Plasmaspheric Structure Revealed by Radio Sounding

M. Salvati, D. L. Carpenter, U. S. Inan, T. F. Bell
Space, Telecommunications and Radioscience Laboratory
Stanford University

R. F. Benson, S. F. Fung
Goddard Space Flight Center

B. W. Reinisch
University of Massachusetts Lowell

The plasmasphere as a target for radio sounding from high altitude differs
markedly from expectations prior to the launch of the IMAGE satellite and
operation thereon of the Radio Plasma Imager (RPI).  Although the RPI team
expected to find evidence of density structure in the plasmapause  region
and outer plasmasphere , it was also expected that on many occasions the
echoes would be discrete, with each pulse returning from a single range.
Instead the typical echo patterns received at a given point as IMAGE
approaches the plasmasphere  at low magnetic latitudes exhibit range
spreading over distances of up to two Re.  However they may also
show a well defined minimum range that decreases  with approach to the
plasmasphere.  The echoes appear to reflect large scale features 
(> 0.1 Re) of the radial electron density profile as well as the
presence of smaller scale irregularities.  Our work on quantitative
modeling of the observed effects will benefit from our ability to compare 
sounding records taken at multiple points as IMAGE approaches and then 
penetrates the plasmasphere.