Scattering Effects Of Electromagnetic Ion Cyclotron Waves On Radiation Belt Electrons And Ring Current Protons

Electromagnetic ion cyclotron(EMIC)waves,commonly distributed in the inner magnetosphere,can be categorized into three different bands by the ion cyclotron frequencies:H~+band,He~+band,and O~+band EMIC waves.Distinct from other plasma waves,EMIC waves have relatively large amplitudes and can rapidly scatter both radiation belt electrons and ring current protons into the atmosphere through wave-particle interactions.The loss of ring current protons can contribute to the formation of isolated proton auroras in the sub-auroral region.Therefore,EMIC waves play a significant role in the particle dynamics in Earth’s magnetosphere.Although numerous studies have investigated the wave properities,the global distributions,and the scattering effect on particles of EMIC waves,there are still some important problems to be considered.This dissertation aims to study the combined scattering effect on radiation belt electrons,and investigate the evolution of proton energy spectra driven by multi-band EMIC waves.The quasi-linear theory,the test particle simulation,and the observational particle data are utilized to further understand the influence driven by EMIC waves.The principal contents and conclusions of this dissertation are summarized as follows:1.Both quasi-linear theory and test particle simulations have been applied to evaluate the scattering effects in the form of diffusion rates and single particle tracing.Although the analyses on resonant conditions indicate that H~+and He~+band EMIC waves can simultaneously scatter electrons with the same energies and pitch angles at the same latitude,the simulation results demonstrate that scattering effect driven by different wave modes can be linearly superposed.To further verify and generalize this conclusion,we change the background ion composition,and adopt the broadband EMIC waves with amplitudes(1,5,and 10 n T)and monochromatic EMIC waves with amplitudes ranging from 0.1 to 10 n T to investigate both linear and nonlinear resonant scattering effects.We conclude that the combined scattering rates driven by simultaneous two-band waves are consistent with the linear superposition of independent scattering rates caused by single-band waves.In conclusion,our results suggest that the linear superposed scattering results can effectively evaluate the combined resonant scattering effects driven by two-band EMIC waves.2.Based on quasi-linear theory and kinetic simulation,the resonance region between EMIC waves and ring current protons are displayed,indicating that EMIC waves tend to scatter lower energy protons with increasing frequencies and L-shells.Then we quantify the scattering diffusions driven by H~+and He~+band EMIC waves on ring current protons,and we simulate the associated time scale of proton loss.We find that protons with energy less than～10 ke V tend to be scattered by H~+band EMIC waves,while He~+band EMIC waves are mainly contributing to protons with energy greater than～10 ke V.The scattering effects on proton energy spectra are simulated by solving the two-dimensional Fokker-Plank diffusion equation.The simulation results show that EMIC waves lead to a new form of proton spectra.Using the initial proton flux data observed by Van Allen Probes and the scattering coefficients driven by EMIC waves,we simulate the evolution of proton energy spectra at different pitch angles.The comparisons between observational and simulated results show that EMIC waves can change the proton energy spectra at low and middle pitch angles from the monotonically decreasing trend to the reversed trend,providing a possible mechanism for the formation of reversed proton energy spectra.3.Based on high-quality Van Allen Probes RBSPICE measurements,we investigate the global distribution of the reversed proton energy spectra using the data from January 1,2013 to July 15,2019.Our results show that the reversed proton energy spectrum is prevalent inside the plasmasphere,with the occurrence rates>90%at L～2–4 during geomagnetically quiet periods.The occurrence also manifests a significant decrease trend with increasing L-shell and enhanced geomagnetic activity,which is on account of the connection between plasmaspause location and geomagnetic activity.The reversed proton energy spectra are characterized by the distinct flux maxima around 200–400 ke V,flux minima around 50–100 ke V,and the ratio between flux maxima and flux minima around 3–30.All of the three parameters show a decreasing trend with the increasing L-shells.Based on the statistical results,we further discuss the possible mechanisms that lead to the formation of reversed proton energy spectra,including the wave-particle interactions,field curvature scattering,charge exchange,and Coulomb collision.This study is benefit to further understand the loss of high-energy electrons and the evolution of ring current protons in the inner magnetosphere,and is of great significance to the dynamic changes of inner magnetosphere particles and space weather.