A Brief Overview of Results from the RPI on IMAGE

J. L. Green
NASA/Goddard Space Flight Center, Greenbelt, MD


The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global
Exploration (IMAGE) spacecraft was designed as a long-range magnetospheric radio
sounder, relaxation sounder, and a passive plasma wave instrument. The
long-range sounder echoes from RPI allows for the remote sensing of a variety of
plasmas structures and boundaries in the magnetosphere. A profile inversion
technique for RPI echoes has been developed and provides a method for
determining the density distribution of the plasma from either direct or
field-aligned echoes. This technique has enabled the determination of the
evolving density structure of the polar cap and the plasmasphere under a variety
of geomagnetic conditions.  New results from RPI show that the plasmasphere
refills significantly faster (by a factor of ~2) than theories have predicted. 
In addition plasma resonance observations from RPI at large radial distances
over the polar cap have been made allowing for the determination of the plasma
density to within an accuracy of less than a few percent. 

RPI's passive receive-only mode measures natural emissions such as the continuum
radiation and auroral kilometric radiation (AKR). Plasmaspheric measurements
from RPI have been compared extensively with the Extreme Ultraviolet (EUV)
Imager on IMAGE. These combined observations show that kilometric continuum is
generated at the plasmapause, from sources in or very near the magnetic equator,
within a bite-out region of the plasmasphere.
 
Finally, the RPI has been used to successfully test the feasibility of
magnetospheric tomography. During perigee passage of the Wind and Cluster
spacecraft, RPI radio transmissions at two frequencies have been observed and
Faraday rotation was measured and occurs when the received electric field is
observed to rotate with time due to the changing density of plasma. Many future
multi-spacecraft missions propose to use Faraday rotation to obtain global
density pictures of the magnetosphere.

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LEP Seminar, Goddard Space Flight Center, 6 September 2002