Can Field-aligned Propagation Satisfy the Propagation Requirement of Index of Refraction? 

Huang, X., Song, P., Sales, G., Reinisch, B.W. 


The radio plasma imager (RPI) on the IMAGE satellite actively sounds the magnetosphere, 
measuring time delays of the returning echoes for a sequence of sounding frequencies. 
When the satellite is located in the plasmasphere the measurements generally show two 
distinct echo traces with the same polarization (identified as the X-mode). One of the 
traces is returned from the closer hemisphere and the other from the conjugate 
hemisphere. Assuming that the waves propagate along the magnetic field line, Reinisch 
et al. (GRL, 4521, 2001) showed that the plasma density distribution along the field 
could be inverted from some echo traces. The inverted density distribution can be used 
to predict other traces observed. A critical assumption made in the inversion technique 
is the exact parallel propagation. The propagation direction of a wave in space depends 
on the spatial variations of the index of refraction, which cannot be determined by a 
single line of measurements. We use the inversion technique to a time sequence of RPI 
measurements during a single pass; each complete measurement represents a density 
profile along a field line. The series of line profiles provides a 2-dimensional 
distribution. Based on this distribution, we derive an empirical model that best fits 
the measurements while satisfying the propagation requirement from the changes in the 
index of the refraction. The deduced function, varying with radial distance and 
magnetic latitude, describes this plasma density distribution with only a small number 
of parameters. The six profiles are fitted with a simplified version of this function 
and the results are discussed.  

_______________
Presented at the 2002 Spring A.G.U. Meeting, 
Washington, D.C., U.S.A., 28-31 May 2002