The results of a computer simulation of the evolution of the plasmasphere (dark grey region) during a 12-hour period following the onset of a geomagnetic storm. The view is of the equatorial plane as seen from a position above the Earth's north pole. The Sun is to the right. The dashed line indicates the position of the magnetopause; the solid line identifies the location of geosynchronous orbit. After one hour, increased convection has begun to transport plasma in the dusk-afternoon sector outward and sunward toward the magnetopause. After 6 hours, the plasmasphere has contracted, and a broad region of entrained plasma extends to the magnetopause and spans most of the afternoon-dusk sector at geosynchronous orbit. As convection weakens, this extended plume narrows and begins to rotate eastward with the Earth (panel d). Although the model illustrates the gross features of plasmaspheric evolution, it does not capture the complex structure of the outlying plasma regions that have been eroded from the main body of the plasmasphere.