The Whistler- and Z-Modes: New Tools For Radio Sounding of the Plasmasphere and Polar Regions from Middle to Low Altitudes D. L. Carpenter (Stanford University, Standford CA 94305), T. F. Bell, V. S. Sonwalkar, and B. W. Reinisch When the Radio Plasma Imager (RPI) on the IMAGE satellite operates in the inner plasmasphere and at moderate to low altitudes over the polar regions, pulses emitted at the low end of its 3-kHz to 3-MHz sounding frequency range can propagate in the whistler mode and/or in the Z mode. Whistler-mode echoes have been observed on at least half of the IMAGE orbits during which soundings are made with single 3.2-ms pulses at stepped frequencies. Z-mode echoes are essentially ubiquitous, appearing during essentially all soundings using single 3.2-ms pulses within the allowed (sometimes quite narrow) Z-mode propagation range. We show how Z-mode echoes can be used to sound upward along the geomagnetic field lines into regions of decreasing ionization density, and also used to investigate density irregularities in high latitude regions. We illustrate the diagnostic power of both discrete and diffuse whistler-mode echoes for study of magnetospheric density and plasma structure. ________________ Presentation at the Yosemite Conference of Inner Magnetospheric Interactions, 3 - 6 February 2004, Yosemite, California, USA