Nonlinear sub-cyclotron resonance as a formation mechanism for gaps in banded chorus

Abstract

An interesting characteristic of magnetospheric chorus is the presence of a frequency gap at $\omega \simeq 0.5{\Omega }_e$, where ${\Omega }_e$ is the electron cyclotron angular frequency. Recent chorus observations sometimes show additional gaps near $0.3{\Omega }_e$ and $0.6{\Omega }_e$. Here we present a novel nonlinear mechanism for the formation of these gaps using Hamiltonian theory and test-particle simulations in a homogeneous, magnetized, collisionless plasma. We find that an oblique whistler wave with frequency at a fraction of the electron cyclotron frequency can resonate with electrons, leading to effective energy exchange between the wave and particles.