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

S. F. Fung (1), R. F. Benson (1), J. L. Green (1), B. W. Reinisch (2), 
D.M. Haines (2), I.A. Galkin (2), J.-L. Bougeret (3), R. Manning (3), 
P.H. Reiff (4), D.L. Carpenter (5), D.L. Gallagher (6), 
and W.W.L. Taylor (7)

1 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
2 University of Massachusetts Lowell, Massachusetts, USA
3 Observatoire de Paris, F-92195 Meudon Cedex, France
4 Rice University, Houston, Texas, USA
5 Stanford University, Palo Alto, California, USA
6 NASA Marshall Space Flight Center, Huntsville, Alabama, USA
7 Raytheon ITSS, NASA GSFC, Greenbelt, Maryland, USA


The Imager for Magnetopause-to-Aurora Global Exploration mission (IMAGE,
see http://image.gsfc.nasa.gov), launched on March 25, 2000, is the first 
mission dedicated to observing the global-scale structure and dynamics of 
the magnetosphere. Remote-sensing instruments on IMAGE are designed to 
observe the magnetopause, ring current, plasmasphere, polar cusp, and the 
auroral region, in order to reveal the morphologies and interactions 
among these inter-connected regions. In particular, the RPI, a digital 
radio sounder with direction-finding capabilities, can transmit radio 
pulses from 3 kHz to 3 MHz (corresponding to plasma densities of 0.1 - 
10^5 cm-3) to probe different magnetospheric plasmas.  Using two 500-m 
tip-to-tip orthogonal dipole antennas in the spin plane and a 20-m 
tip-to-tip spin-axis dipole antenna, signals reflected at remote plasma 
regions are received as echoes.  The RPI measures the echo amplitude, 
phase, range, polarization, Doppler shift, and angle of arrival as a
function of sounding frequency. Advanced digital techniques allow echoes 
of low-power transmitted signals to be detected over magnetospheric
distances. With a nominal 2-minute IMAGE spin period in a highly inclined
(90 deg.) elliptical orbit (apogee of 8 RE), the RPI (with a nominal 
1-min resolution) situated in the magnetospheric cavity observes the 
inner and outer magnetospheric boundaries.  Its observations can reveal 
the electron density profiles and dynamics of key magnetospheric plasma 
regions. This paper highlights the science objectives of the RPI on IMAGE 
and presents some early RPI observations.

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Adv. Space Res., Vol. 30, No. 10, pp. 2259-2266, 2002