The extreme ultraviolet imager investigation for the IMAGE mission

The Extreme Ultraviolet Imager (EUV) of the IMAGE Mission will study the di stribution of He+ in Earth's plasmasphere by detecting its resonantly-scatt ered emission at 30.4 nm. It will record the structure and dynamics of the cold plasma in Earth's plasmasphere on a global scale. The 30.4-nm feature is relatively easy to measure because it is the brightest ion emission from the plasmasphere, it is spectrally isolated, and the background at that wa velength is negligible. Measurements are easy to interpret because the plas maspheric He+ emission is optically thin, so its brightness is directly pro portional to the He+ column abundance. Effective imaging of the plasmaspher ic He+ requires global 'snapshots' in which the high apogee and the wide fi eld of view of EUV provide in a single exposure a map of the entire plasmas phere. EUV consists of three identical sensor heads, each having a field of view 30 degrees in diameter. These sensors are tilted relative to one anot her to cover a fan-shaped field of 84 degrees x30 degrees, which is swept a cross the plasmasphere by the spin of the satellite. EUV's spatial resoluti on is 0.6 degrees or similar to 0.1 R-E in the equatorial plane seen from a pogee. The sensitivity is 1.9 count s(-1) Rayleigh(-1), sufficient to map t he position of the plasmapause with a time resolution of 10 min.