Observations and Analysis of Discrete and Diffuse Whistler Mode Echoes Received
by RPI on IMAGE 

Sonwalkar, V. 
University of Alaska Fairbanks, P.O.B. 755915, University of Alaska Fairbanks,
Fairbanks, AK 99775 United States 

Carpenter, D. 
Stanford University, Electrical Engineering, Stanford University, Stanford, CA
94305 United States 

Harikumar, J. 
Los Alamos National Laboratory, Space Data Systems, Los Alamos National
Laboratory, Los Alamos, NM 87545 United States 

Huntzinger, R. 
University of Alaska Fairbanks, P.O.B. 755915, University of Alaska Fairbanks,
Fairbanks, AK 99775 United States 

Spasojevic, M. 
Stanford University, Electrical Engineering, Stanford University, Stanford, CA
94305 United States 

Bell, T. 
Stanford University, Electrical Engineering, Stanford University, Stanford, CA
94305 United States 

Inan, U. 
Stanford University, Electrical Engineering, Stanford University, Stanford, CA
94305 United States 

Taylor, W. 
Goddard Space Flight Center, Code 630, Goddard Space Flight Center, Greenbelt,
MD 20771 United States 

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


We report on observations and analysis of discrete and diffuse whistler mode
echoes received by RPI on IMAGE.  Discrete whistler mode echoes have been
identified on a number of days during the period when IMAGE was at low altitude
(<2000-6000 km) near its perigee in the southern hemisphere.  The observed
whistler echoes have frequencies below local electron cyclotron frequency.  The
echoes are in the frequency range ~13-300 kHz and show time delays of a
fraction of a second, and longer time delays at lower frequencies, typical of
whistler mode propagation. Out of the ~300 cases examined, discrete echoes were
detected in 24 cases. Ray tracing simulations performed in a limited number of
cases indicate that these echoes are the result of reflections of RPI signals
from the Earth-ionosphere boundary.  By comparing measured time delays of RPI
signals with those from ray tracing simulations it is possible to determine (1)
nature of propagation, i.e., ducted or nonducted, and (2) the electron density
along the ray paths.  In one case, 5 May 2000, when IMAGE was at an altitude of
~4000 km and invariant latitude of ~ 70 Deg, the ray tracing analysis leads to
an electron density of ~800 el/cc at the satellite altitude (~4000 km) with a
R-4.5 dependence. These results indicate that whistler mode propagation
analysis performed on discrete whistler mode echoes obtained on IMAGE at high
latitude can be used to obtain improved electron density models in the high
latitude magnetosphere.  The diffuse whistler echoes have frequencies below
local electron cyclotron frequency and are characterized by an apparent spread
in their time delays. The echoes are in the frequency range ~10-300 kHz and
show well defined lower and upper cutoff frequencies. Out of 35 cases examined,
diffuse echoes were observed in 16 cases, with 11 of them identified during the
period when IMAGE was at low altitude (<1500 - 5000 km) near its perigee in the
southern hemisphere, and five when IMAGE was at relatively high altitude
(<10,000-15,000 km) and mostly at geomagnetic latitudes ranging between 40-50
Deg. In several instances, the diffuse echoes were accompanied by z-mode
radiation and in some cases by free space mode echoes. The Z-mode radiation
upper and lower cutoff are used to obtain local electron plasma frequency and
gyrofrequency. We propose that diffuse whistler mode echoes are the result of
scattering of RPI signals by field aligned small-scale (~10-100 m) plasma
density irregularities, commonly found in the low altitude magnetosphere.
Preliminary analysis indicates that the small scale irregularities responsible
for diffuse echoes are located within ~1000 km of the satellite.  A linear
mode conversion mechanism, first proposed to explain spectral broadening of
ground transmitter signals observed on satellites, can account for many
observed features of these echoes including the diffuse nature and cutoff
frequencies. The lower and upper cutoff frequencies and the time delay and its
spread as a function of frequency can be used to determine the location and
nature of plasma irregularities responsible for wave scattering.

_______________
Fall 2002 Meeting of the American Geophysical Union
San Francisco, CA, USA, 6-10 December 2002