Just beyond the ionosphere lies the plasmasphere, a ring of cold plasma that encircles the planet on closed geomagnetic field lines. Built up from charged ions ejected from the upper ionosphere at equatorial and midlatitudes, the plasmasphere grows and shrinks in response to the solar wind and the dynamic magnetosphere. How the plasmasphere responds to magnetic storms can in turn affect the behavior of the radiation belts. Working from a model of the ionosphere known as SAMI3, Huba and Krall have developed the first comprehensive three-dimensional model of the plasmasphere that is built up from first-principles physics. The authors' plasmasphere model is a simplified dynamic model that seeks to represent a region stretching out to roughly eight Earth radii.