Fung, S. F., R. F. Benson, J. L. Green, M. F. Smith, D. L .Carpenter,
W. Calvert, D. L. Gallagher, P. H. Reiff, B. W. Reinisch, and W. W. L.
Taylor, Remote Sensing of the Magnetopause and Boundary Layers by
Radio Sounding, presented at the AGU Spring Meeting, Baltimore, May
23-27, 1994.

Remote Sensing of the Magnetopause and Boundary Layers by
Radio Sounding 

S F Fung, R F Benson, J L Green, and M F Smith (NASA Goddard
Space Flight Center, Greenbelt, MD 20771; fung@nssdca.gsfc.nasa.gov); 
D L Carpenter (Starlab, Stanford University, Stanford, CA 94305);
W Calvert (Dept. of Physics and Astronomy, University of Iowa,;
Iowa City, IA 52240); D L Gallagher (NASA Marshall Space Flight
Center, Huntsville, AL  35812); P H Reiff (Dept. of Space Physics and
Astronomy, Rice University, Houston, TX 77251); B W Reinisch
(University of Massachusetts Lowell, Lowell, MA  01854);
W W L Taylor (Nichols Research Corp, Arlington, VA 22209)

This paper investigates the use of the radio sounding technique
to observe the magnetopause and the plasma boundary layers (BL).
Due to recent advances in radio sounder design and
instrumentation, it is feasible to perform remote radio sounding of
the magnetosphere by using techniques perfected for ionospheric
studies.  Like ionospheric sounding, free-space electromagnetic
waves at 3-100 kHz, launched from a space-based sounder toward
the magnetopause, reflect specularly at their cutoffs where the local
refractive indices vanish.  The locations and plasma parameters of
the remote reflection points can be derived from the measurements
of the delay times and frequencies of the returned echoes.  From a
sounder situated outside the plasmasphere, it will be possible to
probe and monitor the structure and dynamics as well as the global
configuration of the magnetopause/BL, thereby yielding much
information on the dynamic coupling between the solar wind and
the magnetosphere.  Using ray tracing calculations based on a model
magnetosphere, we will demonstrate that the return of multi-
frequency radio echoes from the magnetopause/BL to a space-based
sounder/receiver is possible.  Radio echo signatures of irregularities
in the boundary layers will also be investigated.  Furthermore, it
will be shown that observations of swept-frequency radio echoes
arriving from different directions can be used to determine the
three-dimensional plasma structure, shape and nearly instantaneous
location of the magnetopause.  Thus by using radio sounders in
space, it will be possible to monitor the global magnetospheric
dynamical response to the time varying solar wind.