Identification of Long-Range IMAGE/RPI Echoes Based on Received Power Levels

R. F. Benson, S. F. Fung, J. L. Green
NASA GSFC

B. W. Reinisch,  I. Galkin, G. Khmyrov
U. Mass. Lowell

D. Carpenter, M. Salvati
Stanford

H. G. James
CRC

The radio plasma imager (RPI) on the Imager for Magnetopause-to-Aurora Global
Exploration (IMAGE) satellite detects well-defined long-range echoes in the
vicinity of the plasmapause. These echoes correspond to virtual ranges (i.e.,
assuming free-space speed-of-light velocities) often greater than an earth radius
(RE) when the distance to the plasmapause is only of the order of 0.1 RE. The
virtual ranges are consistent with distances to the polar-cap ionosphere.  The
location of IMAGE relative to the polar-cap ionosphere suggests that the RPI
echoes may result from waves that are guided (or ducted) along electron-density
irregularities that are maintained along the direction of the background magnetic
field. Such field-aligned propagation is common in the topside ionosphere and can
result in an increase in the echo signal by 20 to 40 dB [G. E. K. Lockwood, J.
Geophys. Res., 78, pp. 2244-2250, 1973] over that expected from inverse-square
path loss. We investigate the received signal strengths of the RPI echoes in
order to identify which ones can be attributed to wave ducting. We first
demonstrate the technique by using ISIS 1 ionospheric topside-sounder ionograms
recorded near apogee (3500 km) at low latitudes in order to obtain the longest
possible propagation paths across the direction of the terrestrial magnetic
field. This geometry represents a difficult sounding condition because there is
no field-aligned propagation to the ionospheric target below. Earlier
investigations of echo signal strength were based on much shorter distances using
either Alouette 1 at 1,000 km [L. E. Petrie et al., J. Geophys. Res., 70, pp.
4347 -4356 1965] or Alouette 2 at about 1,500 km [C. A. Franklin and M. A.
Maclean, Proc. IEEE, 57, pp. 897-929, 1969]. The results of the investigation of
the signal strengths of the RPI echoes obtained when IMAGE is in the vicinity of
the plasmapause will be compared to the Alouette/ISIS ionospheric topside-sounder
results and to the types of echoes expected for a magnetospheric radio sounder
near the plasmapause.

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Submitted to the Spring 2001 AGU Meeting, Boston, Massachusetts