Abstract

The butterfly distribution is one of the characteristic features of the electron pitch angle distribution in electron radiation belts, and it has the maximum electron flux intensity at the pitch angle between the loss cone and 90 degrees. In this Letter, we report the results of test-particle simulations to reveal that the lower-band whistler chorus can be an important cause of the butterfly distribution of relativistic electrons. Our simulations show that the combination of two nonlinear scattering processes, that are phase-trapping and phase-bunching, contributes to the wave-driven butterfly distribution. The butterfly pitch angle distribution of relativistic electrons is proposed to be a signature of the nonlinear scattering processes due to intense whistler chorus waves in the heart of the outer radiation belt.