Conjugate Asymmetry in the Plasmaspheric Density Distributions at Solstice -
RPI Case Studies

Xueqin Huang, Bodo W. Reinisch, Paul Song, Ivan Galkin, George Cheney
Center for Atmospheric Research, University of Massachusetts Lowell

James L. Green, Robert F. Benson, 
NASA Goddard Space Flight Center

Jiannan Tu, James Horwitz
Department of Physics, Center for Space Plasma and Aeronomic Research 
University of Alabama in Huntsville

The Radio Plasma Imager (RPI) on the IMAGE satellite [Burch et al., EOS
Trans. American Geophys. Union, 82, 241-145, 2001] performs sounding in the
frequency range from 3 kHz to 3 MHz. The observations during the first two
years of operation frequently show echo traces from signals that have
propagated along a magnetic field line and are reflected in the topside
ionosphere. A newly developed profile inversion program calculates the
electron density distribution along the field line. The RPI sounding
observations make it possible to measure the entire field-aligned profile
from the satellite location to the ends of the field line in the topside
ionosphere [Reinisch et al., Geophys. Res. Lttrs., 28(24), 4521-4524, 2001]
within the time it takes to step through the frequencies that cover the
plasma frequencies.

Sounding data for two different periods were analyzed: equinox and solstice.
For June 2001, a sequence of eight consecutive profiles was determined for
one morning pass when IMAGE changed position from L=2.2 to L=3.3. Soundings
were made every two minutes. In March 2001, noon profiles were determined
for L values between 2.4 and 3.6. Each sounding provides a complete
hemisphere-to-hemisphere density profile. By piecing together these profiles
along different L shells we were able to construct two-dimensional density
distributions as a function of latitude and L-shell.  The results show a
symmetric north-south profile during the equinox period, and an asymmetric
distribution at solstice with higher densities in the winter hemisphere.

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To be presented at the 10th International Ionospheric Effects Symposium
Alexandria, Virginia, USA, May 7-9, 2002.