Observations of Magnetospheric Plasmas by the Radio Plasma Imager (RPI) on the IMAGE Mission S. F. Fung (1), R. F. Benson (1), J. L. Green (1), and B. W. Reinisch (2) 1 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. 2 University of Massachusetts Lowell, Massachusetts, USA Email: shing.fung@gsfc.nasa.gov Fax: 1-301-286-1771 The Imager for Magnetopause-to-Aurora Global Exploration mission (IMAGE, see http://image.gsfc.nasa.gov), scheduled for launch on February 15, 2000, is the first satellite mission designed to observe the global-scale structure and dynamics of the magnetosphere. Remote-sensing instruments on IMAGE will observe the magnetopause, ring current, plasmasphere, polar cusp, and the auroral region, in order to reveal the morphologies and interactions among these inter-connected regions. In particular, the RPI, a digital radio sounder, will transmit radio pulses from 3kHz to 3MHz (corresponding to remote plasma densities of 0.1 - 10^5 cm-3) by two crossed-dipole antennas (tip-to-tip lengths of 500 m) in the spin plane. A third 20-m spin-axis dipole antenna will also be used for echo reception. The three-axis measurements will allow the determinations of echo strength, range, polarization, Doppler shift, and angle of arrival as a function of sounding frequency. Advanced digital techniques will allow echoes of low-power transmitted signals to be detected over magnetospheric distances. With a nominal 2-minute IMAGE spin period in a highly inclined (> 45) elliptical orbit (apogee of 8 RE), the RPI (with a nominal 1-min resolution) situated in the magnetospheric cavity will observe the inner and outer magnetospheric boundaries, revealing the density profiles and dynamics of key magnetospheric plasma regions. This paper highlights the science objectives of the Radio Plasma Imager (RPI) on IMAGE and presents some early RPI observations. _______________________________________________ The 33nd COSPAR Scientific Assembly, Warsaw Poland, July 19-23, 2000 Symposium: Advances in Global Magnetospheric Structure, Dynamics, and Region Coupling Submitted to COSPAR Proceedings, September 2000.