A new approach to tracking the boundary of the plasma-sphere (i.e., the plasmapause) in a time-series of satellite images of the Earth is described. The approach is based on the active contour models (snakes) and exploits prior knowledge of the plasmasphere to automatically initialize and refine the snake's position in each image. It then uses a greedy minimization scheme to drive the snake toward the plasmapause in each image. A Voronoi diagram is used to restrict the snake evolution to ensure that the snake does not loop over itself. Correspondences between successive images are established using a radial alignment process. The approach aids in quantifying plasmasphere changes, which are key indicators of the impact of solar events on the Earth's magnetosphere.