Simulation Study of Whistler-Mode Chorus Wave Generation in the Earth's Inner Magnetosphere

Whistler-mode chorus waves play an important role to control electron dynamics in the Earth’s radiation belt. Most of the existing theoretical and simulation studies on the chorus waves assume a one-dimensional field-aligned wave propagation. Physics of the chorus wave excitation and evolution in the multi-dimensional dipole field geometry still remains a challenge. The oblique propagation will be subject to wave attenuation at higher latitudes and introduce additional harmonic resonances. We have conducted simulations of two-dimensional chorus waves excited by hot anisotropic electrons interacting with cold dense plasma in a dipole magnetic field. It is found that the rising tone element of chorus waves with frequency chirping from low frequency to up to higher than the half electron gyro-frequency is generated at low latitudes. As the chorus wave propagates toward high latitudes, the wave becomes oblique and both the Landau and cyclotron resonance become significant. Two bands chorus waves are thus formed. In addition, we have found that a strong wave-particle interaction process presents in multi-dimensional electron distribution during the formation of chorus wave subpackets. The nonlinear physics associated with the wave growth and wave frequency chirping has been quantitatively evaluated in the process of chorus wave development.