Radio Sounding in Space: Magnetosphere and Topside Ionosphere

B. W. Reinisch (1), D. M. Haines (1), R. F. Benson (2), J. L. Green (2), 
G.S. Sales (1), and W. W. L. Taylor (3)

(1) Center for Atmospheric Research, University of Massachusetts Lowell, 
    600 Suffolk Street, Lowell MA  01854, USA 
(2) NASA Goddard Space Flight Center, Code 630, Greenbelt MD  20771 
(3) Raytheon ITTS/GSFC, Code 630, Greenbelt, MD  20771

ABSTRACT
Modern sounding techniques have been developed for the space-borne exploration 
of Earth's magnetosphere and topside ionosphere. Two new satellite instruments 
will use the advanced techniques of the groundbased Digisondes. The Radio 
Plasma Imager (RPI), a low frequency sounder with 500-m dipole antennas 
designed to sweep from 3 kHz to 3 MHz, will be part of NASA's IMAGE mission to 
be launched in February 2000 into an elliptical orbit with an altitude at 
apogee of 7 Re. While in the magnetospheric cavity RPI will receive echoes 
from the magnetopause and the plasmasphere and will measure the direct 
response of the magnetosphere's configuration to changes in the solar wind. 
With three orthogonal dipole antennas (two 500 m tip-to-tip antennas in the 
spin plane used for transmission and reception, one 20 m antenna along the 
spin axis for reception only) the arrival angle of returning echoes can be 
determined with high accuracy. The other instrument is the TOPside Automated 
Sounder (TOPAS), which was originally conceived for the Ukrainian WARNING 
mission with a launch date in 2001. Using one antenna for transmission and 
three orthogonal 10-m antennas for reception, TOPAS will be able to determine 
the arrival angle of ionospheric echoes and their wave polarization. It will 
then be possible to automatically scale the topside ionograms and calculate 
the electron density profiles in real time. Operating as a high-frequency 
radar, TOPAS will for the first time measure topside plasma velocities by 
tracking the motions of plasma irregularities. 

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Journal of Atmospheric and Solar-Terrestrial Physics, v. 63, pp. 87-98, 2001