On The Generation Of Chorus And Its Interaction With Electrons

There is a dipole magnetic field around the Earth.Both electrons and ions from the solar wind plasmas will be trapped by this dipole field when they come to the Earth.These trapped particles formed the radiation belts and magnetosphere.The magnetosphere of the Earth is largely controlled by the solar wind and exhibits high dynamic processes,such as diffusion,convection and wave-particle interaction.Chorus waves,which consist of discrete elements with frequency chirping,play an important role in these processes.One of the most interesting features of chorus waves is that their spectrograms usually exhibit a structure of two frequency bands with a power minimum near 0.5Ωe,where Ωe is the electron gyro-frequency.Chorus can be resonant with electrons and this mechanism can explain many phenomena in the radiation belts.Chorus waves have been widely studied for these unique characters since they were discovered.However the detailed physical description of wave-particle interaction,as well as the generation mechanism of chorus waves,are still unsolved.This work focuses on these two questions and studies generation of chorus wave under different magnetic configurations.Firstly,we derived the equations of motion of the electrons in the phase space in the presence of chorus waves.We found that there are mainly two factors which deter-mine the motion of electrons:the wave field and the mirror force of the ambient magnetic field.If the wave field is dominant,the electrons will experience phase trapping or phase bunching in the phase space.If the mirror force is dominant,the electrons will experience an adiabatic process.The competitive relation of the two forces is sensitive to the initial condition of electrons.A lot of research work about the initial conditions has been done,such as studies on the initial launching latitude and pitch angle.However the influence of initial gyro-phase has not been revealed yet.In this paper,test particle method is applied to study the influence of initial gyro-phase on the motion of electrons.It can be seen that some electrons with special initial gyro-phase will be trapped first under a given wave amplitude.Then more electrons with their initial gyro-phase around this special phase be-come trapped when the wave amplitude increases.By studying the phase space trajectory of three typical electrons,we find the electrons trapped at first have a smaller trap radius than the electrons trapped later when the wave amplitude becomes large.Test particle method is not self-consistent,because the electrons have no feedback to the waves.Then we use a new method solving the Maxwell equations self-consistently to study the interaction between electrons and chorus waves.The electrons are isotropic in the terrestrial magnetophere,leading to the whistler instability.This instability is be-lieved to be the cause of chorus.We derive the linear growth rate of the whistler instability and find that the starting frequency of chorus is about the frequency with maximum linear growth rate by a group of numerical experiments.Then we use a two-component electron-s with different temperatures to excite chorus in an inhomogeneous mirror field.Chorus with a gap at 0.5 Qce is generated and the results are in agreement with observations.We found that the gap can be anywhere by changing the temperatures of the two-component electrons.So we conclude that the position of the chorus gap is determined by the linear properties of electrons.The observed electrons with temperature about～keV are isotrop-ic in the radiation belts,and the location of the gap of chorus waves excited by electrons with this distribution is 0.5 Qce.Then we simulated the generation process of chorus in a launching device named Dipole Research EXperiment Device(DREX).The simulation results show that chorus can be generated under the designed parameters of DREX and the generation process of chorus in the DREX device is the same with that in the magnetosphere.We studied the influences of three parameters(namely the cold plasma density,the hot plasma density and the parallel thermal velocity)on the generation of chorus,and got the optimal value of the three parameters for chorus generation in DREX.Our conclusion can be the theoretical basis for the future experiments on the DREX device.Falling tone chorus is generated in the PIC simulation for the first time.An electron hill is formed in the phase space and its position is coincident with the theory.We found that falling tone may be a result of propagation effect by studying the propagating properties of the excited falling tone chorus.In the last chapter,we studied the generation of chorus on the Saturn and Jupiter.Comparison of chorus waves on the two planets with the Earth is also carried out.Chorus wave are commonly observed on the outer space of these three planets.The radius and magnetic strength of these three planets are different,thus the chorus may have different properties among the three planets.We discussed the influences of radius and magnetic strength on chorus.By varying the radius and fixing magnetic strength,we found that the sweep rate,frequency range and intensity of the excited chorus decrease with the increase of radius.When varying the magnetic strength and fixing the radius,we found that the sweep rate of chorus increases with the increase of the magnetic strength.So we predict that the sweep rate of chorus observed on the Jupiter is lager than that on the Earth because of a lager magnetic strength on the Jupiter.