High Latitude Topside Sounding Observations by the Radio Plasma Imager Henize, V. K., P. H. Reiff, J. Tu, J. Horwitz, B. Reinisch Among other capabilities, the Radio Plasma Imager (RPI) on IMAGE can be used effectively as a topside sounder. Inversions performed on returns from X-mode waves guided along magnetic field lines can yield electron densities along that field line. At L-shells above the plasmapause, these measurements during quiet times are used to construct an empirical model of the polar cap electron density profile in three dimensions. Average power law distributions for geocentric distance are found, with latitudinal and longitudinal variations discussed. Analysis of RPI data during storm time reveals greatly enhanced densities during the main phase, gradually returning to normal values during the recovery phase. As power law distributions are maintained throughout, outflow from ionospheric heating is suggested. This is supported by the agreement between RPI observations and results from an ionospheric outflow model. It is occasionally necessary to fit observed profiles with two power laws. In these cases, the interpretation is that one power law represents the electrons associated with O+ ions, and the other with the H+ distribution. _______________ To be presented at the Magnetospheric Imaging Workshop, Yosemite National Park, California, U.S.A., Feb. 5-8, 2002.