| Accurately understanding and modeling the transport process of charged particles in collisionless plasmas is one of the fundamental issues in space science.Although the scattering process of particle energy and pitch angle caused by the resonant interaction between charged particles and plasma waves is one of the most effective ways to affect particle transport,however,other physical processes may still cause the effective transport of charged particles under different conditions.This paper focuses on two particular and important physical processes:the nonlinear resonance of charged particles caused by large amplitude waves,and the magnetic field line curvature scattering in the absence of waves.The first part of this thesis is about nonlinear wave-particle interactions.The classical theory that quantitatively describes the resonant interaction between charged particles and plasma waves is quasi-linear diffusion theory,in which the effect of waves on particles is regarded as a diffusion process,and the evolution of phase distribution of particles with time can be obtained by solving the diffusion equation.However,the quasi-linear theory has its limitations due to the assumption of waves being small amplitude and incoherent.The diffusion coefficients are calculated along the unperturbed orbits of particles.The well-known difficulties encountered by quasi-linear theory include the estimation of particle diffusion coefficients for particles with pitch angle near 90° or when the plasma wave amplitude is large.Here we use a relativistic test particle simulation model in a uniform background magnetic field to study the cyclotron resonant interactions between relativistic electrons and parallel propagating whistler mode waves.First,we verify that the quasi-linear theory can accurately describe the pitch angle and energy scattering of relativistic electrons when its assumptions are met.Second,we show the inconsistency between the quasi-linear theory expectation and the test particle simulation results for pitch angles near 90° or when the wave amplitude is large,and analyze the limitations of the quasi-linear diffusion theory.After analyzing the previous theoretical work,we test a nonlinear resonance broadening theory by Karimabadi et al.and use it to describe the diffusion behavior of relativistic electrons in the above cases.Finally,we find that the test particle simulation results in general agree with the results of the nonlinear resonance broadening theory.However,the resonance width in nonlinear theory needs further study to better describe the diffusion process of particles.Thus,our results show that taking the nonlinear resonance broadening effect into account in the radiation belt modeling is helpful to improve the accuracy of the relativistic electron dynamics description,especially in the active period when large amplitude waves are frequently observed.In the second part of this thesis,the magnetic field line curvature scattering effects are studied.In the Earth’s magnetosphere,the effects of field line curvature scattering have important effects on the distribution of energetic electrons in radiation belts,the precipitation of ring current ions,and the formation of the isotropic boundary of ions.The single and multiple jumps of the magnetic moment are studied by test particle simulations,and the multiple jumps of the magnetic moment are regarded as a diffusion process to calculate the curvature scattering diffusion coefficients of test particle.The diffusion coefficients obtained from the simulation are compared with those obtained from Birmingham’s non-adiabatic theory to verify the correctness of the theory in describing the curvature scattering process.However,the role of curvature scattering in particle transport in other planetary space environments has not been thoroughly studied.Among the several planets in the solar system,the space plasma environment of Mars is unique and complex due to the presence of crustal magnetic field and the lack of a global magnetic field.In previous observations,the Martian ion precipitation flux is significantly enhanced at the closed and radial magnetic field configuration of the crustal magnetic field,suggesting that the precipitation ions are most likely from particle pitch angle scattering.To test the hypothesis that this pitch angle scattering is caused by curvature scattering effects,we carried out further quantitative studies.First,based on a spherical harmonics model of the Martian crustal magnetic field,we trace and obtain the global magnetic field lines from planes at different heights,and select the completely closed magnetic field lines.Second,for different species and energies of ions,the adiabatic coefficients are calculated and the corresponding curvature scattering rates are obtained according to Birmingham theory.Finally,we investigate the relationship between crustal magnetic magnitude,elevation angle,and ion scattering rate and compare it with statistical observations of ion precipitation fluxes.Our results show that the time scale from curvature scattering of energetic pick-up protons is fast enough to reach 10s,which provides some basis for the hypothesis that curvature scattering can cause ion precipitations.For heavy ions such as oxygen ions,the pitch angle scattering rate is significantly related to the magnetic field magnitude,and the curvature scattering rate is lower in the region with the stronger crustal magnetic field.The comparison with the statistics of ion precipitation fluxes shows that the ion precipitation in the regions of medium magnetic field magnitude and small magnetic elevation angle may be due to curvature scattering effects.This work should be useful to further understand particle precipitation at Mars. |
