Theory Analysis And Simulation Research On The Generation Of Electromagnetic Ion Cyclotron Waves Influenced By Thermal Ions

Electromagnetic ion cyclotron(EMIC)wave is an important plasma waves in inner magnetosphere,which can cause pitch angle scattering and energy variation of charged particles through resonant interaction with them.The generation of EMIC wave is influenced by many space environmental factors,such as magnetic field structure,plasma density gradient,ion implantation and magnetic storm activity,among which the influence of thermal ion composition is very important.Based on the observation data of RBSP satellite,the influence of thermal ions on the generation of EMIC waves is studied by using linear theory and hybrid simulation method,and the influence of thermal hydrogen ions on the amplitude threshold of rising tone EMIC wave generation is analyzed by using the nonlinear theory.Firstly,69 EMIC wave events observed by RBSP-B satellite from August 2017 to February 2018 are extracted.Satellite observations show that the fluctuations are more easily excited at the plasmapause boundary and external region,and there are fluctuations during the occurrence of substorms and geomagnetic calm periods.In the linearity theory,we treat the thermal hydrogen ions with energy less than 200 eV approximately as cold plasma,and take the satellite observations data for other thermal ions.The calculated wave generation frequency bands are in good agreement with the satellite observations.In this paper,the effect of thermal ions on generation of EMIC wave in oxygen band is studied.The results show that temperature anisotropy of thermal oxygen ions(O+),thermal helium ions(He+)and thermal hydrogen ions(H+)can excite EMIC wave in the oxygen band,and the corresponding wave frequencies increase in turn.When the temperature anisotropy or concentration ratio of different kinds of thermal ions increases,the growth rate of oxygen band waves stimulated by them increases correspondingly.In addition,the intensity of helium and hydrogen band waves can be weakened by increasing the proportion of thermal heavy ions,whereas the intensity of helium and hydrogen band waves can be enhanced by increasing the proportion of thermal hydrogen ions.It is shown that the variation of thermal ion composition plays an important role in modulating the wave energy distribution in different frequency bands.Using hybrid simulation method,we simulated the process of helium band EMIC wave excited by thermal hydrogen ion with temperature anisotropy of 0.5.It was found that the higher the concentration of thermal hydrogen ion,the greater the intensity of helium band wave,which is completely consistent with the results of linear theory.Finally,the generation process of rising tone EMIC wave is studied by using the nonlinear theory.It is found that the increase of the concentration or concentration ratio of thermal hydrogen ion can reduce the amplitude thresholds of helium and hydrogen band waves,more conducive to wave generation.The influence of temperature anisotropy of thermal hydrogen ion is more complex.For example,when the temperature anisotropy of thermal hydrogen ion increases,the amplitude threshold near the oxygen ion cyclotron frequency side will be increased,while the amplitude threshold near the helium ion cyclotron frequency side will be reduced.Various research methods have shown that the state of thermal ions has an important influence on the excitation of electromagnetic ion cyclotron waves in different frequency bands.Accurate understanding of the characteristics of thermal ions is helpful to understand the evolution law of the key parameters such as the frequency,growth rate and wave amplitude of electromagnetic ion cyclotron waves in the inner magnetosphere.