Magnetospheric Radio Sounding

R F Benson, S F Fung and J L Green (All at NASA/Goddard Space Flight 
Center, Greenbelt, MD 20771; 301-286-4037; e-mail: 
u2rfb@lepvax.gsfc.nasa.gov); W Calvert, D M Haines and B W Reinisch 
(All at University of Massachusetts, Lowell, Lowell, MA 01854); D L 
Carpenter (Stanford University, Stanford, CA 94305); D L Gallagher 
(NASA/Marshall Space Flight Center, Huntsville, AL 35812); P H Reiff 
(Rice University, Houston, TX 77251); W W L Taylor (Hughes 
STX/Goddard Space Flight Center, Greenbelt, MD 20771); J-L Bougeret 
and R Manning (Both at Paris Observatory, Meudon, France)

One of the instruments selected for flight in January, 2000 on IMAGE 
(Imager for Magnetopause-to-Aurora Global Exploration), NASA's first 
MIDEX (Medium-Class Explorer) mission, is a radio sounder with 
direction-finding capability known as RPI (Radio Plasma Imager).  In a 
recent publication  [Calvert et al., Radio Sci., 30, 1577, 1995], the 
feasibility of magnetospheric radio sounding of the type proposed to be 
performed by the RPI was considered.  It was shown that a radio sounder 
outside the plasmasphere could provide remote density measurements of 
the magnetopause and plasmasphere, as well as other important density 
features elsewhere in this region. Using digital integration and tuned 
reception at frequencies from 3 kHz to 3 MHz with 500-m tip-to-tip 
dipole antennas and 10 W transmitter power, such a sounder (using three 
mutually orthogonal antennas for reception) would be capable of 10% 
density resolution and 500 to 1300 km spatial resolution in only a few 
minutes at distances of up to 4 RE.  These claims have been recently 
challenged  [Greenwald, Radio Sci., 32, 277, 1997]  and defended  [Calvert 
et al., Radio Sci., 32, 281, 1997] .  Here, the justifications for the 
RPI expectations will be reviewed, including our response to questions 
raised concerning applying the well-proven ionospheric radio-sounding 
technique to the magnetosphere.  The presentation will include material 
used in a Non-Advocate Review of the RPI held at the University of 
Massachusetts Lowell on 28 January 1997.