Gap Formation Around 0.5Ω_e in the Whistler-Mode Waves Due To the Plateau-Like Shape in the Parallel Electron Distribution: 2D PIC Simulations

The power gap around 0.5Ω_e (where Ω_e is the equatorial electron gyrofrequency) of whistler-mode waves is commonly observed in the Earth's inner magnetosphere, but its generation mechanism is still under debate. By performing two-dimensional particle-in-cell simulations in a uniform background magnetic field, we investigate the spectral properties of whistler-mode waves excited by temperature anisotropic electrons. The waves have positive growth rates in a wide range of normal angles (θ ≈ 0°–35°), resulting in the generation of both parallel and nonparallel waves. Although the nonparallel wave modes are weaker than the parallel ones, they can cause the plateau-like shape around 0.5 V_Ae (where V_Ae represent the electron Alfven speed) in the parallel direction of electron velocity distribution. The plateau-like electron component can then lead to severe damping in the waves around 0.5Ω_e via the cyclotron resonance, and the power gap is formed. This mechanism is called as “spectrum bite”. Our study sheds fresh light on the well-known gap formation at ∼0.5Ω_e in the whistler-mode waves, which is ubiquitously detected near the equator in the inner magnetosphere.