New Perspectives on the Earth's Plasma Environment from the IMAGE Satellite

D. L. Carpenter
Space, Telecommunications and Radioscience Laboratory
Stanford University

A new satellite, called IMAGE (Imager for Magnetopause-to-Aurora Global Exploration)
was launched in March, 2000 and is now producing global-scale images of the Earth's
space environment from various locations along a polar orbit with apogee about 8
Earth radii and perigee about 1200 km. Among the instruments on board are EUV, a
photon imager operating at 30.4 nM (U. of Arizona, W. Sandel, PI) and the Radio
Plasma Imager (RPI) (U. Mass. at Lowell, B. Reinisch, PI), designed to act as a
radio sounder of various important plasma boundaries within space near the Earth.
This talk will emphasize the operation of the RPI.

The Earth's plasmasphere, a near-Earth region of cold plasma with a
geomagnetic-field-aligned outer boundary called the plasmapause, is a major object
of study for both EUV and RPI. By collecting scattered sunlight at 30.4 nM from He+,
a minor but important constituent of the plasmasphere,  EUV is able to obtain global
images of that region during several-hour periods as IMAGE ascends toward apogee at
high northern latitudes. Meanwhile, RPI is able to obtain data on the plasmasphere
by the classical radio sounding technique developed during the "golden age" of
ionospheric topside sounders in the 1960's and early 1970's. RPI is also able to
make local measurements of plasma parameters along the IMAGE orbit.

The performance of RPI thus far has yielded many surprises. It was widely believed
before launch that although it would be difficult to sound the magnetopause, due in
part to the dynamic nature of the boundary, it should be easy to sound the
plasmapause, the region of steep density gradients at the outer limit of the
plasmasphere.  In fact, most of the echoes from the plasmapause region are spread in
range rather than discrete, as had originally been expected, for reasons that remain
to be understood. Another surprise is the extent to which the sounder echoes are
dominated by propagation along the geomagnetic field, rather than in the directions
that ray tracing in simple models of the medium would predict. 

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Presentation at the meeting of the Belgian Physical Society, May 16, 2001