Research On Nonlinear Excitation And Saturation Of Whistler Waves And ECH Waves In The Magnetosphere

The magnetosphere is generated by the earth’s dipole field,which is the outmost shell to protect the earth from the solar wind.The magnetosphere is dominated by ionized particles(the so-called plasma),and it has various complicated physical processes.The magnetospheric environment is essential to the solar-terrestrial environment by linking solar radiation and solar wind to the earth’s ionosphere and atmosphere.Besides,the radiation in the magnetosphere directly affects the orbiting spacecraft.There are various types of plasma waves in the magnetosphere,including electromagnetic and electrostatic waves.These waves can interact with charged particles,leading to their acceleration and scattering.The accelerated energic particles are hazardous to the spacecraft instruments,and the scattered particles could precipitate into the atmosphere to form auroras,leading to the energy transport down to the ionosphere and atmosphere.In the past decades,linear and quasilinear theories have studied the formation mechanism,distribution properties,and impact on the space environment of these plasma waves.However,there remains uncertainty in the nonlinear physical processes.Consequently,we focus on one of the most challenging but critical nonlinear problems,i.e.,the nonlinear excitation and saturation of the magnetospheric plasma waves.Our investigation is mainly based on Particle-In-Cell(PIC)simulation because it is a self-consistent numerical method that resolves all the nonlinear plasma phenomena.In this dissertation,we mainly discuss the nonlinear excitation and saturation of two types of plasma waves:whistler mode waves and Electron Cyclotron Harmonic(ECH)waves,while the whistler mode waves are further categorized as broadband whistler waves and chorus waves.Firstly,we investigated the saturation properties of broadband whistler waves in a plasma with both cold and hot electron components,including plasma parameters and wave properties at saturation.Under low plasma β,We confirmed the previously proposed anti-correlation between electron temperature anisotropy(A)and β‖.However,we found the relation significantly different under high β conditions.Besides,we found that the saturation amplitudes of the waves are positively correlated to the initial linear growth rates and can be roughly described by a power law function.The research provided a reliable estimation of the upper bound of the equilibrium plasma parameters,and it may benefit the global magnetosphere models by linking initial plasma parameters to the saturated wave properties.Secondly,we went into the nonlinear frequency chirping of chorus waves.We for the first time found the controlling role of the background magnetic field topology in the direction of the chorus chirping.That is,rising tone chorus waves were generated in the dipole field,falling tone chorus in the reversed dipole field,while both rising and falling tone chorus in a uniform magnetic field.The finding inspired the new theoretic framework,the TaRA model,which greatly improved our understanding of the nonlinear chirping of chorus waves.Further,we investigated the nonlinear phase space structure correlated to the falling tone chorus and found it consistent with nonlinear theories.Besides,we studied the location of the source region of chorus waves and found that the source was not fixed but moving.Moreover,the motion of the source region was related to the change in the chirping rate,consistent with the TaRA model.The research on chorus waves revealed the underlying mechanism and provided a reasonable explanation for the observed direction of chorus chirping and variation of chirping rate in a single chorus element.Thirdly,we for the first time succeeded in generating ECH waves through loss cone distribution in PIC simulation and analyzed the excitation and saturation processes.We found the excitation of ECH waves is consistent with linear theory in terms of the wave frequency,the growth rate,and dispersion relation.On the other hand,the saturation of ECH waves is caused by the scattering of electrons and the refilling of the loss cone,which leads to the decrease of the linear growth rate in turn.Compared with previous simulation studies of ECH waves which used ring distribution electrons as linear drive,our saturation mechanism is different,because the heating of cold electrons was thought essential to the saturation of ECH waves,but the heating was negligible in our simulation.Our studies enhanced the understanding of nonlinear excitation and saturation of the three plasma waves above,and the results might also apply to other plasma waves.The studies improved the nonlinear plasma wave theory,expanded our knowledge about basic plasma physics,and could be potentially helpful to the forecast of the magnetospheric environment.