Abstract
The plasmasphere is a region of cold (less than ~ 1 - 2 eV), dense (~ 10 - 10^4 electrons cm^-3), plasma that encircles the Earth and extends to geocentric equatorial distances ranging from ~ 2 to 7 Earth radii (RE). At the outer boundary of the plasmasphere, there typically exists a sharp gradient in the equatorial electron density profile known as the plasmapause. The size and shape of the plasmasphere vary considerably due to recurring geomagnetic storm and substorm activity.

The plasmasphere erosion and recovery processes have been studied but only from the relatively limited perspective of individual (or, occasionally, multiple) ground stations and satellite crossings of the plasmapause and plasmasphere. The global-scale, multi-hour images of the plasmasphere now available from the Extreme Ultraviolet Imager (EUV) of the IMAGE mission allow us to address ongoing and fundamental questions about the structure and dynamics of the plasmasphere system.

The EUV instrument images the He+ distribution in the Earth's plasmasphere by detecting resonantly scattered solar 30.4-nm radiation and produces images with ~ 0.1 RE spatial resolution every 10 minutes. Tracking the location of the plasmapause in sequences of EUV images allows quantitative measurements of radial and azimuthal motions of the boundary during various phases of geomagnetic disturbances. Specifically, we show inward motions and steepening of the plasmapause boundary on the nightside of the Earth, the formation and evolution of plasmaspheric drainage plumes, and the tendency for the development of mesoscale azimuthal irregularities in the plasmapause radius in a specific local time sector. In addition, we calculate the total amount of He+ (and thus infer the total amount of plasma) removed from the plasmasphere during specific periods of enhanced geomagnetic activity.