Relating Magnetospheric Passive Dynamic Spectral Emission Peaks to Plasma and 
Upper-Hybrid Frequencies Determined from Active RPI Sounding on IMAGE

R. F. Benson, P. A. Webb (NRC) and J. L. Green
NASA/Goddard Space Flight Center

B. W. Reinisch
University of Massachusetts Lowell


The electron density is often derived from an intense narrowband emission on 
passive radio-wave dynamic spectra.  The derivation is usually based on the 
assumption that the peak in this emission either corresponds to the electron 
plasma frequency f(pe) or the upper-hybrid frequency f(uh) which is related 
to f(pe) and the electron cyclotron frequency f(ce) by the expression 
f(uh)^2 = f(pe)^2 + f(ce)^2.  The Radio Plasma Imager (RPI) on the Imager for 
Magnetopause-to-Aurora Global Exploration (IMAGE) satellite provides an ideal 
opportunity to investigate the relationship of this peak to f(pe) and f(uh). 
Plasma resonances and wave cutoffs stimulated by RPI active soundings during 
5 plasmapause crossings were used to make accurate (within a few %) 
determinations of f(pe) and f(uh).  These values were then related to the 
observed emission peaks in the dynamic spectra obtained during passive RPI 
operations (interspersed among the active soundings) as a function of the 
ambient f(pe)/f(ce) value.  While it was found that the peak can occur at 
either f(pe) or f(uh) (more often at f(pe)), it often occurred between f(pe) 
and f(uh).  It is typically within 5% of f(pe) or f(uh) for large f(pe)/f(ce) 
(~ 4) and within 20% for smaller f(pe)/f(ce) (< 3).

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To be presented at the 2002 Spring A.G.U. Meeting, 
Washington, D.C., U.S.A., 28-31 May 2002