Plasma Distribution as a Function of Latitude and L-shell in the plasmasphere: A RPI Case Study X. Huang, B. Reinisch, P. Song, G. Sales, G. Khmyrov, A. Kozlov Center for Atmospheric Research, University of Massachusetts Lowell In the past it has been difficult to assess the plasma density (NE) distribution along magnetic field lines with in-situ measurements because it is rare to have a few satellites along the same field line. It is also challenging to derive the distribution from theory because the existence of field-aligned flow makes a static approximation invalid. The new sounder observations by the radio plasma imager (RPI) on IMAGE [Burch et al., EOS Trans. American Geophys. Union, 82, 241-145, 2001], make it possible to measure the entire field-aligned profile within the time it takes to step through the frequencies that cover the plasma frequencies from the satellite location to the ends of the field line in the topside ionosphere [Reinisch et al., Geophys. Res. Lttrs, in press, 2001]. This takes approximately 10 s, i.e., the measurement is made almost instantaneously. A special profile inversion technique inverts the field-aligned propagation echo traces to profiles along the field line. Each profile is validated by recalculating the echo traces for different wave modes using the derived profile. This paper discusses the application of the new technique to one IMAGE pass in the morning sector when the sequence of plasmagrams shows clearly defined echo traces. A case study shows a sequence of eight consecutive profiles while IMAGE changed position from L = 2.22 to L = 3.23. Soundings were made every two minutes. Each sounding provides a complete hemisphere-to-hemisphere density profile. By piecing together these profiles along different L shells we were able to construct two-dimensional density distributions as a function of latitude and L-shell. _______________ Presented at the January 2002 URSI meeting, Boulder, Colorado