Butterfly Distributions of Energetic Electrons Driven by Ducted and Nonducted Chorus Waves

Bursts of electron butterfly distributions at 10s keV correlated with chorus waves are frequently observed in the Earth's magnetosphere. Strictly ducted (parallel) upper-band chorus waves are proposed to cause them by nonlinear cyclotron trapping. However, chorus waves in these events are probably nonducted or not strictly ducted. In this study, test-particle simulations are conducted to investigate electron scattering driven by ducted (quasi-parallel) and nonducted upper-band chorus waves. Simulation results show butterfly distributions of 10s keV electrons can be created by both ducted and nonducted upper-band chorus waves in seconds. Ducted upper-band chorus waves cause these butterfly distributions mainly by accelerating electrons due to cyclotron phase trapping. However, nonducted waves tend to decelerate electrons to form these butterfly distributions via cyclotron phase bunching. Our study provides new insights into the formation mechanisms of electron butterfly distributions and demonstrates the importance of nonlinear interactions in the Earth's magnetosphere.