Wave Polarization of Field-Aligned Propagating Modes Using IMAGE/RPI

G. Sales, X. Huang, B. Reinisch, I. Galkin
Center for Atmospheric Research
University of Massachusetts Lowell

Field-aligned propagation (FAP) modes have been observed extensively during
the lifetime of the IMAGE/RPI satellite project.  Both direction-of-arrival
and time delay measurements have supported the hypothesis that these modes
are directed along the geomagnetic field lines from the satellite down to
altitudes where reflection back to the satellite occurs.  The plasmagram
inversion technique, developed at UMLCAR, has determined that these "guided"
modes have, primarily, extraordinary polarization.  With the orthogonal
antenna system on the IMAGE satellite, used first to determine the arrival
angle, it is now also possible to analyze the polarization of the arriving
waves over a range of frequencies.  From the measured amplitude and phase on
the three antennas we can reconstruct the polarization ellipse at the
satellite.  

We have selected one of the many sequences of plasmagrams to analyze the
polarization characteristics of these FAPs.  These FAP modes are
predominately observed for 2 < L < 5 with a median value of L = 3. Of these,
approximately 40% are in sequences of more than one plasmagram and the
median is between two and three plasmagrams.  On Aug. 25, 2000 as IMAGE/RPI
approached the plasmasphere, at L = 4.75, a sequence of three plasmagrams
recorded these field aligned propagation modes over a period of 8 minutes. 
The sequence ended at L = 4.45, a significant travel distance for the
satellite from the beginning to the end.  We discuss the extensive passive
and active calibration analyses that were required to determine the correct
arrival-angles for these modes.  Building on the success in the
arrival-angle analysis we turned our attention to the polarization
characteristics of the FAP modes.  The RPI plasmagram is used to determine
the local electron gyro frequency and the local plasma frequency.  Combining
these with the arrival-angle measurements and with the received amplitude
and phase on the antenna system we can determine the received polarization
as a function of frequency and time.   

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Presented at the January 2002 URSI meeting, Boulder, Colorado