DRAFT

Empirical models of the plasma density in the plasmasphere and polar cap

Xueqin Huang, Bodo W. Reinisch, Paul Song, Gary S. Sales, Patrick Nsumei
Environmental, Earth, & Atmospheric Sciences Department., 
Center for Atmospheric Research 
University of Massachusetts Lowell

James J. Green, Robert F. Benson, Shing Fung
NASA Goddard Space Flight Center

Dennis L. Gallagher
NASA Marshal 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 Imager for Magnetopause-to-Aurora
Global Exploration (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 using three orthogonal antennas, 500-m long
dipoles in the spin plane, and a 20-m dipole along the spin axis. IMAGE is
on an elliptical polar orbit with an altitude of 7.2 RE at apogee, and
1,200 km at perigee. Echo traces recorded on plasmagrams in the polar cap
and the plasmasphere have been inverted into electron density profiles.
When sounding from inside or outside the plasmapause, the echoes received
from the plasmasphere are often diffuse in nature exhibiting substantial
range spread and it is difficult to exactly determine the electron density
profile. The direct echoes received from the topside ionosphere when IMAGE
is in the polar cap, however, form very well defined traces that can be
inverted to profiles.
 
When IMAGE is sounding inside or close to the plasmapause there are very
often clear echo traces with virtual ranges of up to 7 RE. Reinisch et al.
[Geophys. Res. Lttrs., 28(24), 4521-4524, 2001] have shown that the waves
producing these traces have propagated along the magnetic field lines and
are reflected in the topside ionosphere. A newly developed profile
inversion program calculates the electron density distribution either
along the direct path from the spacecraft to the topside ionosphere in the
polar cap, or along the magnetic field line through the spacecraft in the
plasmasphere. The RPI sounding observations of the field aligned
propagation echoes 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. This measurement is made within the time it takes to
step through the frequencies that cover the plasma frequencies, generally
less than 20 seconds.

The polar cap soundings show the plasma density distribution from ~1Ð5 RE.
The measurements show a clear dependence of the densities on magnetic
activity. For the plasmasphere density distribution, we analyzed sounding
data for two different periods: 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 27th General Assembly of International Union of 
Radio Science (URSI), Maastricht, The Netherlands, August 17-24, 2002.