Remote Sensing of Magnetospheric Plasma Density from the Analysis of
Whistler Mode Echoes Received by RPI on IMAGE

V. S. Sonwalkar, Electrical and Computer Engineering Department, University 
of Alaska Fairbanks, Fairbanks, AK 99775, USA (email: ffvss@uaf.edu)

J. Li, Electrical and Computer Engineering Department, University of Alaska 
Fairbanks, Fairbanks, AK 99775, USA

D. L. Carpenter, Electrical Engineering Department, Stanford University, 
Stanford, CA 94305, USA

A. Venkatasubramanian, Electrical and Computer Engineering, University of 
Alaska Fairbanks, Fairbanks, AK 99775, USA

T. F. Bell, Electrical Engineering Department, Stanford University, Stanford, 
CA 94305, USA

U. S. Inan, Electrical Engineering Department, Stanford University, Stanford, 
CA 94305, USA

R. F. Benson, MC 692, NASA Goddard Space Flight Center, Greenbelt, MD 20771,
USA.

B. Reinisch, Center for Atmospheric Research, University of Massachusetts at
Lowell, Lowell, MA 01854, USA


Whistler mode wave injection and reception using the RPI Instrument on IMAGE
satellite has led to a new remote sensing method to measure the plasma
density and density structures in the magnetosphere. During 2000-2002
period, RPI frequently recorded discrete (characterized by discrete spectral
form) and diffuse (characterized by multiple receptions at each frequency)
whistler mode echoes in ~10-400 kHz frequency range when IMAGE was at low
altitudes (< 6000 km) in the inner plasmasphere or near its perigee in the
southern hemisphere. In most cases, the whistler mode echoes were
accompanied by Z-mode echoes. The discrete and diffuse echoes, were observed
~ 5% to 10% and ~ 10% to 20%, respectively, of the total number of
transmissions at low altitude. Ray tracing simulations indicate that the
discrete echoes may result from reflections of RPI signals from the
Earth-ionosphere boundary and that the diffuse echoes may result from
scattering of RPI signals from small scale (~ 10-100 m) plasma density
irregularities. 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 10 cases
from the period 21 April to 28 August 2000, when the satellite was at >50 deg.
latitude, indicated that the electron density varied between 500 to 1000
el/cc at ~4000 km altitude and showed r^-4.5 dependence with altitude. Two
cases from the period of 20 May to 31 May 2002, when discrete echoes were
accompanied by Z-mode echoes, were analyzed to determine the electron
density. In these cases, it was possible to determine the local electron
density at the satellite from the Z-mode cutoffs and the local
gyrofrequency. This allowed determination of electron density at the F2
layer peak. The simulations showed that the electron density at the F2 layer
peak (~250 km) was about ~3-4 e5 el/cc in either cases, and the electron
density at ~4000 km altitude was about 500 el/cc in one case and about 75
in the other case. 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.
One diffuse case, 06 August 2000, was analyzed. Simulation result indicated
the presence of . 10 . 100 m scale plasma irregularities within . 1000 km of
the IMAGE satellite (Altitude ~2500 km, lambda m ~65 degrees) 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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Presentation, Fall Meeting, American Geophysical Union, San Francisco, USA, 
8-12 December 2003