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

When the Radio Plasma Imager (RPI) on the IMAGE satellite operates in the inner plasmasphere and at moderate to low altitudes over the poles, pulses emitted at the low end of the RPI 3 kHz to 3 MHz sounding frequency range can propagate in the Z mode as well as the whistler mode. At medium altitudes within the plasmasphere, discrete Z-mode echoes with turning points Earthward of IMAGE are often observed, analogous in form to the regular and oblique Z-mode echoes found on topside sounder records. In the polar regions, where f(pe)/f(ce)<1 usually obtains, the Z-mode echoes tend to be diffuse and to exhibit properties such as an intensity decrease at the local value of f(ce), where there is a significant change with frequency in the topology of the Z-mode refractive index surface. At altitudes up to a few thousand km in polar or near-polar regions, Z-mode soundings regularly provide information on the local value of f(pe), whereas passive recordings that depend upon indications of the upper hybrid resonance frequency f(uh) usually fail to do so. Within the transition region from the auroral zone to the plasmasphere and within the plasmasphere itself, at altitudes from approximate to 2000 km upward, we find evidence of the Z-mode trapping phenomenon, noted previously in connection with natural wave emissions detected at polar latitudes. This phenomenon involves echoes returning both from turning points in denser regions Earthward of the sounder and from more tenuous regions above. Within Z-mode "cavities,'' discrete Z-mode echoes are found to propagate back and forth along geomagnetic-field-aligned paths between 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 determination of the electron density profile along geomagnetic field lines to distances of thousands of km above the location of IMAGE, all on the basis of echo delay information within frequency bands only approximate to 20 to 30 kHz wide. The forms of such density profiles are in general agreement with results reported from conventional RPI X-mode sounding from higher altitudes, as well as with predictions based upon a diffusive equilibrium model of density along plasmasphere field lines.