Plasma Resonances Observed by the Radio Plasma Imager on the IMAGE Satellite

R F Benson, Goddard Space Flight Center, Greenbelt, MD 20771, 301-286-4037, 
u2rfb@lepvax.gsfc.nasa.gov; 
V A Osherovich, RITSS/Goddard Space Flight Center, Greenbelt, MD 20771, 
301-286-3649; 
J L Green, Goddard Space Flight Center, Greenbelt, MD 20771, 301-286-7354, 
green@nssdca.gsfc.nasa.gov; 
B W Reinisch, University of Massachusetts Lowell, Lowell, MA 01854, 
978-934-4903, bodo_reinisch@uml.edu

The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global 
Exploration (IMAGE) satellite was designed for long-distance magnetospheric 
radio sounding.  The RPI consists of a swept-frequency (3 kHz to 3 MHz) 
digital radio sounder with direction-finding capability.  IMAGE was launched 
into a polar orbit with an apogee radial distance of 8 RE and a perigee 
altitude of 1,000 km on 25 March 2000.  In order to receive long-range echoes 
from magnetospheric plasma structures, pulse compression and spectral 
integration techniques are employed.   The combination of spectral 
integration and a short pulse repetition frequency enables weak long-range 
echoes to be detected.  This capability, however, comes at the expense of 
frequency resolution if the frequency sweep is to be completed in a time 
significantly less than a satellite spin period (2 minutes).  It is difficult 
to identify the local plasma resonances stimulated by the sounder pulses under 
these conditions.  These resonances can be beneficial in the inversion of the 
magnetospheric reflection traces into electron-density profiles. When RPI is 
programmed to transmit short (3.2 ms) pulses, with a linear stepping size 
between frequency transmissions comparable to the RPI receiver bandwidth of 
300 Hz over a limited portion of the sweep range, the plasma resonances at the 
harmonics of the electron cyclotron frequency fce  are clearly identified.  
These resonances have been stimulated before by magnetospheric radio sounders 
but never at such great distances at high latitudes.  Even at the IMAGE apogee 
distance, resonances up to 18 fce are observed and the time duration of the 
lower harmonics can exceed 100 ms.  These resonances, and other plasma 
resonances observed by RPI, will be presented and discussed in analogy with 
the plasma resonances  stimulated by ionospheric topside sounders.

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Presented at the URSI meeting, Boulder, Colorado, U.S.A., Jan 2001