Benson, R. F., B. W. Reinisch, J. L. Green, J.-L. Bougeret, W. Calvert,
   D. L. Carpenter, S. F. Fung, D. L. Gallagher, D. M. Haines, R. Manning,
   P. H. Reiff and W. W. L. Taylor, Magnetospheric radio sounding on the
   IMAGE mission, Radio Sci. Bull., v. 285, pp. 9-20, June 1998.

Radio sounding can be used to obtain accurate remote, as well as in situ,
electron number density (Ne) measurements in magnetized space plasmas. It is
based on the reflection of radio waves at the location where the refractive
index goes to zero. The Ne structure of the terrestrial ionosphere has been
routinely investigated by both ground-based and satellite-borne sounders. The
terrestrial magnetosphere, on the other hand, has never been subject to routine
radio-sounding investigations. 

Recently, NASA selected Imager for Magnetopause-to-Aurora Global Exploration
(IMAGE) as the first Medium-class-Explorer (MIDEX) mission. IMAGE is scheduled
to be launched on January 1, 2000 and has the scientific objective of
understanding the global dynamics of the terrestrial magnetosphere and its
response to changing solar-wind conditions. The IMAGE payload includes a radio
plasma imager (RPI) in addition to far ultraviolet (FUV), extreme ultraviolet
(EUV) and neutral atom (NAI) imagers. The RPI will consist of a swept-frequency
digital radio sounder covering the frequency range from 3 kHz to 3 MHz. It will
be designed similar to modern state-of-the-art ground-based digital ionospheric
sounders. It will employ on-board digital signal-processing techniques and will
be capable of measuring echo amplitude, phase, time delay, Doppler spectrum,
polarization and direction-of-arrival as a function of sounding frequency. Most
RPI soundings will be taken near apogee at a geocentric distance of 8 RE between
45 and 90 degrees north geographic latitude. In this region, the spacecraft 
will be immersed in the Ne cavity extending from the plasmapause to the 
magnetopause. The RPI will often be able to simultaneously determine the Ne 
profiles leading up to these two boundaries. In addition, it will be able to 
produce echo maps showing magnetospheric Ne structures. The combination of 
such information with the FUV, EUV and NAI images, will provide unprecedented 
global descriptions of magnetospheric dynamic plasma processes.