Investigations of Magnetospheric Plasmas by the Radio Plasma Imager (RPI) on the 
IMAGE Mission

S. F. Fung, R. F. Benson, and J. L. Green
NASA Goddard Space Flight Center
Mail Code 632
Greenbelt, Maryland, 20771 USA
Tel: 301 286 6301, Fax: 301 286 1771, E-mail: shing.fung@gsfc.nasa.gov

B. W. Reinisch
University of Massachusetts Lowell

S. A. Boardsen, and W. W. L. Taylor
Raytheon ITSS


Background
	The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), scheduled 
for launch in January 2000, is the first satellite mission designed to observe the global 
structure and dynamics of the magnetosphere (see http://image.gsfc.nasa.gov). Remote-
sensing instruments on IMAGE will observe key magnetospheric regions such as the 
magnetopause, ring current, plasmasphere, polar cusp, and the auroral region, in order to 
reveal interactions among these inter-connected regions. The morphologies and inter-
relationships of the different parts of the magnetosphere and their responses to the variable 
solar wind will be seen through global images produced by energetic neutral atoms (ENA), 
FUV, EUV and radio waves. For the first time, we will be able to test our impressions of 
the global magnetosphere resulting from syntheses of many years of in situ measurements. 
This paper highlights the science objectives of the Radio Plasma Imager (RPI) on IMAGE 
and presents some simulated data to illustrate the magnetospheric observations expected to 
be obtained by the RPI.

Instrumentation
	The RPI is a digital sounder with its design based on the Digisonde Portable 
Sounder developed at the University of Massachusetts Lowell. The RPI will transmit radio 
pulses by two crossed dipole anatennas with tip-to-tip lengths of 500 m in the spin plane. 
The echoes will be received by these antennas and a 20-meter spin-axis dipole antenna. The 
three-axis measurements will allow the determinations of the strength, range, polarization, 
Doppler shift, and angle of arrival of individual echoes as a function of sounding 
frequency. Advance digital signal processing techniques will allow echoes of low-power 
transmitted signals to be detected over large magnetospheric distances. In addition, the RPI 
will operate passively to measure thermal noise and natural plasma emissions.
Observations
	With a 2-minute nominal spin period of the IMAGE spacecraft in a highly inclined 
(> 45 degrees) elliptical orbit (apogee of 8RE), the RPI will be well situated in the 
magnetospheric cavity to observe the inner and outer magnetospheric boundaries. Using 
coded pulse transmissions, the RPI will operate from 3kHz to 3MHz to observe remote 
plasma densities from about 0.1 to 105 cm-3. Doppler shift and range measurements will 
yield line-of-sight bulk velocities and locations from all radio wave signals returning from 
remote plasma structures. At high latitudes, the 1-minute time resolution RPI measurements 
will reveal the density profiles and dynamics of the magnetopause, polar cusp, and 
plasmasphere. Simulated data will be presented to illustrate these observations.