The plasmasphere is a low β plasma, meaning that magnetic forces dominate thermal forces, and the plasma is effectively “tied” to the geomagnetic field. Even as the geomagnetic field interacts with the solar wind, field lines closest to the Earth remain “closed.” The onset, propagation, and saturation of plasma waves in the plasmasphere is known to have a significant impact on the radiation belts and the energetic ring current ions. Specifically, electromagnetic ion cyclotron waves, plasmaspheric hiss, and ultralow frequency waves can play a critical role in the evolution of radiation belt particles. Diurnal oscillations in the plasmasphere density versus time at fixed local time, if present, indicate that the plasmasphere has a density structure that is rotating with the Earth. The plasmaspheric mass density is important because of its impact on space weather. Radiation belt energetic particle populations are very sensitive to the core plasmasphere distribution and to the position of the plasmapause.