Overview of the Observations from the Radio Plasma Imager on IMAGE

James L. Green (NASA/Goddard Space Flight Center) and the RPI Team

An overview of the observations from the Radio Plasma Imager (RPI) on the Imager for 
Magnetopause-to-Aurora Global Exploration (IMAGE) will be presented. RPI is the first 
magnetosphere sounder designed to remotely sense a variety of plasma boundaries and 
density structures in the Earth's magnetosphere.  The RPI utilizes two 500 m tip to 
tip X and Y antennas and a 20 m Z axis antenna. RPI generates coded radio pulses with 
the X axis antenna and receives the return echoes with its 3 axis dipole antenna 
system, providing direction of arrival measurements.  Since it became fully 
operational in early May 2000 RPI has obtained echoes from the plasmapause, 
ionosphere, polar cusp and magnetopause during a variety of storm time conditions and 
at distances sometimes exceeding 5 Earth radii (Re).  No instrument on IMAGE has 
reported any interference caused by RPI transmissions. 

In addition, RPI also observes local (near the spacecraft) plasma resonances following 
the transmission of pulses as short as 3.2 ms.  During these times the electron 
cyclotron frequency and the plasma frequency can be determined. Since the RPI antennas 
are 4 or 5 times the Debeye length of the local plasma, even at apogee at 7 Re over 
the polar cap, RPI observes the local thermal plasma noise during the times of its 
passive measurements. These measurements enable accurate insitu local densities and 
temperatures of the polar cap and topside plasmasphere to be determined. Finally, 
whistler mode echoes have also been observed with RPI during its low altitude passes 
over the southern polar cap and plasmasphere along with a number of ducted echoes.  It 
is clear that RPI provide a unique view of the dynamical magnetosphere. 

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Presented at the Fall American Geophysical Union Meeting, 
San Francisco, CA., December 15-19, 2000