Spread echoes from the plasmapause region and main plasmasphere received during
high altitude radio sounding by the RPI instrument on the IMAGE spacecraft

D. L. Carpenter, M. A. Spasojevic, T. F. Bell, U. S. Inan 
Space, Telecommunications and Radioscience Laboratory, Stanford University

B. W. Reinisch, I. A. Galkin 
Center for Atmospheric Research, University of Massachusetts Lowell

R. F. Benson, J. L. Green, S. F. Fung  
NASA Goddard Space Flight Center

S. A. Boardsen
EMERGENT Information Technologies, Inc.


Since May, 2000, the Radio Plasma Imager (RPI) on IMAGE has been sounding the
Earth's plasmasphere from various points along the satellite's polar orbit,
with apogee ~8 Earth radii geocentric distance and perigee near 1200 km
altitude. RPI also measures local plasma parameters along the IMAGE orbit
through passive measurements of natural wave activity. A principal new result
is that both the plasmapause region and the main plasmasphere tend to be
"rough" targets at sounding frequencies ranging from about 50 kHz to 1 MHz.
Echoes that return from directions generally Earthward or transverse to the
geomagnetic field are usually not the discrete traces on range-versus-frequency
records (plasmagrams) that ray tracing simulations in smooth magnetospheric
density models predict. Instead, they exhibit various amounts of spreading,
from  about 0.5  to 2 Earth radii in virtual range (range assuming free-space
speed of light propagation). For echo turning points within the main
plasmasphere, the range spreading is attributed to scattering from, partial
reflection from, and partial-path propagation along geomagnetic-field -aligned
electron density irregularities with cross-field scale sizes ranging from about
200 m to over 10 km and electron density within  about 10 percent of
background. For turning points within the plasmapause region, the spreading
appears to be partially attributable to a longitudinal distribution of
irregularities along the plasmapause "surface." That the irregularities are
field-aligned is suggested by the efficiency with which RPI, in addition to the
spread "direct" echoes noted, excites other echoes that are guided, sometimes
into both hemispheres, along the geomagnetic field, and which return with
minimal range spreading. Although the occasional existence of the
irregularities in question is supported by evidence from other radio
experiments such as topside sounders and whistler-mode instruments, and is also
supported (somewhat less directly) by in situ measurements on satellites such
as CRRES, ISEE, synchronous satellites, and CLUSTER, the apparently new aspect
of the RPI data is the regularity with which the spreading is observed, even
during periods of deep magnetic quieting. In future studies that may incude
full wave treatments and ray tracing in density models that include field
aligned irregularities, it is hoped to learn about the specific ways in which
irregularities act to produce the observed range spreading effects. Once
progress is made in that area, high altitude radio sounding may provide a means
of studying the scale sizes and occurrence properties of plasmasphere
structures that are otherwise not easily susceptible to measurement. The RPI
data should add impetus to an already existing body of theoretical work on
mechanisms that may naturally give rise to irregular structures within the
plasmasphere and plasmapause regions, such as interchange and quasi interchange
instabilities, shear flow instabilities, and the perturbing action of electric
fields with various spatial scales.

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To be presented at the 27th General Assembly of International Union of 
Radio Science (URSI), Maastricht, The Netherlands, August 17-24, 2002.