Bridging the gap between the Earth's ionosphere and plasmasphere

Cold plasma in the ionosphere-plasmasphere system controls various physical processes in the near-Earth space, including energization and loss of particles, coupling mechanisms and propagation of radio waves. The physical processes in the ionosphere and plasmasphere are different but tightly coupled. Ionospheric particles fill the plasmasphere on the dayside, while the plasmaspheric particles descend into the ionosphere at night and help sustain it in the absence of photoionization. However, how exactly these populations evolve and transfer into one another remains poorly understood due to two longstanding challenges. First, the measurements have been separated by a gap at altitudes of ~800-8000 km. Second, fundamentally different measurement techniques have been used in the two regions, and they have never been compared. We address both of these issues and show that two of the most prominent techniques to measure Ne in the two regions - using wave spectra and radio occultations - yield near-identical results. Using observations by the Arase mission at altitudes 440–32,000 km, we demonstrate that it is possible to bridge the gap between the ionosphere and plasmasphere. This result opens many opportunities for studying fundamental scientific questions that relate to the lifecycle of cold plasma in geospace, as well as coupling with the thermosphere and magnetosphere, influence of cold plasma on magnetic reconnection, and setting the inner boundary conditions for physics-based simulations, including global magnetospheric MHD codes. We additionally discuss the current state and the future improvements of empirical and physics-based models of the ionosphere-plasmasphere system.