Research On The Excitation Of Electromagnetic Ion Cyclotron Waves In The Inner Magnetosphere And Its Effects On The Plasma Field Line Distribution

Electromagnetic ion cyclotron(EMIC)waves play significant roles in the dynamic process of the inner magnetosphere,including the precipitation of Me V relativistic electrons,the decay of the ring current through gyroresonant pitch angle scattering,the heating of cold electrons through Landau damping,and heating of cold ions through wave-particle interactions.As a common wave mode in the inner magnetosphere,the generation mechanism of EMIC waves and their effects on the terrestrial space environment have drawn more and more attentions in recent years.Based on the satellite observations,theoretical analyses,and hybrid simulations,we have investigated the excitation of EMIC waves and its effects on the plasma field line distribution in the inner magnetosphere.The main results of this thesis are shown as follows:1.The observational evidence of the excitation of EMIC waves driven by the solar wind dynamic pressure directly.We report a case where prompt emergence and disappearance of electromagnetic ion cyclotron(EMIC)waves are driven by the sequentially enhanced solar wind dynamic pressure in the dayside inner magnetosphere on November 6,2015.The measured hot protons(>60 ke V)display perpendicular temperature enhancements during compressions,which provides sufficient temperature anisotropies for the EMIC wave generation.During decompression the magnetosphere is expanded and the anisotropyies of hot ions are resuced,which leads to the attenuation of EMIC waves.Our findings suggest that the external pressure pulses can be a direct driver for wave evolution and energetic particle dynamics at multiple points in the dayside magnetosphere.We also give comprehensive analyses on wave characteristics and particle observations,finding that the waves are likely to be locally excited due to offequator anisotropic proton sources.2.Generation of the second-harmonic of EMIC wavesThere are some stuties about the second-harmonic(SH)of EMIC waves which has double frequency of the fundamental wave(FW)in recent years,but the generation mechanism remains uncovered yet.Through multiscale analysis,previous study finds the generation condition of SH,i.e.,the frequency and the wave number of SH must be twic as much as FW,and the propagation direction of FW and SH is identical.EMIC waves with harmonics are observed by the MMS on on November 7,2018.The bicoherent index shows that the FW and SH are coupled in frequency,which satisfies the double-frequency condition needed by the SH generation mechanism.We obtain the wave numbers by applying phase differencing method to MMS2 and MMS3 and find that the wave numbers also have double relationship.Hence the results of theoretical analyses are verified by observations.The generation of SH is reproduced by the 1-D and 2-D hybrid simulation.By applying the bicoherent index to the simulation results,we also verify the theoretically predicted coupling between fundamental and second harmonic in frequency and wave number.3.The observational evidence of the equatorial mass density peaks triggered by the heating of low energy ions by EMIC wavesThe simutaneous observation of toroidal mode ULF waves and two band EMIC waves by the Van Allen Probes provides us favorable conditions to calculate the field line distribution of mass density.According to the wave frequencies of the ULF waves,the wave equation of toroidal mode wave is solved.We get the local ion composition according to the cutoff frequencies of EMIC waves.Combined with the observations and theoretical analyses,we find a local mass density peak near the magnetic equator.According to the frequency spacing of toroidal ULF waves,we find that the appearances of EMIC waves have good correlation with wider ULF frequency gaps.The fluxes of low energy ions are perpendicular enhanced when EMIC waves coexist,indicating heating of these ions by EMIC waves.According to Liouville’s theorem,the field line distribution of these low energy ions also supports a local mass density peak at magnetic equator.Hence we provide an observational evidence which indicates that the formation of local mass density peaks at the equator should be due to enhancements of equatorial ion concentrations which are triggered by EMIC waves’ perpendicular heating on low energy ions.