An ovwerview of results from RPI on IMAGE

J.L. Green and B.W. Reinisch


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 RPI is a
highly flexible instrument that can be programmed to perform these types of
measurements at times when IMAGE is located in key regions of the magnetosphere.
RPI is the first radio sounder ever flown to large radial distances into the
magnetosphere. The long-range sounder echoes from RPI allow remote sensing of a
variety of plasmas structures and boundaries in the magnetosphere. A profile
inversion technique for RPI echo traces 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 in slightly greater than a day at L values of 2.8 and that ion heating
is probably playing a major role in the overall density distribution along the
field-line. In addition, RPIÕs plasma resonance observations at large radial
distances over the polar cap provided in situ measurements of the plasma density
with an accuracy of a few percent. For the first time in the magnetosphere, RPI
has also observed the plasma D resonances. RPIÕs long antennas and its very low
noise receivers provide excellent observations in the passive receive-only mode
when the instrument measures the thermal plasma noise as well as natural
emissions such as the continuum radiation and auroral kilometric radiation
(AKR). Recent passive measurements from RPI have been compared extensively with
images from the Extreme Ultraviolet (EUV) imager on IMAGE resulting in a number
of new discoveries. For instance, these combined observations show that
kilometric continuum can be generated at the plasmapause from sources in or very
near the magnetic equator, within a bite-out region of the plasmasphere. The
process by which plasmaspheric bite-out structures are produced is not
completely understood at this time. Finally, RPI has been used to successfully
test the feasibility of magnetospheric tomography. During perigee passages of
the Wind spacecraft, RPI radio transmissions at one and two frequencies have
been observed by the Waves instrument. The received electric field vector was
observed to rotate with time due to the changing density of plasma, and thus
Faraday rotation was measured. Many future multi-spacecraft missions propose to
use Faraday rotation to obtain global density pictures of the magnetosphere. 

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Space Science Reviews, Special issue on IMAGE results, 109, 183-210, 2003.