The Earth's plasmasphere is a major object of study for both EUV and RPI. By collecting scattered sunlight at 30.4 nM from He+, a minor but important constituent of the plasmasphere, EUV is able to obtain global images of that region during several-hour periods as IMAGE ascends toward apogee at high northern latitudes. Meanwhile, RPI is able to obtain data on the plasmasphere by the classical radio sounding technique developed during the "golden age" of ionospheric topside sounders in the 1960's and early 1970's. RPI is also able to make local measurements of plasma parameters along the IMAGE orbit.
The performance of RPI thus far has yielded many surprises. It was widely believed before launch that although it would be difficult to sound the magnetopause, due in part to the dynamic nature of the boundary, it should be easy to sound the plasmapause, the region of steep density gradients at the outer limit of the plasmasphere. In fact, most of the echoes from the plasmapause region are spread in range rather than discrete, as had originally been expected, for reasons that remain to be understood. Another surprise is the extent to which the sounder echoes are dominated by propagation along the geomagnetic field, rather than in the directions that ray tracing in simple models of the medium would predict.
I will present some of the first images of the plasmasphere from EUV, and also sequences of records from RPI showing the response to the sounder as IMAGE moves inbound toward the plasmasphere, passes through the plasmasphere to low altitudes over the southern polar region, and then moves outward through the other lobe of the plasmasphere and begins to probe the northern polar region.