Electron Density Distributions along Magnetic Field Lines in the Magnetosphere 
Deduced from RPI Plasmagrams

Xueqin Huang, Bodo W. Reinisch, and Paul Song
Environmental, Earth, and Atmospheric Sciences Department
Center for Atmospheric Research, University of Massachusetts Lowell

Robert F. Benson, James L. Green, and Shing F. Fung
NASA Goddard Space Flight Center

Don L. Carpenter
Stanford University

The Radio Plasma Imager (RPI) on the IMAGE satellite (launched on March 25, 2000) 
transmits short pulses into all directions and signals are reflected back to the 
satellite from the locations where the electron plasma frequency is equal to the 
radio frequency (for the ordinary mode).  RPI data are displayed in the form of 
plasmagrams showing echo amplitude as a function of range and frequency.  The 
echo arrival direction can be calculated from measurements of the three orthogonal 
dipole antennas.  An algorithm has been developed that inverts the observed echo 
traces into electron density profiles.  Using a model magnetic field, the 
inversion algorithm takes into account the variations of the magnitude and 
direction of the field along the ray path.  In a first iteration the inversion 
program assumes straight-line propagation from the spacecraft to the reflecting 
plasma contour in the direction given by the arrival angle measurement. This 
procedure produces plausible Ne profiles if the echoes propagate in a straight 
line from the polar cap or plasmapause. If, however, there are field-aligned ducts 
in the magnetosphere linking the hemispheres, echoes can be trapped inside a duct 
and propagate along the magnetic field, resulting in the so-called ducted echoes. 
In this case the measured delay times must be expressed as an integral along the 
magnetic field line. Inversion of ducted echo traces along model magnetic field 
lines has recently been developed. It provides the density profile along the 
magnetic field. By analyzing a series of plasmagrams the spatial and temporal 
plasma distributions can be determined. This paper will show RPI observations of 
ducted waves and describe the inversion procedures for obtaining field-aligned 
density profiles. 

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To be presented at the URSI meeting, Boston, Mass, July, 2001