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. The pitch angle distributions (PADs) of energetic electrons in the Earth's outer radiation belt are often modified by wave-particle interactions. In recent years, the bursts of butterfly PADs of 10s keV electrons correlated with upper-band chorus waves are frequently observed. By assuming these chorus waves are strictly ducted (parallel) along the geomagnetic field, previous test-particle simulations suggest nonlinear cyclotron trapping is mainly responsible for the bursts of these butterfly PADs. In this study, test-particle simulations in combination with two-dimensional (2-D) electron magnetohydrodynamics simulations are carried out to investigate how ducted (quasi-parallel) and nonducted upper-band chorus waves modify the electron PADs. Simulation results show that ducted chorus waves cause the bursts of butterfly PADs of 10s keV electrons by cyclotron phase trapping, consistent with previous simulation studies on strictly ducted chorus waves. However, nonducted chorus waves cause those by cyclotron phase bunching. Our study provides new insights into the formation mechanisms of the electron butterfly distributions. Simulations show butterfly pitch angle distributions (PADs) of tens of keV electrons are formed by ducted and nonducted upper-band chorus waves in seconds Ducted upper-band chorus waves tend to accelerate electrons to form butterfly PADs of tens of keV electrons via phase trapping Nonducted upper-band chorus waves tend to decelerate electrons to form butterfly PADs of tens of keV electrons via phase bunching