Gap Formation Around 0.5 Omega(e) in the Whistler-Mode Waves Due To the Plateau-Like Shape in the Parallel Electron Distribution: 2D PIC Simulations
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The power gap around 0.5 Omega(e) (where Omega(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 (theta approximate to 0 degrees-35 degrees), 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 0.5 Omega(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 similar to 0.5 Omega(e )in the whistler-mode waves, which is ubiquitously detected near the equator in the inner magnetosphere.