Measuring the Plasma Density Along the Magnetic Field: A Radio Sounding Technique P. Song, X. Huang, B. W. Reinisch, J. L. Green, S. F.Fung, D. Gallagher, and V. M. Vasyliunas A new technique is introduced that remotely measures the plasma density profile in the plasmasphere. Radio plasma imager (RPI) on board of the IMAGE satellite transmits coded signals in the magnetosphere and receives the echoes. The frequency and time delay of an echo carry the information about the plasma density and distance at the reflection point of the echo. Echoes of the same mode forms a distinct trace on a diagram of echo amplitude as a function of frequency and time delay. From such a trace and a given dispersion relation, a density profile can be derived from a best-fit. Sometimes, multiple echo traces can appear in a single diagram of a complete frequency scan. We demonstrate that these traces can be resulted from echoes of different propagating modes along the field from both hemispheres. Therefore, the density profile from one hemisphere to the other along the field is derived. _______________ To be presented at the Magnetospheric Imaging Workshop, Yosemite National Park, CA, Feb. 5-8, 2002.