Investigation of Quasi-Static Electric Fields and Coupled Plasma Populations in the Inner Magnetosphere

Electric fields are a critical component to understanding the dynamics of plasma in the Earth’s magnetosphere – large-scale electric fields control the shape of the low-energy (~1 eV) plasmasphere, they are responsible for transporting plasma sheet particles (~keV) into the inner magnetosphere, forming much of the trapped energetic particle population, and they affect the high-energy radiation belts (100s keV to MeV) both directly and through coupled interactions with the lower-energy populations. This dissertation focuses on in situ electric field measurements and particle dynamics in the inner magnetosphere with an emphasis on the most inner region below ~4 RE in the equatorial plane, where extensive particle and fields observations have only recently been available through the Time History of Events and Macroscale Interactions during Substorms (THEMIS) and the Van Allen Probes missions. We address the measurement of electric fields using double-probe instruments onboard spacecraft, including some of the unique challenges for making accurate measurements in the inner magnetosphere. Next, we explore the average structure of the quasi-static electric field in the inner magnetosphere using many years of in situ measurements near the equatorial plane from THEMIS. These results reveal a spatially structured electric field with stronger electric fields in the dusk sector, which we connect to a magnetopshere-iononsphere coupling mechanism through a detailed event study. Finally, we relate electric field measurements deep within the inner magnetosphere to 100s keV electron enhancements in the slot region