Gap formation around 0.5Ωe of whistler-mode waves excited by electron temperature anisotropy

With a 1-D PIC simulation model, we have investigated the gap formation around 0.5Ω_e of the quasi-parallel whistler-mode waves excited by an electron temperature anisotropy. When the frequencies of excited waves in the linear stage cross 0.5Ω_e, or when they are slightly larger than 0.5Ω_e but then drift to lower values, the Landau resonance can make the electron distribution form a beam-like/plateau population. Such an electron distribution only slightly changes the dispersion relation of whistler-mode waves, but can cause severe damping around 0.5Ω_e via cyclotron resonance. At last, the wave spectrum is separated into two bands with a power gap around 0.5Ω_e. The condition under different electron temperature anisotropy and plasma beta is also surveyed for such kind of power gap. Besides, when only the waves with frequencies lower than 0.5Ω_e are excited in the linear stage, a power gap can also be formed due to the wave-wave interactions, i.e., lower band cascade. Our study provides a clue to reveal the well-known 0.5Ω_e power gap of whistler-mode waves ubiquitously observed in the inner magnetosphere.