Z-mode sounding within propagation "cavities" and other inner magnetospheric 
regions by the RPI instrument on the IMAGE satellite

D. L. Carpenter, T. F. Bell, U. S. Inan, R. F. Benson, V. S. Sonwalkar, 
B. W. Reinisch, D. L. Gallagher 


When the Radio Plasma Imager (RPI) on the IMAGE satellite operates in the
inner plasmasphere and at moderate to low altitudes over the poles, regions
in which the ratio of the local electron plasma frequency fpe to the local
electron gyrofrequency fce may exceed or be less than unity, pulses emitted
at the low end of the RPI 3 kHz to 3 MHz sounding frequency range can
propagate in the the Z mode as well as the whistler mode. At medium altitudes
within the plasmasphere, discrete echoes with turning points Earthward of
IMAGE are often observed, analogous to the regular and oblique Z-mode echoes
found on topside sounder records. In the polar regions, where fpe/fce < 1
usually obtains, the echoes tend to be diffuse and to exhibit properties such
as an intensity decrease at the local value of fce, where there is a
significant topological change in the Z-mode refractive index surface. At low
altitudes near the plasmasphere and in the polar region, passive recordings
usually fail to provide clear identification of local plasma parameters,
while Z-mode soundings can regularly do so. Within the transition region from
the auroral zone to the plasmasphere and the plasmasphere itself, at
altitudes from ~2000 km upward, we find evidence of the Z-mode cavity
phenomenon that has been noted previously in connection with natural wave
emissions detected at polar latitudes. Within a Z-mode cavity, discrete
Z-mode echoes can be trapped as they propagate along field-aligned paths
beween upper- and lower-altitude reflection points. The echoes present unique
forms, depending upon whether IMAGE is located above or below a minimum in
the altitude profile of the Z-mode cutoff frequency. Through an inversion
process, such echoes make possible remote determination of the field-line
electron density profile in regions where that profile is poorly known. In
two examples, the electron density distribution along the field lines was
determined to distances of thousands of km above the location of IMAGE, all
on the basis of echo delay information within frequency bands only ~ 30 kHz
wide.

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Submitted to Journal of Geophysical Research, 2003