Controlling the Chirping of Chorus Waves via Magnetic Field Inhomogeneity

Whistler mode chorus waves are coherent electromagnetic emissions in planetary magnetospheres, characterized by rising-tone or falling-tone chirping elements. Understanding the cause of different chirping directions and their properties is an important step in resolving the long-standing problem of nonlinear chorus generation. We report here, for the first time, particle-in-cell simulations of bidirectional chirping of whistler waves in a uniform magnetic field and falling-tone-only chirping in an inhomogeneous field. Combined with previous simulations of rising-tone-only emissions, we demonstrate that the background magnetic field inhomogeneity is not required for chirping of chorus, but it plays a key role in determining the chirping direction. The findings of the present work also unveil the critical role of the dipole geometry of Earth's magnetic field in causing the statistical predominance of rising-tone chorus and the oblique propagation of falling-tone chorus.