Remote Sensing of Electron Density and Density Irregularities from Whistler and 
Z-Mode Sounding Experiments by RPI on IMAGE    

V. S. Sonwalkar, R. Proddaturi, A. Venkatasubramanian, D. L. Carpenter, 
R. F. Benson, B. W. Reinisch  
 
 
Discrete and diffuse whistler mode echoes are regularly observed when the Radio 
Plasma Imager (RPI) on the IMAGE satellite operates at the low end of its 3-kHz
to 3MHz sounding frequency range in the plasmasphere and at moderate to low
altitudes over  the polar regions.  In regions poleward of the plasmasphere,
diffuse Z-mode echoes are  found to accompany both discrete and diffuse
whistler-mode echoes 90% of the time.  Previous research indicates that discrete
whistler-mode echoes are a consequence  of RPI signal reflections at the bottom
side of the ionosphere.  The diffuse whistler-mode  echoes are a result of
scattering of RPI signals by geomagnetic-field-aligned electron  density
irregularities located within ~2000 km Earthward of the satellite and in
directions  close to that of the field line passing through IMAGE.  Diffuse
Z-mode echoes are  believed to be due to the scattering of RPI signals from
electron density irregularities  within ~3000 km of the satellite, particularly
those in the direction perpendicular to the  geomagnetic field.  Measurements of
electron density and density structures close and  remote to IMAGE location are
possible from the analysis of whistler and Z-mode echoes:  (1) The observed
upper cutoff near the local upper hybrid resonance frequency and the  observed
ÒgapÓ near the gyrofrequency in the plasmagram of diffuse Z-mode permit 
determination of the local electron density. (2) The measured dispersion of
discrete  whistler mode echoes combined with raytracing analysis provides
electron density profile  along the field line passing through the satellite. 
(3) Diffuse whistler and Z mode echoes  provide location and scale size of
geomagnetic-field-aligned electron density  irregularities located up to a few
thousand kilometers from the satellite location in the  direction parallel and
perpendicular to the geomagnetic field.  We have analyzed several cases of
whistler and Z-mode echoes to measure  electron densities and to identify
locations and scale sizes of density structures present in  the mid to high
latitude magnetosphere up to 5000 km altitude.  The results obtained are  in
general agreement with those reported from previous spacecraft measurement of 
electron density and density structures.   

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Sounding Experiments by RPI on IMAGE, XXVIII General Assembly of International 
Union of Radio Science (URSI), New Delhi, India, 23-29 October 2005