Active/Passive Investigations of Magnetospheric Emissions Based on IMAGE/RPI Data

R. F. Benson, J. L. Green, S. F. Fung, S. A Boardsen, L. Garcia, P. A. Webb, and
B. W. Reinisch


On passive radio-wave dynamic spectra, the magnetospheric electron density Ne is
often determined by identifying the electron plasma frequency fpe (~ Ne^1/2) or,
after the electron cyclotron frequency fce is determined from a magnetic-field
model, the upper-hybrid frequency fuh = (fpe^2 + fce^2)^1/2. These frequencies
are identified by assuming that they correspond to the upper or lower frequency
limits of prominent magnetospheric emissions that are propagating in certain
characteristic plasma wave modes. Here we report on the results of
investigations of magnetospheric emissions using interleaved active and passive
observations from the Radio Plasma Imager (RPI) on the IMAGE satellite (Imager
for Magnetopause-to-Aurora Global Exploration). In particular, we discuss the
accuracy of deducing Ne from: (1) the upper- and lower-frequency limits of an
intense narrowband Z-mode emission identified as the upper-hybrid band, (2) the
lowerfrequency limit of the ordinary-mode emission known as continuum, and (3)
the upperfrequency limit of whistler-mode emissions. In addition, we
investigated the geographical distribution of, and the fpe/fce values associated
with, the banded magnetospheric emissions, commonly referred to as "(n + 1/2)fce" 
emissions. These values are necessary for understanding the generation
mechanisms responsible for these emissions that have been observed in every
planetary magnetosphere visited by properly-instrumented spacecraft. In the RPI
investigations, fpe and fce are determined to an accuracy ~ 1% and 0.1%,
respectively, from the sounder-stimulated plasma resonances and wave cutoffs.
This accuracy, obtained from active operations, enables proper wave-mode
identifications of natural emissions received during passive operations. It also
enables commonly-accepted interpretations of the frequency limits of these
emissions to be tested.

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Presentation at the 28th URSI General Assembly, New Delhi, India
23 - 29 October 2005