Whistler and Z mode radio sounding of the magnetosphere at altitudes < 5000 km by 
RPI on IMAGE

V. Sonwalkar (1), J. Li (1) , D. Carpenter (2) , A. Venkatasubramanian (1), 
R. Benson (3), B. Reinisch (4)

(1) University of Alaska Fairbanks (ffvss@uaf.edu),
(2) Stanford University
(3) NASA Goddard Space Flight Center
(4) University of Massachusetts at Lowell


Radio Plasma Imager (RPI) on the IMAGE satellite can be used for the whistler and
Z mode radio sounding of the magnetosphere using the low end of its 3-kHz to 3-MHz
sounding frequency range. During soundings with both 25.6-ms pulses and 3.2-ms
pulses, whistler-mode echoes have been observed in (1)  discrete, 
lightning-whistlerlike forms, and in (2) diffuse, widely time-spread forms
suggestive of mode coupling at the boundaries of density irregularities. Discrete
echoes, observed at altitudes less than "H 5000 km both inside the plasmasphere
and over the auroral and polar regions, are believed to be RPI signals reflected
at the Earth-ionosphere boundary. Diffuse echoes have also been observed at
altitudes less than 5000 km, being most common poleward of the plasmasphere. In
regions poleward of the plasmasphere, diffuse Z-mode echoes were found to
accompany both discrete and diffuse whistler-mode echoes 90% of the time, and were
also present during 90% of the soundings when no whistler-mode echoes were
detected. The upper frequency cutoff, identified as the upper hybrid frequency, of
the Z mode echo, and the gyrofrequency, as estimated from the gap in the Z mode
echo, are used to calculate the local plasma density. By comparing the measured
dispersion of discrete echoes with that from ray tracing simulations, it is
possible to determine the plasma density along the ray path as well as to
determine the nonducted or ducted modes of propagation. The ray tracing
simulations carried out for several cases indicated that the electron density
varied between 50 to 1000 el/cc at "< 4000 km altitude and showed R" 4.5
dependence with altitude. By comparing measured dispersion of diffuse echoes with
that from ray tracing simulations, it is possible to determine the locations and
spatial sizes of density irregularities responsible for diffuse echoes. In one
case, 06 August 2000, simulation result indicated the presence of "< 10"100 m
scale plasma irregularities within "<2000 km of the IMAGE satellite
(Altitude"<2500 km, Èm "< 65%¾) at the time of observation. These results are in
general consistent with previous observations of plasma density in the low
altitude magnetosphere.

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Presented at the 35th COSPAR Scientific Assembly, Paris, France, July 18-25, 2004.