The Radio Plasma Imager (RPI) will be a first-of-its-kind instrument designed to use radio wave sounding techniques to perform repetitive remote sensing measurements of electron number density (Ne) structures and the dynamics of the magnetosphere and plasmasphere. RPI will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched in the year 2000. The design of the RPI is based on the recent advances in radio transmitter and receiver design, and modern digital processing techniques perfected for ionospheric sounding over the last two decades. Free-space electromagnetic waves transmitted by the RPI located in the low density magnetospheric cavity will be reflected at distant plasma cutoffs. The location and characteristics of the plasma at those remote reflection points can then be derived from measurements of the delay time, frequency, Doppler shift, and direction of an echo. The 500 m tip-to-tip X and Y (spin plane) antennas and 20 m tip-to-tip Z axis antenna on RPI will be used to measures echoes coming from perhaps as great as 10 RE. RPI will operate at frequencies between 3 kHz to 3 MHz and will provide quantitative Ne values from 0.1 to 105 cm-3.
Using ray tracing calculations, combined with specific radio imager instrument characteristics, enables simulations of what RPI will measure. These simulations have been performed throughout an IMAGE orbit and under different model magnetospheric conditions and dramatically show that radio sounding can be used quite successfully to measure a wealth of magnetospheric phenomena. The radio imaging technique will provide a truly exciting opportunity to study global magnetospheric dynamics in a way which was never before possible.
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