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.