Analysis of Energy and Mass Distributions of ENAs Measured by the MENA Imager Throughout the IMAGE Mission H. O Funsten (1), R. M. Skoug (1), M. F. Thomsen (1), E. Santiago (1), D. J. McComas (2), and C. J. Pollock (2) 1 - Los Alamos National Laboratory, Los Alamos, NM 2 - Southwest Research Institute, Instrumentation and Space Research Division, San Antonio, TX The composition and energy distribution of magnetospheric plasmas change with geomagnetic activity, and these variations can be observed by neutral atom imaging. A subset of the telemetry from the Medium Energy Neutral Atom (MENA) Imager on the IMAGE mission consists of detailed information about individual coincident events from which we assess composition and energy. In particular, mass is identified by a combination of time-of-flight and pulse height magnitude, and energy is either calculated directly from the time-of-flight assuming all ENAs are hydrogen or determined by time-of-flight combined with knowledge of mass. In this study, we use these direct event data to investigate variations in both the mass and energy distributions throughout the mission to date. From a systematic evaluation of these parameters, we can identify the variations which are due to viewing perspective or temporal evolution of activity. _______________ Presented at the Fall American Geophysical Union Meeting, San Francisco, CA., December 15-19, 2000