Envelope Soliton in Multi-ion Plasma and Ion-Ion Hybrid WaveExcited by Energetic Electron Beam

Another envelope soliton event below the H⁺ gyrofrequency and localized density depletion was discovered in low auroral region ( ~1760 km ) by Freja satellite. This envelope soliton has a characteristic frequency at ~190 Hz, which is also closed to the resonance frequency of hydrogen ion-oxygen ion hybrid wave. This event is correlated in time with the observations of the sharp increase of the ratio of oxygen ion density to hydrogen and with the electrons energization along the magnetic field. A theoretical model on the ion-ion hybrid wave excited by energetic electron beam has also been presented. It is found that the ion-ion hybrid wave is mainly excited by the Cherenkov instability in auroral region.