Electron Density Profiles in the Magnetosphere Measured by the Radio Plasma Imager on IMAGE

X. Huang, B.W. Reinisch, P. Nsumei, I.A. Galkin, S. Boardsen, W.W.L. Taylor, J.L. Green, 
R.F. Benson, S.F. Fung 

The Radio Plasma Imager (RPI) on NASA's IMAGE satellite is able to measure the radial 
electron density profiles as seen from the spacecraft. RPI transmits short radio pulses 
(3.2 ms) into all directions and receives echoes that are reflected at locations where the 
electron plasma frequency is equal to the radio frequency (for the ordinary mode) and the 
electron density gradient is parallel to the wave normal. By measuring the echo delay time 
as function of frequency, RPI generates plasmagrams that are similar to the ionograms known 
from groundbased and topside ionospheric sounding. After it has been verified that a given 
echo trace arrives from a fixed direction, the electron density distribution in this 
direction can be calculated. As in ionospheric sounding, the echo delay time t depends on 
the distance r to the reflector, and on the plasma distribution and the magnetic field along 
the propagation path:

            t = (2/c)Int{G[f, fN(r),fH(r),T(r)]dr}
  	 
Here c is the free space speed of light, G the group index of reflection, fN the plasma 
frequency, fH the gyrofrequency, and T the angle between the wave normal and the magnetic 
field. Using a magnetospheric field model, the integral equation can be solved numerically 
for the reflection distance r as function of f. Since the signal with frequency f is 
reflected where fN = f, the profile can be constructed as function of distance from the 
satellite. These radial profiles can then be transformed into a GSM coordinate system to 
describe the plasma distribution in the magnetosphere.  The observed plasma resonances in 
the plasmagram specify the electron density at the spacecraft location, avoiding the 
"starting height" uncertainties encountered for groundbased ionogram inversion. Plasmagram 
traces observed during the first few months of RPI observations are used to illustrate the 
technique.

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Presented at the URSI meeting, Boulder, Colorado, U.S.A., Jan 2001