Controlling the Chirping of Chorus Waves via Magnetic Field Inhomogeneity

Abstract:

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 (or tones) and their properties is an important step in resolving the long-standing problem of nonlinear chorus generation. We here report successful 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. We then inspect the propagation of the excited waves and find the source region of chorus waves locates upstream, where the inhomogeneity term of field tends to balance the frequency variation term in the chorus equation. 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, according to our results that rising tone chorus waves are excited in the parallel simulation system and dipole-type background field.