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

Sonwalkar, V S,  Harikumar, J,  Carpenter, D L,  Bell, T F,  Spasojevic,
M,  Inan, U S,  Taylor, W W,  Reinisch, B W

We report on observations and analysis of diffuse whistler mode echoes
received by RPI on IMAGE. 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 on 13
cases, with 9 of them identified during the period when IMAGE was at low
altitude (<= 1500 - 5000 km) near its perigee in the southern hemisphere,
and four when IMAGE was at relatively high altitude (~ 10,000 km) and at
geomagnetic latitudes ranging between 40-50 degrees. In several instances,
the diffuse echoes were accompanied by z-mode radiation and in some cases
by free space mode echoes. We propose that these echoes are the result of
scattering of RPI signals by meter scale irregularities, commonly found in
the low altitude magnetosphere. 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.