The Extreme Ultraviolet Imager Investigation for the IMAGE Mission B. R. Sandel, A. L. Broadfoot, C. C. Curtis, R. A. King, T. C. Stone Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA R. H. Hill, J. Chen Baja Technology LLC, 1040 E. Fourth St., Tucson, AZ 85721-0077 O. H. W. Siegmund, R. Raffanti Siegmund Scientific, Walnut Creek, CA 94595 DAVID D. Allred, R. STEVEN Turley Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602} D.L. Gallagher Space Science Department, NASA Marshall Space Flight Center, Huntsville, AL 35812 The Extreme Ultraviolet Imager (EUV) of the IMAGE Mission will study the distribution of He+ in Earth's plasmasphere by detecting its resonantly-scattered 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 wavelength is negligible. Measurements are easy to interpret because the plasmaspheric He+ emission is optically thin, so its brightness is directly proportional to the He+ column abundance. Effective imaging of the plasmaspheric He+ requires global "snapshots" in which the high apogee and the wide field of view of EUV provide in a single exposure a map of the entire plasmasphere. EUV consists of three identical sensor heads, each having a field of view 30 degrees in diameter. These sensors are tilted relative to one another to cover a fan-shaped field of 84 degrees x 30 degrees, which is swept across the plasmasphere by the spin of the satellite. EUV's spatial resolution is 0.6 degrees or ~0.1 Re in the equatorial plane seen from apogee. The sensitivity is 1.9 count sec**-1 Rayleigh**-1, sufficient to map the position of the plasmapause with a time resolution of 10 minutes. _______________ Space Science Reviews, IMAGE Special Issue, Vol. 91, pp. 197-242, February, 2000