  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.
