First results from the Radio Plasma Imager on IMAGE

The Radio Plasma Imager (RPI) is a 3 kHz 3 Mhz radio sounder. incorporating modern digital processing techniques and long electronically-tuned antennas, that is flown to a large radial distances into the high-latitude magnetosphere on the Imager for Magnetopause-to Aurora Global Exploitation (IMAGE) satellite. Clear echoes, similar to those observed by ionospheric topside sounders, are routinely observed from the polar-cap ionosphere by RPI even when IMAGE is located at geocentric distance up to approximately 5 Earth radii. Using an inversion technique, these echoes have been used to determine electron-density distributions from the polar-cap ionosphere to the location of the IMAGE satellite. Typical echoes from the plasmapause boundary, observed from outside the plasmasphere, are of a diffuse nature indicating persistently irregular structure. Echoes attribute to the cups and the magnetopause have also been identified, those from the cups have been identified more often and with greater confidence.