Generation of He+ and O+ Emic Waves by the O+ Bunch Distribution Associated with Fast Magnetosonic Shocks in the Magnetosphere
K. H. Lee1*, L. C. Lee1
1Institute of Earth Sciences, Academia Sinica, Nangang, Taiwan
* presenting author:Kun-Han Lee,
Electromagnetic ion cyclotron (EMIC) waves are often observed in the magnetosphere with frequency usually in the H+ and He+ cyclotron bands and sometimes in the O+ band. The temperature anisotropy, caused by injection of energetic ions or by compression of magnetosphere, can efficiently generate H+ EMIC waves, but not as efficient for He+ or O+ EMIC waves. We propose a new generation mechanism for He+ and O+ EMIC waves associated with weak fast magnetosonic shocks, which are observed in the magnetosphere. These shocks can lead to the formation of a “bunch” distribution of O+ ions in the perpendicular velocity plane. The O+ bunch distribution can excite intensive He+ EMIC waves and weak O+ and H+ waves. The He+ EMIC waves dominate in quasi-perpendicular propagation and show harmonics in frequency spectrum of Fourier analysis. The proposed mechanism can explain the generation and some observed properties of EMIC waves in He+ and O+ cyclotron bands in the magnetosphere.

Keywords: fast shock, helium and oxygen cyclotron waves, oxygen bunch distribution