Radio Sounding of the Plasmapause M. Salvati, D.L. Carpenter, U.S. Inan, T.F. Bell Space, Telecommunications and Radioscience Laboratory Stanford University B.W. Reinisch Center for Atmospheric Research Univerisity of Massachusetts, Lowell The Radio Plasma Imager (RPI) on the IMAGE mission is a low- power radar which operates in the radio frequency bands which contain the plasma resonance frequencies characteristic of the Earth's magnetosphere (3 kHz to 3 MHz). RPI can locate regions of various plasma densities by observing radar echoes from the plasma that are reflected where the radio frequency is equal to the plasma frequency. For sounding densities in the region of the plasmapause, the appropriate frequency range is approximately 10-300 kHz. The original expectations of the RPI team were that echoes from the plasmapause region would be discrete, consisting of a single return echo received at each frequency. However, the typical echo patterns received as IMAGE approaches the plasmasphere at low magnetic latitudes include diffuse echoes, involving returning signals which reflected at ranges of up to two Earth radii, and highly efficient field aligned ducting within or adjacent to the region of steep plasmapause gradients. These initial results suggest that irregular structure is regularly present in the plasmapause boundary region. Such structure has been identified in in-situ radio data from a number of satellites. We will present examples of diffuse plasmapause echoes and interpret them in terms of overall characteristics of plasmapause structure. We will also present results of initial attempts to quantitatively model the reflections from the irregular plasmapause density structures in terms of normal incidence of ordinary and extraordinary electromagnetic plasma waves on a horizontally stratified multi-layered interface. __________ To be presented at the URSI meeting, Boston, Mass, July, 2001