Radio Imaging in the Earth's Magnetosphere

J L Green ( NASA Goddard Space Flight Center, Greenbelt, MD 20771; 
ph.301-286-7354; fax: 301-286-1771; email: green@bolero.gsfc.nasa.gov); 
S Boardsen, W W L Taylor, S F Fung, R F Benson 
(All at NASA Goddard Space Flight Center, Greenbelt, MD 20771); 
B W Reinisch, W Calvert 
(All at University of Massachusetts, Lowell, Lowell, MA 01854)

Chapman Conference on Space Based Radio Observations at Long Wavelengths
Paris, France, October 19-23, 1998.
Invited Paper

The Radio Plasma Imager (RPI) will be a first-of-its-kind instrument 
designed to use radio wave sounding techniques to perform repetitive 
remote sensing measurements of electron number density (Ne) structures 
and the dynamics of the magnetosphere and plasmasphere.  RPI will fly 
on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) 
mission to be launched in the year 2000.  The design of the RPI is 
based on the recent advances in radio transmitter and receiver design, 
and modern digital processing techniques perfected for ionospheric 
sounding over the last two decades.  Free-space electromagnetic waves 
transmitted by the RPI located in the low density magnetospheric cavity 
will be reflected at distant plasma cutoffs.  The location and 
characteristics of the plasma at those remote reflection points can then 
be derived from measurements of the delay time, frequency, Doppler shift, 
and direction of an echo.  The 500 m tip-to-tip X and Y (spin plane) 
antennas and 20 m tip-to-tip Z axis antenna on RPI will be used to 
measures echoes coming from perhaps as great as 10 RE. RPI will operate 
at frequencies between 3 kHz to 3 MHz and will provide quantitative Ne 
values from 0.1 to 100,000 cm-3.

Using ray tracing calculations, combined with specific radio imager 
instrument characteristics, enables simulations of what RPI will measure.  
These simulations have been performed throughout an IMAGE orbit and under 
different model magnetospheric conditions and dramatically show that 
radio sounding can be used quite successfully to measure a wealth of 
magnetospheric phenomena. The radio imaging technique will provide a 
truly exciting opportunity to study global magnetospheric dynamics in a 
way which was never before possible.