Benson, R. F., W. Calvert, D. L. Carpenter, S. F. Fung, D. L.
Gallagher, J. L. Green, P. H. Reiff, B. W. Reinisch, M. F. Smith, and
W. W. L. Taylor, Simulations of magnetospheric radio soundings,
presented at the National Radio Science Meeting (URSI), University of
Colorado at Boulder, Boulder, Colorado, January 3-7, 1994.

SIMULATIONS OF MAGNETOSPHERIC RADIO SOUNDINGS

Robert F. Benson, Goddard Space Flight Center Greenbelt, MD  20771 W.
Calvert (Univ. of Iowa), D. L. Carpenter (Starlab, Stanford Univ.), S.
F. Fung (GSFC), D. L. Gallagher (MSFC), J. L. Green (GSFC), P. H.
Reiff (Rice Univ.), B. W. Reinisch (Univ. of Mass., Lowell), M. F.
Smith (GSFC), and W. W. L. Taylor (Nichols Research Corporation)

	This paper will present the results of simulations of global
mag-netospheric sounding from a space-borne radio sounder located
between the plasmapause and the magnetopause.  Three-dimensional ray
tracing was performed using diffusive equilibrium electron density Ne
and plasmapause models (Angerami and Thomas, J. Geophys. Res., 69,
4537 , 1964; Aikyo and Ondoh, J. Radio Res. Labs., 18, 153, 1971,
respectively) combined with a Maxwellian Ne fall-off from the
magnetopause boundary.  This boundary was determined using the model
of Roelof and Sibeck (accepted in J. Geophys. Res., 1993) which is
based on the IMF Bz and the solar wind dynamic pressure.

	The ray tracing results for frequencies from 30 to 90 kHz are
summarized on simulated magnetospheric plasmagrams (records of echo
delay time vs. frequency), similar to the ionograms from ionospheric
sounding which have been studied for the last three decades.  The
similarities include the ability to interpret multiple echoes from
different directions that appear on the same record.  The differences
include the predominance of curved reflection surfaces in
magnetospheric sounding as compared with the usually justified
assumption of horizontal stratification in the case of ionospheric
sounding.  The curved magnetospheric surfaces can result in defocusing
near the plasmapause and focusing near the mag-netopause.  Such
focusing and defocusing can also result from boundary irregularities. 
Magnetopause irregularities will lead to multiple reflections from the
same general direction.  A smooth magnetopause, on the other hand,
will yield several echoes from signifi-cantly different directions. 
The capability to interpret such multiple echoes means that it will be
possible, from such a sounder mission, to monitor the global
configuration of the magnetopause while pro-viding nearly simultaneous
radial Ne profiles in different directions in order to yield
unprecedented 3-dimensional information on the mag-netopause and
plasmasphere.