Dispersive And Resonant Properties Of Obliquely Propagated Electromagnetic Ion Cyclotron Waves In Multi-ion Plasmas

Charge exchange and Coulomb collision processes are two major mechanisms responsible for the ring current decay. Previous studies have shown that pitch angle scattering of ring current ions induced by wave-particle interaction can also play an important role in ring current decay. In particular, gyroresonance between left-hand polarized electromagnetic ion cyclotron (EMIC) waves and ring current protons produces pitch angle scattering, yielding the rapid ring current decay with a time scale to about an hour, much faster than those of the charge exchange and the Coulomb collision. Currently, much attention has been paid to the pitch angle scattering resulting from parallel propagated EMIC waves. Hence, we shall present detailed analysis on the characteristics of oblique propagatied EMIC waves in multi-ion plasmas, including the dispersion relation, the minimum resonant energy and the resonant curves.The Stix notations D ,S is calculated for high and low geomagnetic activities; and different fractional compositions of the ion species. Particularly, the dispersion relation, the minimum resonant energy and the resonant curves of EMIC waves are investigated. It is shown that EMIC waves separate at the zero point of Stix notation D , and the stop-bands increase as wave normal angle increasing in He +-band and H +-band. This is a new result different from that in a single ion plasma, indicating that Stix notations D perhaps play an important role in dispersive properties of obliquely propagated EMIC waves. For harmonic resonance n < 2, as wave frequencyωincreases, the minimum resonant energy Em in is found to drop rapidly form energies 100 MeV to a few keV in all three bands: H +-band, He +-band and O +-band. For harmonic resonance n≥2, Em in increases asωincreasing in He +-band and H +-band. For fixed kinetic energy Ek = 100keV, we present a detailed analysis on the resonant curves of ions H +, He +and O +. We find that at harmonic n = 1, the resonant curves of ion H +exists only for the wave normal anglesθ> 90; the resonant curves of ion He + exist forθ< 90 in the H +-band, but θ> 90 in both O +-band and He +-band. Resonant curves of ion O~+ locate forθ< 90 in both H +-band and He +-band, but forθ> 90 in the O~+-band.