Benson, R. F., J. L. Green, S. F. Fung, M. F. Smith, W. Calvert, D. L. Carpenter, D. L. Gallagher, P. H. Reiff, B. W. Reinisch, and W. W. L. Taylor, Wave injection experiments during planned magnetospheric radio soundings, presented at the 1995 IUGG/IAGA Meeting, Boulder Colorado, July 2-14, 1995. Wave injection experiments during planned magnetospheric radio soundings R F Benson, J L Green, S F Fung, M F Smith, NASA, Goddard Space Flight Center, Greenbelt, MD, 20771 (u2rfb@lepvax.gsfc.nasa.gov) W Calvert, Dept. of Physics and Astronomy, University of Iowa, City, IA, 52240 (wynne@calvert.physics.uiowa.edu) D L Carpenter, STAR Laboratory, Stanford University, Stanford, CA, 94305 (dlc@nova.stanford.edu) D L Gallagher, Marshall Space Flight Center, ES83, Huntsville, AL, 35812 (dennis.gallagher@msfc.nasa.gov) P H Reiff, Dept. of Space Physics and Astronomy, Rice University, Houston, TX, 77251 (reiff@spacvax.edu) B W Reinisch, University of Massachusetts Lowell, Lowell, MA, 01854 (reinisch@cae.uml.edu) W W L Taylor, Nichols Research Corp, Arlington, VA, 22209 (taylor@nssdca.gsfc.nasa.gov) The concept of radio sounding in the magnetosphere has recently attracted considerable attention [e.g., see Reiff et al., EOS, 75, 129, 1995]. A magnetospheric radio sounder would be able to obtain true-range profiles and orbit-plane contours of the electron density in the magnetosphere as were obtained in the ionosphere by the Alouette and ISIS and later ionospheric topside sounders. A sounder located between the plasmapause and the magnetopause would be able to obtain electron density profiles between the spacecraft location and each of these boundaries based on the frequency and delay time of the echoes of electromagnetic waves transmitted by the sounder. Sounder-excited electrostatic waves and sounder-stimulated plasma emissions will also be received from the near vicinity of the spacecraft. As in the case of the ISIS ionospheric sounders, these electrostatic waves (or "resonances") and sounder-stimulated emissions will provide accurate measurements of the ambient electron density and magnetic field strength. This local measuring technique has also been employed by relaxation sounders which were not designed for remote sensing of distant plasma distributions. The magnetospheric GEOS 1 relaxation sounder made such local measurements using sounder-excited electrostatic waves. The relaxation sounder carried by the Ulysses spacecraft was used to determine the electron density and magnetic field strength along the trajectory through a portion of Io's plasma torus. In this case the measurements were based mainly on sounder-stimulated plasma emissions that were found to have the same spectral components as the ones produced by the much more powerful ionospheric ISIS sounders [Osherovich et al., J. Geophys. Res., 98, 18751, 1993]. Later work has shown that the Ulysses sounder-stimulated emissions clearly coincide with natural emissions in the torus. This paper will discuss the questions that remain concerning the interpretation of sounder-stimulated electrostatic echoes and sounder-stimulated plasma emissions resulting from space-borne sounders, the diagnostic capabilities of these signals and their significance to the interpretation of data from future magnetospheric radio sounders. The major questions concerning the sounder-stimulated plasma emissions concern the excitation mechanism, how dependent it is on ambient plasma conditions, and whether it involves the creation, or enhancement, of field-aligned electron density irregularities by the sounder.