Relative Power Levels of Multiple RPI/IMAGE Field-Aligned Magnetospheric Echoes

R. F. Benson, Goddard Space Flight Center, Greenbelt, MD 20771,
  301-286-4037, u2rfb@lepvax.gsfc.nasa.gov;
S. F. Fung, Goddard Space Flight Center, Greenbelt, MD 20771,
  301-286-6301, fung@nssdca.gsfc.nasa.gov;
J. L. Green, Goddard Space Flight Center, Greenbelt, MD 20771,
  301-286-7354, green@nssdca.gsfc.nasa.gov;
B. W. Reinisch, University of Massachusetts Lowell, Lowell, MA 01854,
  978-934-4903, bodo_reinisch@uml.edu

The Radio Plasma Imager (RPI) (B. W. Reinisch et al., Geophys. Res
Lett., 28, 1167-1170, 2001) on the Imager for
Magnetopause-to-Aurora Global Exploration (IMAGE) satellite (J. L.
Burch, EOS, Trans. AGU, 82, 241 and 245, 2001) often detects
discrete long-range echoes when IMAGE is located in the general vicinity
of the plasmapause. The long time delays indicate that these echoes,
which extend over a few hundred kilohertz up to at least 600 kHz, do not
correspond to signal returns from the nearby plasmapause. Some echoes
include traces that do not extend toward zero virtual range. These
"floating" traces often have an "epsilon" shape similar to those
observed at low altitudes in the topside ionosphere by ISIS 1 when it
was immersed within equatorial plasma bubbles. The ISIS 1 observations
were interpreted in terms of ducted propagation along field-aligned
electron-density irregularities (FAI) that were maintained from one
hemisphere to the other (P. L. Dyson and R. F. Benson, Geophys. Res
Lett., 5, 795-798, 1978) following the earlier work based on
Alouette 1 observations at higher (1,000 km) altitude (D. B. Muldrew,
J. Geophys. Res., 68, 5355-5370, 1963).

An investigation of the relative power levels of these magnetospheric
epsilon signatures was carried out in order to determine the degree of
signal loss between the various echo components (which are attributed to
different magnetospheric path lengths). At each frequency, the power
level corresponding to the signal received on the x dipole antenna (the
same one used for transmission) for each echo component was recorded. 
The results indicate that the path length of the signal is not a major
contributor to differences observed in the received power among the echo
components. Thus the magnetospheric "epsilon" shaped echoes appear to
be due to nearly loss-free propagation along FAI that are maintained
from one hemisphere to the other in the vicinity of the plasmapause in
a manner similar to their ionospheric counterparts.