The Earth is surrounded in space by a torus-shaped cloud of ionized gas (or plasma) called the plasmasphere. The plasmasphere arises from outflow of plasma from the ionosphere, into space along Earth's magnetic field lines. The outer edge of the plasmasphere, known as the plasmapause, is an ever-changing boundary whose precise location and shape depend upon electromagnetic conditions in Earth's space environment (geospace). Active conditions in geospace (known as space storms) cause erosion of the outer layers of the plasmasphere, and quiet conditions allow ionospheric outflow to refill the depleted layers. Storm-time plasmaspheric erosion has two very concrete and dramatic effects on space weather, the component of geospace activity that affects human activities and technology. The first space weather effect involves a side-effect of plasmaspheric erosion. When the plasmasphere is eroded, the Earth's outer Van Allen (or radiation) belt contracts Earthward, bringing hazardous radiation in closer proximity to our array of military and communications satellites, and limiting the scope and duration of extravehicular activities by astronauts on the international space station (ISS). This contraction of the outer Van Allen belt occurs because normally the energization processes that create the outer Van Allen belt are severely hindered by the presence of near-Earth plasmaspheric plasma. Thus, erosion of the outer plasmasphere allows near-Earth energization to proceed unhindered, causing enhanced levels of energetic particles at lower altitudes and latitudes. The second space weather effect of plasmaspheric erosion involves the erosion process itself. During erosion events, plumes of eroding plasmaspheric plasma can extend several tens of thousands of km into space. These plumes leave a characteristic signature in the ionosphere that often extends northwest across the U.S. and Canada. This ionospheric signature interferes with the clear transmission of signals from the Global Positioning System (GPS) satellites, either by bending or scattering (scintillating) the signals. This interference can result in GPS range errors of tens of meters, enough to affect navigation systems used by private automobiles, commercial airlines, and the military. Thus, in a modern civilization which relies increasingly upon space-based technology for everyday civilian and national defense activities, it is of compelling importance to understand and mitigate the two main plasmaspheric space weather effects.