Generation of Highly Oblique Lower Band Chorus Via Nonlinear Three‐Wave Resonance

Abstract

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 2‐D 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.