Abstract
The harmonic structure of lower hybrid waves (LHWs) driven by energetic ions can be generated through non-linear wave-wave coupling. We investigate the parameter dependence of the excitation and time evolution of this structure, using one-dimensional electromagnetic particle-in-cell (PIC) simulations. Focusing on two parameters, ωpe/Ωe ${\omega }_{pe}/{{\Omega }}_{e}$ (ratio of the electron plasma to electron gyro frequencies) and u⊥/vA<1 ${u}_{\perp }/{v}_{A}< 1$ (ratio of energetic-ion to Alfvén velocities), we analyze the fluctuation spectra in the wavenumber-frequency plane and demonstrate that the harmonic structure can be excited across wide parameter ranges of 0.25≤ωpe/Ωe≤4 $0.25\le {\omega }_{pe}/{{\Omega }}_{e}\le 4$ and u⊥/vA<1 ${u}_{\perp }/{v}_{A}< 1$, indicating a weak parameter dependence. However, the excitation region and time evolution of the harmonic structure can be significantly affected by these parameters. We find that conditions of low ωpe/Ωe ${\omega }_{pe}/{{\Omega }}_{e}$ and intermediate u⊥/vA ${u}_{\perp }/{v}_{A}$ are preferable for the excitation and survival of the harmonic structure. Previous observations have reported the harmonic structure of LHWs in the polar region at 4,000 km altitude and in the plasma sheet at XGSM ∼ ${\sim} $ 17 RE ${R}_{\mathrm{E}}$. Nevertheless, this study predicts that the harmonic structure can also be excited in other regions of the magnetosphere where energetic ions are present.
