Gap Formation Around 0.5Ωe 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 Omega(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 0.5 Omega(e), or when they are slightly larger than 0.5 Omega(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 0.5 Omega(e) via cyclotron resonance. At last, the wave spectrum is separated into two bands with a power gap around 0.5 Omega(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 Omega(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 0.5 Omega(e) power gap of whistler-mode waves ubiquitously observed in the inner magnetosphere. Plain Language Summary The whistler-mode waves, which have a great influence on electron dynamics in the I 7 ,arth's magnetosphere, are ubiquitously observed with a power minimum around 0.5 Omega(e), This phenomenon is also called the 0.5 Omega(e) power gap. How the power gap forms is still one of the remaining issues since whistler waves were in-situ detected by satellites about 50 years ago. In this study, with a PIC simulation model, we propose a promising and natural generation mechanism of the power gap in the quasi-parallel whistler waves. When the frequencies of excited waves in the linear stage cross 0.5 Omega(e), or when they are slightly larger than 0.5 Omega(e) but then drift to lower values, a pronounced beamlike/plateau electron distribution in the parallel direction is formed due to the Landau resonance. Such an electron distribution can cause a severe damping around 0.5 Omega(e), via cyclotron resonance. At last, the power gap naturally generates. Besides, when only the waves with frequencies lower than 0.5 Omega(e) are excited in the linear stage, a power gap can also be formed due to the wave-wave interactions. Our study can shed light on the formation of 0.5 Omega(e) power gap in the whistler-mode waves observed in the inner magnetosphere.

Automated summary

Based on a one-dimensional particle-in-cell simulation model, this study investigates the formation mechanism of power gap around 0.5 Omega(e) in quasi-parallel whistler-mode waves excited by electron temperature anisotropy. The Landau resonance-induced beam-like/plateau electron distribution can lead to severe damping and generate a power gap around 0.5 Omega(e). Additionally, wave-wave interactions can also contribute to the formation of such power gap when only waves with frequencies lower than 0.5 Omega(e) are excited.