Time: Thursday, April 6th, 03:30pm
Venue: Rm. 413, Bld. 11#, Yuquan Campus
Title about: Generation of helium and oxygen EMIC waves by the bunch distribution of oxygen ions associated with fast magnetosonic shocks in the magnetosphere
Speaker: 李罗权院士,Institute of Earth Science, Academia Sinica, Nankang, Taiwan
Abstract:
Electromagnetic ion cyclotron (EMIC) waves are often observed in the magnetosphere with frequency usually in the proton and helium cyclotron bands and sometimes in the oxygen band. The temperature anisotropy, caused by injection of energetic ions or by compression of magnetosphere, can efficiently generate proton EMIC waves, but not as efficient for helium or oxygen EMIC waves. Here we propose a new generation mechanism for helium andoxygen EMIC waves associated with fast magnetosonic shocks, which are observed in the magnetosphere. These shocks can be associated with either dynamic pressure enhancement or shocks in the solar wind and can lead to the formation of a “bunch” distribution in the perpendicular velocity plane of oxygen ions. The oxygen bunch distribution can excite strong helium EMIC waves and weak oxygen and proton waves. The dominant helium EMIC waves are strong 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 helium and oxygen EMIC waves in the magnetosphere.
