Greenwald, R. A., Limitations of Radiowave Imaging of the
Magnetopause, presented at the American Geophysical Union Fall
Meeting, San Francisco, December 15-19, 1996.

Limitations of Radiowave Imaging of the Magnetopause

R. A. Greenwald (Johns Hopkins Applied Physics Laboratory, Laurel, MD
20723; 301-953-5408; e-mail: ray_greenwald@jhuapl .edu)

Last year, Calvert et al. [1995] published a paper in Radio Science
proposing the use of a VLF radio sounder to probe the structure and
dynamics of the magnetopause.  The paper claimed that with a sounder
of modest capability,  10W transmitter and 500 m tip-to-tip antenna,
it would be possible to image the magnetopause with an angular
resolution of 0.4 to 4 degrees. The purpose of this paper is to point
out that some of the assumptions used by Calvert et al.[1995] in
determining their sensitivity, at best, are overly optimistic and
more likely are incorrect.  Two assumptions are of particular 
importance. Firstly, Calvert et al. [1995] claim that over most of
the frequency range for magnetopause sounding, the noise level of the
system will be determined by receiver noise.  Observations reported in
the literature indicate that the noise level will actually be determined
by the level of trapped and escaping continuum radiation.  This level
is more than one order of magnitude larger than the noise level assumed
by Calvert et al. [1995]. Secondly, Calvert et al. [1995] claim that
they will be able to enhance their signal-to-noise ratios by ~20 dB
through coherent digital integration.
 
Actually, little if any coherent gain will be achieved due to the
dynamic nature of the magnetopause.  The effect of these two performance
reductions will be to reduce the potential signal-to-noise ratio of a
magnetopause sounder to less than unity. Instead of being able to image the
magnetopause with high angular resolution, a radio sounder will have difficulty
detecting any magnetopause return.

Calvert, W. et al., The feasibility of radio sounding in the magnetosphere,
Radio Sci., 30, 1577-1595, 1995.