Kleinheubacher Tagung, Main, Germany, Sept. 28 - Oct. 2, 1998.
Invited Paper

Radio Sounding in the Magnetosphere and Topside Ionosphere

B. W. Reinisch (1), D. M. Haines (1), R. F. Benson (2), K. Bibl (1), 
W. Calvert (1), G. Cheney (1), S.  F. Fung (2), J. L. Green (2), 
J. Grebowsky (2), X. Huang (1), R. Manning (3) and W. W. L. Taylor (4) 

(1) University of Massachusetts Lowell, (2) NASA Goddard Space Flight 
Center, (3) Observatoire Paris-Meudon, (4) Raytheon STX/GSFC.

Two new space borne probes are being developed to remotely sense 
plasma distributions in the magnetosphere and the topside ionosphere. 
The Radio Plasma Imager (RPI), a low frequency sounder designed to 
sweep from 3 kHz to 3 MHz, will be part of the NASA's IMAGE satellite 
mission to be launched in January 2000. While in the magnetospheric 
cavity (7 Re altitude), 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 angle of arrival of returning echoes 
can be determined with high accuracy. Similar to modern groundbased 
ionosondes, RPI will operate like a low-frequency radar system 
measuring range, location, plasma density, and motion of the 
reflecting plasma structures. The TOPside Automated Doppler Sounder 
(TOPADS) is being developed for the Ukrainian WARNING mission 
scheduled for launch in 2001. At a high inclination circular orbit 
(600 km altitude), TOPADS will measure the topside electron density 
profiles every 75 km. Three orthogonal 20 m tip-to-tip dipole antennas 
will be used for reception, one of them for transmission. Operating as 
an HF radar, TOPADS will be providing for the first time topside 
plasma velocities by tracking the motions of plasma irregularities.  
Automatic processing of the ionogram data will provide real time 
electron density profiles that will be made freely available as a 
space weather diagnostics tool. IMAGE and WARNING will be flying at a 
most propitious time, that of near maximum and declining solar 
activity, to understand coupling between the ionosphere and the 
magnetosphere and the solar wind.