Combined Scattering Effects Of Inner Magnetospheric Waves On Radiation Belt Electrons

The discovery of the Earth’s radiation belts by Explorer 1 in 1958 was a major milestone in geophysics and astronomy,which marked the birth of magnetospheric physics.Owing to continuous accumulations of the high-resolution wave and particle measurements from multiple satellite missions in geospace and the development of the state-of-art modeling during the Van Allen Probe Era,significant advances have been made in understanding the dynamic properties of the energetic electrons and the underlying physical mechanisms in the radiation belts.It has been well acknowledged that wave-particle interactions play an important role in altering electron dynamics.Most previous studies focused on the wave-particle interaction effects of individual wave modes in the inner magnetosphere,while only rather limited investigations looked into the competition and cooperation among various kinds of plasma waves upon their modulation of the radiation belt electron dynamics.This dissertation focuses on the combined scattering effects of inner magnetospheric waves on radiation belt electrons based on both observations and simulations.In the first chapter,we introduce the Earth’s radiation belts,motions of radiation belt electrons in the magnetic field,and multiple plasma waves in the inner magnetosphere including magnetosonic(MS)waves,plasmaspheric hiss,lightninggenerated whistlers,whistler mode chorus,and very-low-frequency(VLF)transmitter waves,and the general wave-particle interactions in the radiation belts including resonant effects and nonlinear effects.In the second chapter,we introduce the Van Allen Probe mission and its wave and electron flux data adopted in this dissertation,and describe the basic wave-particle interactions in the radiation belts in quasi-linear regime.In the third chapter,we first study the combined scattering effects of various plasma waves on the inner belt electrons.Recent studies suggested that human-made ground-based VLF transmitters leaked into the inner magnetosphere can efficiently scatter energetic electrons,bifurcating the inner electron belt,which typically exhibits a single-peak radial structure in the inner belt.Using 6-year measurements from Van Allen Probes during 2013 – 2018,we first present a comprehensive survey of the statistical distributions of the bifurcation of the Earth’s energetic electron(tens of keV)belt in terms of its dependence on geomagnetic activity,electron energy,and season.Our statistics demonstrates that the bifurcation strongly depends on electron energy,which is mostly observed under relatively quiet geomagnetic conditions,with higher occurrence rate during winter than summer.Then,we further report a radially bifurcated electron belt formation at energies of tens of keV at altitudes of ～0.8 – 1.5 Earth radii on timescales over 10 days.Using Fokker-Planck diffusion simulations,we provide quantitative evidence that VLF transmitter waves are primarily responsible for the bifurcation of inner energetic electron belt for the first time.Our results provide quantitative direct evidence to link operations of VLF transmitters at ground to changes of the energetic electron environment in geospace.Identification of the capability of VLF transmitters to precipitate a considerable portion of energetic electron population over a ～10-day period demonstrates a remarkable feasibility of mitigation of energetic electron fluxes.In the fourth chapter,we quantify the electron scattering effects of simultaneous plasmaspheric hiss and MS waves inside the plasmasphere based on both case study and parametric study.We first report a typical event that plasmaspheric hiss and MS waves occurred in two neighboring time intervals but with distinct wave intensity profiles on 21 August 2013 based on Van Allen Probe A.By quantifying the individual and combined scattering effects of electrons by these two waves,we find that their combined scattering is capable of causing electron distribution variations largely distinguishable from the consequences of individual waves.The net effect of electron diffusion strongly relies on the relative dominance of the two wave intensities,which also controls the relative contributions of each wave mode.Then we perform a parametric study to quantitatively investigate the net electron scattering effects and the relative contributions of simultaneously occurring hiss and MS waves with 255 groups of different wave amplitude combinations.Our results show that the combined scattering effects are dominated by pitch angle scattering due to hiss emissions at L =4,when their amplitude is comparable to or stronger than that of MS waves,thereby producing the butterfly,top‐hat,flat‐top,and pancake pitch angle distributions,while the butterfly distributions can evolve over a broader energy range when MS waves join the combined scattering effects.Our results demonstrate that the relative intensities of various plasma waves play an essential role in controlling the radiation belt electron dynamics.In the fifth chapter,we investigate the combined scattering of outer radiation belt electrons outside the plasmasphere by multiple simultaneous plasma waves.We report a typical event that chorus waves,exohiss,and MS waves occurred simultaneously on the dayside observed by Van Allen Probes on 25 December 2013.By combining calculations of electron diffusion coefficients and 2-D Fokker-Planck diffusion simulations,we quantitatively analyze the combined scattering effects of multiple waves to demonstrate that the net impact of the combined scattering does not simply depend on the wave intensity dominance of various plasma waves.Although the observed MS waves have the most intense wave amplitude,the electron butterfly distribution is inhibited by exohiss and chorus,and electrons are considerably accelerated by the combined scattering effects of MS and chorus waves.The simulated electron pitch angle distributions exhibit the variation trend consistent with the observations.Our results strongly suggest that competition and cooperation between resonant interactions with concurrently occurring magnetospheric waves need to be carefully treated in modeling and forecasting the radiation belt electron dynamicsIn the last chapter,we summarize the scientific results obtained regarding the combined scattering effects of inner magnetospheric waves on the Earth’s radiation belt electrons,and propose future efforts to pursue improved comprehensive understanding of radiation belt electron dynamics by effectively and accurately including the combined scattering effects of a variety of magnetospheric waves.