Reiff, P. H., J. L. Green, R. F. Benson, S. F. Fung, M. F. Smith, W.
Calvert, D. L. Carpenter, D. L. Gallagher, B. W. Reinisch, W. W. L.
Taylor, The use of radio imaging in substorm studies, presented at the
AGU Spring Meeting, Baltimore, May 23-27, 1994.

The Use of Radio Imaging in Substorm Studies

P. H. Reiff (Dept. of Space Physics & Astronomy, Rice University,
Houston, TX 77251-1892; Internet: reiff@spacvax.rice.edu); J. L.
Green,
R. F. Benson, S. F. Fung, M. F. Smith (all at: Goddard Space Flight
Center,
Greenbelt, MD; email: NCF::GREEN); W. Calvert (Dept. of Physics &
Astronomy, University of Iowa, email:
wynne@calvert.physics.uiowa.edu);
D. L. Carpenter (Star Laboratory, Stanford University, email:
STAR::CARPENTER); D. L. Gallagher (Marshall Space Flight Center, 
Huntsville, AL 35812; email: GOBO::GALLAGHER); B. W. Reinisch (Center
for Atmospheric Research, University of Massachusetts, Lowell, Lowell,
MA 01854, email: reinisch@cae.uml.edu); W. W. L. Taylor (Nichols
Research
Corporation, Arlington, VA 22209; email: NHQVAX::WTAYLOR)

Radio imaging, long used in ground-based and space-based studies of the
ionosphere, holds great promise for application to substorm studies.  
A spacecraft at high latitudes in the magnetospheric cavity or
magnetotail lobes will be in a density minimum with increased densities at the
magnetopause, the plasma sheet, and the plasmasphere.  In the radio
sounding technique, free-space X- and O-mode waves launched at a sequence 
of frequencies will reflect from regions where the local plasma
characteristics yield a low-frequency cutoff of the respective wave mode
that matches the transmitted frequency.  Since the O-mode cutoff is at the
plasma frequency, the frequency of the returned echo is proportional to
the square root of the remote density. The X-mode waves, with cutoff
frequency dependent on both the electron gyrofrequency and the plasma
frequency, can be used to obtain remote magnetic field information when
combined with the O-mode echo analysis.  A sounder located on the dayside
can monitor the three-dimensional size of the magnetosphere on ~minute
time scales, allowing use and testing of quantitative, global models of
the substorm process.  In addition, from a dayside location one may
observe plasmasphere erosion and refilling during and after a substorm
cycle.  A sounder located on the nightside or in the magnetotail lobe can
quantitatively monitor both the size of the magnetotail and the lobe field
strength, again quantitatively measuring globally, not just locally, the
magnetic flux changes in the growth and expansion phases.  It may even be
possible to infer the presence of plasmoids, either directly or from the
change in orientation of the magnetopause boundary as a plasmoid
passes by.