Observations Of ULF Waves In The Earth's Inner Magnetosphere

This paper is firstly based on the multi-satellite detection method to study the ultra-low frequency?ULF?fluctuation events on September 1,2015.Then the two events of January 26,2016 and February 1,2016 are also studied together.The paper is divided into four parts.The first part is an introduction.Firstly,the structure of the magnetosphere is introduced.Then,the ULF waves in the magnetosphere are introduced,including the source and classification of ULF waves,poloidal ULF waves and toroidal ULF waves,the internal and external sources which excite the ULF waves.Then,the research history and current situation in this field are introduced.Then,we introduce the satellite projects of RBSP,MMS and THEMIS.Finally,the organization of the paper is introduced.The second part is the research of the specific observation event.Firstly,the observations of RBSP,MMS,THEMIS and GOES satellites are presented.ULF waves are observed by these satellites from 23:00 UT on August 31 to 18:00 UT on September 1,2015.Then the spatial distribution of the wave observed by these satellites is studied.Between 23:00 UT on August 31 and 02:30 UT on September 1,the distribution of the firstly observed waves by RBSP-B,THMIS-A and GOES-13 is global.Between 06:00 UT and 18:00 UT on September 1,the distribution of the waves observed by RBSP and MMS-1 is localized in L and MLT.Finally,the properties of the ULF wave event,including polarization,harmonic mode and wave number,are introduced.During the period of fluctuation,the polarization state of the event changes with time.As the ratio|Ba|/|Br|decrease from the initial 0.82 to the final 0.13,the polarization of the event changes from the initial mixed mode to the final purely poloidal mode.According to the phase difference between the time series of magnetic field?Br?and electric field?Ea?of MMS-1,the second harmonic poloidal mode is identified.According to the data from three MMS satellites,the azimuthal wave number is determined to be-232.In the third part,the free energy of the ULF wave is studied.The free energy of the ULF wave can be supplied not only by the bump-on-tail ion distributions,but also by the inward radial density gradient.By comparing the absolute values of_?W^?1)|_,and_dW?L^dL?1)|,(2,we can find out which mechanism works or which one plays the leading role.The data of three orbit in four consecutive orbits observed by RBSP show that the inward radial density gradient of the plasma plays a dominant role in the excitation of the ULF wave.The value of plasma radial density gradient is sometimes positive or sometimes negative.The negative plasma radial density gradient provides the free energy needed to excite the ULF waves.The positive plasma radial density gradient can counterbalance the free energy provided by bump-on-tal ion distribution(_?W^?1)|_,>0).In the fourth part,two ultra-low frequency fluctuation events on January 25 and February 22,2016 are studied by using RBSP and MMS combined observations.Both events occurred in the late recovery phase of the magnetic storm,and the fluctuations were distributed between the morning and noon sector.The two events observed by RBSP occurred in the low L-shell,and the two events observed by MMS occurred in the high L-shell.In high L-shell,the polarization of ultra-low frequency wave is mainly circumferential wave,and the fundamental harmonic wave is along the direction of magnetic force line.In the low L-shell,the polarization of ultra-low frequency wave is mainly radial wave and second harmonic along the magnetic line.In this paper,the excitation mechanism of ULF waves is studied by using multi-satellite observations.The results are of great significance for us to understand the generation and propagation of ULF waves and the influence of ULF waves on the radiation and ring current dynamics.