Local Electron Density Measurements from  Sounding Experiments by RPI on IMAGE

R. Proddaturi, V.S. Sonwalkar, J. Li, A. Venkatasubramanian, D. Carpenter, R.
Benson, and B. Reinisch


RPI sounding experiments lead to a variety of echoes, propagating in various
plasma wave modes, and local resonances. Characteristic frequencies of these
echoes and resonances can be used to determine the local plasma frequency and
thus the local electron density. In this work we have estimated plasma frequency
by two methods: (1) using upper hybrid frequency measured from the diffuse Z
mode echo upper cutoff and gyro-frequency measured from a gap in the diffuse Z
mode echo or from resonances at the multiples of gyrofrequency, (2) upper hybrid
frequency from the diffuse Z mode and the free space cutoff frequency $f_{R=0}$
from the $R-X$ mode echo. Broadband diffuse Z-mode echoes occur 90% of the time
at high latitudes (lambda m > 45 degrees S) near perigee in the southern
hemisphere, where f{pe} << f{ce}. In the middle and low latitudes (lambda m < 45
degrees), where f{pe} >> f{ce}, Z-mode echoes are narrowband and are often
accompanied by Qn and Dn resonances. The free space $R-X$ mode echoes are
commonly observed at both high and low latitudes. Multiples of gyrofrequency are
typically observed at mid- to low-latitude in both the northern and southern
hemisphere and at high latitude in the northern hemisphere. RPI plasmagrams were
analyzed for three orbits (apogee to apogee) in the year 2002. These three
orbits were selected because suitable sounding programs, those that can cover Z
mode bandwidth over a wide range of latitude, were used, and also because a
large number of diffuse Z mode echoes were actually observed. Electron densities
as low as 10 el/cc and as high as 9000 el/cc were measured. The transmission
frequencies place a limitation on the upper and lower limits of measurable
f{pe}. The measured f{pe} values showed good agreement with measurements made
from the thermal noise but showed large deviations when compared with model
f{pe} values. For a particular orbit on August 26, 2002, $N_e$ measured was as
low as ~20 el/cc at higher altitudes outside the plasmasphere (lambda m > 60
degrees , altitude >7000 km, MLT=1.89) and increased as IMAGE approached the
plasmasphere. A maximum of ~8900 el/cc was measured well within the plasmasphere
(L = 1.56, lambda m = 17 degrees N, altitude =2700 km, MLT = 2.44). As the
satellite left the plasmasphere, measured electron density decreased to a
minimum of about 55 el/cc near the auroral zone (L = 6.83, lambda m = 57 degrees
S, altitude = 6277 km, MLT=13.66) and then started to rise again. A sharper
change in N_e was seen at both the inbound and outbound crossings of the
plasmapause. As the satellite again entered the plasmasphere (L = 3.94, lambda m
= 21 degree S, altitude = 15500 km, MLT = 14.34) at a higher altitude the maximum
value of Ne measured was lower (~520 el/cc) as expected. Our results demonstrate
that magnetospheric sounding experiments employing Z mode and free space modes
provide a powerful means of making local plasma density measurements.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004