Magnetospheric Radio Tomography: Theory and CLUSTER Experiments

Y. Zhai, S. A. Cummer, Electrical and Computer Engineering Department, 
Duke University, Durham, NC 27708, USA.
J. Green, Space Science Data Operations Office, NASA Goddard Space Flight
Center, Greenbelt, Maryland, USA.
B. Reinisch, Center for Atmospheric Research, University of
Massachusetts-Lowell, Lowell, Massachusetts, USA.
I. Christopher, R. Mutel, J. Pickett, D. Gurnett, Dept. of Physics and
Astronomy, University of Iowa, Iowa City, Iowa, USA.
C. P. Escoubet, ESA/ESTEC,Noordwijk,The Netherlands.


Recent studies have shown the feasibility and scientific value
of radio tomography for remote sensing Earth's magnetosphere.
Radio tomography uses radio waves and phenomena known as Faraday
rotation and phase/group delay that can be measured to reconstruct
full images of electron density and magnetic field. Faraday rotation
is the rotation of the polarization of a linearly polarized wave as
it travels through the magnetospheric plasma.  We present first a
flexible and robust direct reconstruction method for magnetospheric
radio tomography. We show that for a combined reconstruction of
plasma density and magnetic field the direct reconstruction method
performs as well as popular iterative methods for large number
of satellites, but it performs significantly better when the
number of satellites becomes small. The main advantages of this
method are that extra information, such as in situ measurements,
can be easily and flexibly incorporated into the reconstruction. We
demonstrate the good performance of this method with MHD simulations
in reconstructing electron density and magnetic field using
constellations of relatively few satellites (11 and fewer) in a
single orbit in a variety of magnetospheric regions.  To validate
the feasibility of the measurements behind radio tomography, we
have performed three separate experiments designed using the Radio
Plasma Imager (RPI) on the IMAGE spacecraft as the signal source,
and the WBD instruments on the four CLUSTER spacecraft as the wave
receivers.  To measure Faraday rotation of the transmitted wave
electric field polarization due to propagation through a magnetized
plasma, RPI signals received by WBD instruments in April 2002 and
May 2003 are carefully analyzed and interpreted for the three radio
tomography experiments.  Based on the time-varying spin modulation
of the WBD received signals, and their spin-phase difference with
the RPI transmitted signal, we report the measured Faraday rotation
and the average electron density extracted along the wave propagation
path. We demonstrate that the deduced average electron density agrees
well with empirical statistical models of the northern polar region.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004