Deduction of electron density distribution in the plasmasphere from RPI 
plasmagrams

Xueqin Huang and Bodo W. Reinisch


The Radio Plasma Imager (RPI) on board the IMAGE satellite transmits
pulse-modulated radio signals and receives the returned echoes. The echo
information including amplitude and traveling time as a function of sounding
frequency are recorded in the form of plasmagrams, and more than one million
plasmagrams have been collected since the launching of the satellite in 2000.
Similar to the ionograms obtained in ionospheric sounding, distinct echo traces
appear in the plasmagrams and the electron density distribution of the
plasmasphere can be deduced from the echo trace data. This paper describes the
profile inversion technique.

The plasmagram traces show the frequencies, amplitudes and propagation delays of
signals that are reflected somewhere in the plasmasphere and returned back to
the satellite. Several distinct traces often appear in one plasmagram and the
relative echo delay times indicate that the signals propagated along the
geomagnetic field line (Reinisch et al., 2001). Assuming a dipole magnetic
field, a very general functional description of the density distribution around
a field line is found to show that X/Z-mode signals can exactly propagate along
the field line. Raytracing simulations in the modeled duct structure also show
that a bundle of rays in a cone around the field line can propagate in the duct
along the field. We show that all waves in the cone propagating in the X/Z-mode
can reflect back and return to the satellite with almost identical time delays.
In contrast, O-mode rays cannot be reflected in the duct. The field-aligned
propagation mechanism successfully explains the RPI plasmagrams and is the
foundation for the development of the profile inversion technique.

When a signal travels along the field line intersecting the satellite, it is
reflected at the plasma cutoff and returned back to the receiver. The time
delay, or virtual range, depends on the density distribution along the field
line. This physical process is expressed by an integral equation containing the
density in the integrant. Assuming the density can be expanded in terms of
shifted Chebyshev polynomials, the integral equation can be numerically solved
to determine the density distribution along the field line. This inversion
technique has been applied to a limited number of plasmagrams. The deduced
density distribution from this limited database is used to construct a
preliminary plasmasphere density model.

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General Assembly of International Union of Radio Science, New Delhi, India, 
23-29 October 2005