Characteristics, generation, and role of chorus waves in the Earth's radiation belts: Observations and simulations

This dissertation evaluates characteristics, propagation properties, and generation mechanism of whistler-mode chorus waves, investigates electron distributions responsible for the chorus excitation, and quantifies roles of three important types of plasma waves in radiation belt electron dynamics. Two different sources of spacecraft data including THEMIS and CRRES, together with simulations by a ray tracing code and a 2-D diffusion code, have been utilized. By performing a new global survey of chorus wave distributions using data from the THEMIS spacecraft in the near-equatorial orbits, we find that nightside chorus is confined closer to the Earth with lower occurrence rates, while dayside chorus can extend to larger radial distance with higher occurrence rates. In order to understand the day-night asymmetry of chorus distributions, the generation of chorus on the nightside during injection events is modeled using a ray tracing code (HOTRAY) and compared to satellite observations (CRRES and THEMIS). Simulation results show that nightside chorus waves are confined near the equator and become attenuated in the mid-latitudes due to the strong Landau damping. Furthermore, electron distributions responsible for the chorus generation are analyzed using the THEMIS data, since chorus excitation is thought to be caused by cyclotron resonant instability of anisotropic electron distributions. Statistical results show that on the nightside, large electron anisotropies and intense chorus emissions are confined within 8 RE with a remarkable consistency. Furthermore, as injected plasma sheet electrons drift from midnight through dawn toward the noon sector, their anisotropy increases and peaks at 7 ≤ L ≤ 9 on the dayside, due to combined effects of drift-shell splitting and scattering by waves. In addition, we evaluate the specific roles of three important types of plasma waves in radiation belt electron dynamics and conclude that chorus leads to both acceleration and loss processes, while electromagnetic ion cyclotron (EMIC) waves and hiss waves contribute to electron losses. The combination of observation and simulation results in this dissertation provides a major advance in the understanding of the characteristics, generation, and role of chorus waves in the Earth's radiation belts.