Calvert, W., Benson, R. F., 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, Feasibility of radio sounding in the magnetosphere,
presented at the National Radio Science Meeting (URSI), University of
Colorado at Boulder, Boulder, Colorado, January 3-7, 1994.

FEASIBILITY OF RADIO SOUNDING IN THE MAGNETOSPHERE

W. Calvert, R. F. Benson, 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 c/o W. Calvert Department of Physics and Astronomy
University of Iowa, Iowa City. IA 52242

Providing remote density measurements of unprecedented accuracy and
coverage, pulsed radio sounding in the magnetosphere would permit
almost continuous observation of the plasmapause and magnetopause,
along with complete cross-sectional density images of plasmasphere,
cusp, and auroral plasma cavity.  This technique would thus revitalize
magnetospheric research and answer many important questions that
cannot be addressed by local measurements.

Using modern digital techniques and operating at frequencies of 3 kHz
to 3 MHz, a magnetospheric radio sounder would require 500-meter
tip-to-tip dipole antennas and a transmitter peak pulse power of ten
watts or less.  This sounder would be equipped with three-axis
electric antennas to measure echo directions, including two long-wire
antennas in the spacecraft spin plane and a short, tuned dipole
receiving antenna along the spin axis.  The plasmagram cycle, during
which echo delays and directions are measured as a function of
frequency, would require only a few minutes and include both
range-gate and Doppler integration to enhance the echo signals. 
Density versus distance profiles could thus be calculated at intervals
of roughly two degrees along a satellite orbit at 6 RE, using
techniques previously developed during the Alouette and ISIS
Topside-Sounder Programs [see Franklin and Maclean, Proc. IEEE, 57,
897-928, 1969; J. E. Jackson, ibid., 960-975].

Considering spatial attenuation, focusing, and competing natural and
instrumental noise sources, such a sounder is found to be quite
feasible and practical.  It is therefore considered an outstanding
candidate for future missions to remotely image the magnetosphere.