Observations of Ducts in the Plasmasphere by RPI

G.S. Sales, X. Huang, B.W. Reinisch, P. Song
Center for Atmospheric Research
University of Massachusetts Lowell

D. L. Carpenter
Stanford University

S. Fung, R. Benson, J. Green
NASA-GSFC

RPI sounding observations made with the IMAGE satellite, as 
it approaches and recedes from the plasmasphere, show that 
ducted modes often dominate the propagation process.  These 
ducts are frequently observed on successive plasmagrams 
covering a period of 8 to 10 minutes while the spacecraft 
moves 2000 to 3000 km across the geomagnetic field lines.  
These ducts appear to be a somewhat consistent feature of 
the plasmasphere/plasmapause region.  Based on 
angle-of-arrival measurements it has been determined that 
these reduced plasma density ducts are aligned along the 
geomagnetic field.  The RPI three orthogonal-axis antenna 
system makes it possible to measure the arrival angles of 
reflected signals.  These arrival angle calculations are 
made at each frequency for each identified mode within a 
plasmagram by using the received signal amplitude and phase 
on the three antennas.  The measured angle-of-arrival is 
compared with the geomagnetic field direction at the 
satellite using a model of the earthÕs field.  These 
results are consistent with the magnetic orientation of the 
ducts.  Other propagation modes are also observed in this 
region but there is a natural selection process favoring 
the ducted modes because these modes suffer less spreading 
loss than directly reflected modes.  From the available RPI 
data for approximately 200 orbits acquired over a four 
month period, statistics on the frequency of occurrence, 
duration, the physical location of these ducts and their 
spatial dimensions are presented.  

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To be presented at the URSI meeting, Boston, Mass, July, 2001