Radio sounding the Earth's plasmasphere and excitation of the whistler and Z 
modes by the Radio Plasma Imager (RPI) instrument on the IMAGE satellite

D. L. Carpenter, M. A. Spasojevic, T. F. Bell, and U. S. Inan, 
Space, Telecommunications, and Radioscience Laboratory, Stanford University

V. S. Sonwalkar, Dept. of Electrical Engineering, 
University of Alaska Fairbanks

B. W. Reinisch and I. A. Galkin, 
Center for Atmospheric Research, University of Massachusetts Lowell

R. F. Benson, J. L. Green, and S. F. Fung, 
NASA Goddard Space Flight Center, Greenbelt, MD

W. W. L. Taylor, 
Raytheon ITSS, GSFC, Greenbelt, MD

S. A. Boardsen, 
EMERGENT Information Technologies, Inc., Largo, MD 


Since May, 2000, the Radio Plasma Imager (RPI) on Imager for Magnetopause-to-Aurora 
Global Exploration (IMAGE) has been sounding the Earth's plasmasphere from various 
points along the satellite's polar orbit, with apogee ~ 8 RE  geocentric distance 
and perigee near 1200 km altitude. RPI also measures local plasma parameters along 
the IMAGE orbit through passive measurements of natural wave activity. A principal 
new result is that both the plasmapause region and the main plasmasphere tend to 
be "rough" targets at sounding frequencies ranging from about 50 kHz to 1 MHz. 
Echoes that return from directions generally Earthward or transverse to the 
geomagnetic field are usually not the discrete traces on range-versus-frequency 
records (plasmagrams) that ray tracing simulations in smooth magnetospheric 
density models predict. Instead, they exhibit various amounts of spreading, from 
~ 0.5  to 2 RE in virtual range (range assuming free-space speed of light 
propagation). For echo turning points within the main plasmasphere, the range 
spreading is attributed to scattering from, partial reflection from, and 
partial-path propagation along geomagnetic-field -aligned electron density 
irregularities with cross-field scale sizes ranging from about 200 m to over 10 km 
and electron density within  ~10 % of background. For turning points within the 
plasmapause region, the spreading appears to be partially attributable to a 
longitudinal distribution of irregularities along the plasmapause "surface". When 
RPI operates in the inner plasmasphere and at moderate to low altitudes over the 
polar regions, pulses emitted at the low end of its 3 kHz to 3 MHz sounding 
frequency range can propagate in the whistler mode or in the Z mode. During 
soundings with both 51.2 ms pulses and 3.2 ms pulses, whistler mode echoes have 
been observed both in discrete, lightning-whistler-like forms, and in diffuse 
forms indicative of mode coupling at the boundaries of density irregularities. 
Both of these types of echoes have potential for diagnostics of plasma 
distributions and structures. 

_______________
Proceedings of the Ionospheric Effects Symposium, Alexandria, VA, 2002.