Magnetospheric Electron-Density Values Determined from Plasma Resonances 
Stimulated 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 
J  F Fainberg, Goddard Space Flight Center, Greenbelt, MD 20771, 301-286-6940 
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 
S F Fung Goddard Space Flight Center, Greenbelt, MD 20771, 301-286-6301 
B W Reinisch, University of Massachusetts Lowell, Lowell, MA 01854, 978-934-4903 

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 also stimulates local plasma resonances that can be used to 
accurately deduce both the ambient electron density and the magnetic field strength 
even in very tenuous plasmas where the former is difficult to measure.  The 
information provided by these resonances is beneficial to the process of 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 high-latitude IMAGE apogee radial distance 
of 8 RE, resonances up to 18 fce are observed and the time duration of the lower 
harmonics can exceed 100 ms.  Plasma resonances at the electron plasma frequency fpe 
and the upper-hybrid frequency fuh, where fuh2 = fpe2 + fce2 are also stimulated by 
RPI. These resonances have been used to identify the ambient electron density and the 
magnetic field strength along the IMAGE orbit and the results, and comparisons to 
existing models, will be presented.

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