Inner Magnetosphere Empirical Plasma Density Models Based on Radio Sounder
Observations from the RPI instrument on IMAGE

J. Green (1), B. Reinisch (2), X. Huang (2), P. Song (2), and P. Nsumei (2)

(1) NASA Goddard Space Flight Center, Greenbelt, MD 20771 
(2) Center for Atmospheric Research, University of Massachusetts Lowell, MA 01854 

A new Inner Magnetosphere Empirical Plasma density distribution model (IMEP) is
being developed using the radio plasma imager (RPI) measurements on the IMAGE
satellite. RPI performs radio sounding in the magnetosphere using frequencies
between 3 kHz and 3 MHz. IMAGE is on an elliptical polar orbit with an apogee
altitude of 8.2 RE geocentric radial distance, and a perigee of ~1,000 km. The
primary presentation of RPI echo measurements are in the form of plasmagrams,
which are the magnetospheric analog of ionograms.  A plasmagram is a plot of
echo amplitude as a function of frequency and virtual range (derived from an
echo's delay time).  RPI plasmagrams show echo traces that are formed by signals
propagating over distances of up to 7 RE. Both direct and field-aligned echoes
have been measured. Direct echoes are those signals whose return path, to the
spacecraft, are along the density gradient in the plasma.  The field-aligned
echoes propagate along the magnetic field to both hemispheres, reflect at their
local cutoff frequencies, before returning to the spacecraft. A newly developed
technique inverts the echo traces on the plasmagrams to electron density
profiles. By using consecutive plasmagrams with field-aligned echoes electron
density profiles are obtained at varying L shells while IMAGE moves through the
plasmasphere.  In this way it has become possible to develop an empirical
plasmasphere density model. Our results show a symmetric plasmasphere density
distribution around the magnetic equator at equinox, while higher densities are
contained in the winter hemisphere during solstice. When IMAGE is in the polar
cap region, the vertical polar cap plasma density distribution can be determined
for a variety of geomagnetic conditions.  The resulting polar cap density
models, created from the sounder data, are compared with a number of existing
models.

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Chapman Conference on Physics and Modelling of the Inner Magnetosphere
Helsinki, Finland, 25-29 August 2003