Gap Formation Around 0.5Ω e of Whistler‐Mode Waves Excited by Electron Temperature Anisotropy

With a one‐dimensional particle‐in‐cell simulation model, we have investigated the gap formation around 0.5 Ω eof the quasi‐parallel whistler‐mode waves excited by an electron temperature anisotropy. When the frequencies of excited waves in the linear stage cross 0.5 Ω e , or when they are slightly larger than 0.5 Ω ebut 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 0.5 Ω evia cyclotron resonance. At last, the wave spectrum is separated into two bands with a power gap around 0.5 Ω 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 0.5 Ω eare 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 0.5 Ω epower gap of whistler‐mode waves ubiquitously observed in the inner magnetosphere.