Small-scale field-aligned plasmaspheric density structures inferred from 
RPI on IMAGE

D. L. Carpenter (1), M. A. Spasojevic (1), T. F. Bell (1), U. S. Inan (1), 
B.W. Reinisch (2), I. A. Galkin (2), R. F. Benson (3), J. L. Green (3), 
S. F. Fung (3), and S. A. Boardsen (4)

1 - Space, Telecommunications and Radioscience Laboratory, 
    Stanford University, Stanford, California, USA.
2 - Center for Atmospheric Research, University of Massachusetts, 
    Lowell, Massachusetts, USA.
3 - NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
4 - EMERGENT Information Technologies, Inc., Largo, Maryland, USA.

Among the objectives of the Radio Plasma Imager (RPI) on IMAGE is the 
observation of the Earth's plasmasphere from the satellite's polar orbit, 
with apogee ~8 Re geocentric distance and perigee near 1200 km altitude. 
This objective is here pursued by: (1) remote sounding from high altitude 
regions outside the main plasmasphere; (2) sounding within the 
plasmasphere; (3) in situ passive measurements of natural wave activity. 
During sounding of the plasmasphere from both outside and inside the 
plasmapause, RPI echoes that follow non-field-aligned ray paths are 
usually not the discrete traces on range-versus-frequency records 
(plasmagrams) that are predicted by ray-tracing simulations in smooth 
magnetospheric density models.  Instead, such RPI echoes exhibit various 
amounts of spreading, from ~0.5 Re to ~2 Re in virtual range (range 
assuming free-space speed of light propagation). The range spreading is 
attributed to scattering from, partial reflection from, and propagation 
along geomagnetic-field-aligned electron density irregularities. There 
exists a substantial body of theoretical work on mechanisms that might 
explain the appearance of such irregularities both within and beyond the 
plasmasphere. That the spread-producing irregularities are field-aligned 
is suggested by the efficiency with which RPI excites discrete echoes 
that propagate along the geomagnetic field, sometimes into both 
hemispheres. The spatial distributions and scale sizes of the spread 
producing irregularities remain to be investigated. The RPI echo data, 
however, coupled with earlier evidence from topside sounders and 
whistler-mode instruments, suggest that they can have cross-field scale 
sizes within a range from ~200 m to over 10 km and electron densities 
within ~10% of background. RPI is found capable of detecting plasmapause 
locations from distances of ~3 Re or more. When minimal signal 
integration is used, the location and range of density values of a steep 
plasmapause can be determined from distances of order 1 Re, and echoes 
can at times be returned from points extending inward from the 
plasmapause to locations where the electron density reaches ~ 3000 cm-3, 
which is usually at L < 3.

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Accepted for publication in Journal of Geophysical Research, 2002