Observational And Numerical Study Of Electromagnetic Ion Cyclotron Waves And Whistler-mode Chorus Waves

Both electromagnetic ion cyclotron(EMIC)wave and whistler-mode chorus wave are common emissions in the Earth's inner magnetosphere,which are believed to play important roles in regulating particle dynamics in the radiation belt.In this study,by analyzing the data from Van Allen Probes and POES satellites,we have investigated the typical characters,source regions,and wave-particle interactions of EMIC waves.Besides,by using particle-in-cell(PIC)simulation models,we have also tried to figure out the physical process underlying the formation of power gaps around 0.5 ?e in the whistler-mode waves.Our principal conclusions are summarized as follows:1.The substorm injection and solar wind dynamic pressure are different drivers of EMIC wave excitation.With the data from Van Allen Probes satellites,we have investigated the typical characters and the source region of EMIC waves.The H+band(fcHe+<f<fcH+)EMIC waves usually occur at relatively larger L-shells in the noon sector,with very small wave normal angles and weakly left-hand polarization.He+band(fcO+<f<fcHe+)waves are preferentially detected in the predawn and morning sector at relatively smaller L-shells.These waves have moderate wave normal angles and left-hand polarization,and their magnetic amplitudes are comparable with or even stronger than those of H+ band waves.While the occurrence rate of O+band(f<fcO+)waves is the lowest,which only have a little higher occurrence rate in the pre-dawn sector at small L-shells.These waves are observed to have either very small or very large wave normal angles,with linear or weakly right-hand polarization.The amplitudes of O+band waves seem to be independent of the geomagnetic activities.Moreover,we have separated investigated the roles of substorm injection and the enhancement of solar wind dynamic pressure in the excitation of EMIC waves.During substorms,EMIC waves are mainly generated in the dusk sector near the magnetic equator.While during the periods of large solar wind pressure,the excitation of EMIC waves mainly occurs in the noon sector,but there are also higher-latitude source regions besides the magnetic equator.2.EMIC wave can cause the heating of heavy ions in the perpendicular direction,and is the most important driver of the relativistic electron precipitations in the noon and dusk sectors.We have presented a harmonic EMIC wave event,which contains a fundamental wave and five harmonic waves.The frequencies of these harmonic waves are n times that of the fundamental wave,where n=2,3,4,5,6.The fundamental waves are excited by anisotropic protons,while the harmonic waves are generated due to the coupling between the electromagnetic components and electrostatic components of EMIC waves.This harmonic EMIC wave can modulate the ion pitch-angle distribution through cyclotron resonance and then preferentially heat heavy ions and?keV protons in the perpendicular direction.With the data from POES satellites,we have investigated the global distribution and potential drivers of relativistic electron precipitations(REPs).The events are mainly detected at L=4-6 in the dusk and midnight sectors.Moreover,the dependence of the REP events on the |Dst×Pdd|*and AE*indices has been separately considered,which are defined as the maximum values of |Dst×Pd| and AE in the previous 1 hour.During the low |Dst×Pd|*but large AE*the events are preferentially detected in the dusk sector.While during the low AE*but large |DstxPd|*?they are mainly in the noon and midnight sectors.We have further demonstrated that the EMIC wave is a dominant driver of the REPs in the noon and dusk sectors,while the highly curved field line plays a dominant role on the nightside.3.Verify the lower band cascade theory,and investigate the change of wave number and amplitude ratio during wave evolution.One of the most remarkable properties of whistler-mode waves is the power gap around 0.5 ?e,which can separate their spectrogram into a lower band(0.1-0.5?e),and an upper band(0.5-0.8?e).With a 1-D PIC simulation model,we have investigated the lower band cascade in the whistler-mode waves.These lower band waves are firstly excited by the anisotropy of thermal electrons.The upper band waves are subsequently generated due to the coupling between the electromagnetic and electrostatic components of lower band waves,whose frequencies are twice those of lower band waves.The peak wave number of lower band waves gradually drifts to smaller values due to the decline of the anisotropy,while the peak wave number of upper band harmonic waves remains almost unchanged.Consequently,upper band waves are no longer the second harmonic of lower band waves.Moreover,the amplitude ratio between the upper band and lower band waves is positively correlated with the wave normal angle,but is anticorrelated with the anisotropy of electrons.The parameter survey shows that the banded waves can be formed when the dominant frequency is in the range of-0.3-?0.4 ?e.4.The plateau electron component is the key factor of the wave damping around 0.5 ?e,which can cause the power gap in the whistler waves.With both 1-D and 2-D PIC simulation models,we have investigated the damping of whistler-mode waves around 0.5?e due to the plateau electron component.When the frequencies of the excited waves cross 0.5 ?e,or when they are slightly larger than but then drift to smaller than 0.5 ?e,there is a pronounced plateau component at about±0.5VAe,since the waves are not strictly parallel propagating and the Landau resonance will take effects.Such a component can cause severe damping around 0.5?e via cyclotron resonance,and the energy transfer is mainly in the perpendicular direction.Eventually,the power gap around 0.5 ?e forms.Parameter analysis shows that the power gap can be generated by such a process when the dominant frequency ranges from-0.5-?0.6?e.We have further found that the position of plateau shape(VD)can play the most important role in determining the frequency of the power gap,which decreases as VD increases.Besides,the beam-like shape in one direction can lead to the formation of a frequency gap in the waves propagating in the opposite direction.