RPI sounding as IMAGE skims the plasmapause: Ne from direct & ducted echoes
and plasma resonances

R. F. Benson, P. A. Webb (NRC), J. L. Green, S. F. Fung
NASA/Goddard Space Flight Center
Greenbelt, Maryland

B. W. Reinisch, X. Huang
Center for Atmospheric Research
University of Massachusetts, Lowell
Lowell, Massachusetts

D. L. Carpenter
STAR Lab
Stanford University
Stanford, California


In early January 2003 the Radio Plasma Imager (RPI) on the Imager for
Magnetopause-to Aurora Global Exploration (IMAGE) satellite performed
numerous soundings in the plasmapause region as the outbound IMAGE skimmed
along the dawn-side plasmapause boundary.  Under these conditions it is
possible for the electron cyclotron frequency at the satellite to decrease
by an order of magnitude, while the electron plasma frequency fluctuates by
less than 50%, over a 3/4 hr time interval along the IMAGE orbit.  These
values can be accurately determined from the RPI-stimulated
electrostatic-wave plasma resonances and electromagnetic-wave cutoffs.  The
soundings in this region reveal a striking sequence of short-range diffuse
echoes and long-range discrete echoes.  These echoes have been interpreted
as direct signal returns from the nearby plasmasphere and guided signals
along the magnetic-field direction from the distant denser plasma region
below the satellite, respectively [Reinisch et al., GRL, 28, 1167, 2001;
Fung et al., Adv. Space Res., 30, 2259, 2002, Carpenter et al., JGR, in
press, 2003].  The latter have been used to determine the electron-density
(Ne) distribution along the magnetic-field direction [Reinisch et al., GRL,
28, 4521, 2001] and to make Ne contours along the orbital path [Nsumei et
al., JGR, in press, 2003].  Here we present results using plasma resonances
and both the diffuse and discrete echoes to obtain local and remote Ne
information during outbound IMAGE dawn-side plasmapause skimming
conditions.

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URSI Meeting, Ohio State University, Columbus, Ohio, June 22-27, 2003