Amplitude and Polarization of Field Aligned Propagating Radio Signals in the
Plasmasphere Observed with RPI on IMAGE

Gary Sales, Xueqin Huang, Bodo Reinisch, and Ivan Galkin 
Center for Atmospheric Research, University of Massachusetts Lowell

Robert Benson, 
NASA Goddard Space Flight Center

Magnetic field aligned propagation (FAP) dominates the radio signals from
RPI/IMAGE satellite whenever the satellite enters or leaves the
plasmasphere.  These signals appear to propagate along the magnetic field
passing through the satellite position to both the northern and southern
hemispheres returning to the satellite along the same paths after
reflection. Using the measured voltages on the three orthogonal antenna
system on board the RPI/IMAGE satellite, a technique was developed to
uniquely determine the arrival-angle and the polarization of each echo after
reflection from both the northern or southern hemisphere.  The results of
this analysis show that the received field aligned propagated (FAP) signals
from both hemispheres are predominately X-mode polarization.  This supports
the observation by Huang and Reinisch based on their electron density
profile inversion procedure.  Here it was necessary for them to assume that
both the northern and southern hemisphere echoes were X-mode polarizations
in order to obtain consistent electron density profiles. Statistical
analysis of these FAP modes indicates that they occur about 30% of the time
as the IMAGE satellite moves within the plasmasphere.  These guided modes
are seen at L-shell values between 2 and 4.  They also tend to be seen on
sequential plasmagrams approximately 40% of the time while the satellite
moves a significant distance through the region.  

One mechanism for the guidance of these signals along the earthÕs magnetic
field are plasmaspheric ducts in which these waves propagate with less than
1/r rate of amplitude decrease.  A second mechanism involves the spatial
spreading of near-longitudinally propagating waves with X-mode polarization.
 The refractive surface, using the profiles derived from the electron
density inversion of sequential plasmagrams, indicates that spatial
spreading is least in the direction along the earthÕs magnetic field for the
X-mode.  In order to test these hypotheses, ray tracing at small angles to
the magnetic field direction are analyzed quantitatively to determine the
expected losses.  Using the 3-dimensional HASEL ray tracing code a
comparison is made between O-mode and X-mode propagation losses as a
function of the angle the rays make with the earthÕs magnetic field

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To be presented at the 10th International Ionospheric Effects Symposium
Alexandria, Virginia, USA, May 7-9, 2002.