Reiff, P. H., J. L. Green, S. F. Fung, W. Calvert, and W. W. L.
Taylor, Radio sounding of multi-scale plasmas, presented at the 1995
Cambridge Symposium on Multiscale Phenomena in Space Plasmas, Bermuda,
February 21-25, 1995.

Radio Sounding of Multiscale Plasmas

P. H. Reiff, C. B. Boyle, Dept. of Space Physics & Astronomy, Rice
University, Houston, TX 77251, J. L. Green, S. F. Fung, R. Benson, 
NASA Goddard Space Flight Center, Greenbelt, MD 20771, W. Calvert,
Dept. of Physics & Astronomy, University of Iowa, Iowa City, IA 52242,
W. W. L. Taylor, Nichols Research Corporation, Arlington, VA 22209

Radio sounding, long used in ground-based and space-based studies of
the ionosphere, is a powerful technique which can also be used to
remotely study multi-scale magnetospheric plasmas.  A radio sounder
aboard a spacecraft located in the low density magnetospheric cavity
will be suited for sounding the magnetopause, the cusp, and the
plasmasphere.  Free-space electromagnetic waves transmitted by the
sounder will be reflected from remote plasma regions where the
transmitted wave frequency matches the local O-mode or X-mode cutoff,
and will return almost instantaneously to the sounder as O-mode or
X-mode echoes wherever the density gradient at the reflection point is
approximately parallel to the line of sight from the sounder.  Thus,
swept-frequency echoes from the magnetopause, cusp, plasmapause, and
boundary layers will yield density profiles and magnetic field
information, as well as position and velocity information from these
remote regions.  Despite the large sounding distances, the nearly
parabolic shape of the magnetopause will result in partially focused
return signals.  Using modern digital signal processing techniques, a
radio sounder can observe remote magnetospheric regions using a
fraction of the power used in previously flown topside ionospheric
sounders.  Large-scale (~1 RE) undulations of the magnetopause or
boundary layer, as may be caused by pressure pulses or large-scale
surface waves, will result in multiple echoes, permitting knowledge of
the amplitude, wave length, and velocity of the waves.  These extra
return signals allow the inference of the 3-dimensional configuration
and dynamics of the magnetopause and its boundary layers on very rapid
time scales.