Harmonic Structure of Lower Hybrid Waves Driven by Energetic Ions at 4000 km Altitude: PIC Simulation

Past satellite observations in the polar region at about 4,000 km altitude have reported lower hybrid waves (LHWs) and their harmonics, but the excitation mechanism of the harmonic LHWs has not yet been clarified. We perform one-dimensional, electromagnetic, full particle-in-cell simulations by setting parameter values in the ranges observed by the satellite. The results from the two runs, that is, for omega(pe)/omega(e) = 0.1 and 0.2, show that the energetic ions can generate the harmonic LHWs in both cases. However, in the former case, more harmonic LHWs with larger amplitudes are created and stronger background-ion acceleration is observed. Thus, our present simulations have solved a previously unsolved problem, revealing the possibility that harmonic LHWs are involved in ion acceleration phenomena commonly observed in the polar region, such as ion outflow.