves could be generated by a low-energy beam-like electron component or by temperature anisotropy of keV electrons in the presence of a low-energy plateau-like electron component. This paper, however, presents an alternative mechanism for generation of this highly oblique lower-band chorus. Through a nonlinear three-wave resonance, a quasi-parallel lower-band chorus wave can interact with a mildly oblique upper-band chorus wave, producing a highly oblique quasi-electrostatic lower-band chorus wave. This theoretical analysis is confirmed by 2D electromagnetic particle-in-cell simulations. Furthermore, as the newly generated waves propagate away from the equator, their wave normal angle can further increase and they are able to scatter low-energy electrons to form a plateau-like structure in the parallel velocity distribution. The three-wave resonance mechanism may also explain the generation of quasi-parallel upper-band chorus which has also been observed in the magnetosphere.
Short TitleGeophys. Res. Lett.

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