Plasmasphere Empirical Modeling With the IMAGE Mission

D. L. Gallagher (1), M. L. Adrian (2), S. F. Fung (3), J. L. Green (4), 
B. Sandel (5)

(1) NASA Marshall Space Flight Center, National Space Science & Technology
Center, Mail Code SD50, 320 Sparkman Drive
Huntsville, Alabama, 35805, USA Tel: 256-961-7687, Fax: 256-961-7215, Email:
dennis.gallagher@msfc.nasa.gov <mailto:dennis.gallagher@msfc.nasa.gov

(2) NASA Marshall Space Flight Center, National Space Science & Technology
Center, Mail Code SD50, 320 Sparkman Drive
Huntsville, Alabama, 35805, USA Tel: 256-961-7675, Fax: 256-961-7215, Email:
mark.l.adrian@msfc.nasa.gov

(3) NASA Goddard Space Flight Center, Mail Code 632, Greenbelt, Md 20771,
USA Tel: 301-286-6301, Fax: 301-286-1771, Email: fung@nssdca.gsfc.nasa.gov

(4) NASA Goddard Space Flight Center, Mail Code 630, Greenbelt, Md 20771,
USA Tel: 301-286-7354, Fax: 301-286-1771, Email: green@bolero.gsfc.nasa.gov

(5) University of Arizona, Lunar and Planetary Laboratory,
1040 E. Fourth Street, Room 901, Gould Simpson Building Tucson, Arizona,
85721, USA Tel: 520-621-4305, Fax: 520-621-8364, Email: sandel@arizona.edu

Empirical models of plasmaspheric properties date from the pioneering work
of Storey (Phil. Trans. Of the Royal Soc. (London) A, 246, 113, 1953) where
he developed the analysis of ground whistler observations that lead to his
estimate for the equatorial plasma density at L=3.  The most recent in situ
satellite study takes us to 1000 CRRES satellite passes and a statistical
analysis of the plasmapause location at all local times and for varying
geomagnetic conditions by Moldwin et al. (2002, J. Geophys. Res., in
press)].  These and many other studies over the intervening 49 years have
given us a strong familiarity with the distribution of cold plasmaspheric
ions throughout the magnetosphere.  The major components of inner
plasmasphere, nightside bulge, sunward convection tail, and plasmapause are
all well established.  Storm-time erosion and the resulting ionospheric
refilling has been encompassed, even if not completely understood.
Small-scale density variations near the plasmapause and extending at least
to geosynchronous orbit have been characterized in a variety of ways, even
though we do not yet understand their origin.

The use of whistler observations from the ground and in situ satellite
measurements to characterize plasmaspheric density distributions are an
essentially heroic endeavor.  This relatively successful pursuit is nearly
always strongly aliased in time, space, or both by limited observing
locations and changing magnetospheric conditions.  Statistical derivations
of the properties of density distribution depend on organizing measured
properties into categories of similar plasmaspheric states.  Statistical
scatter in densities and boundary locations are invariably large, owing to a
strong dependence on the historical detail of changing magnetospheric
conditions and the crude indices available to describe those conditions.
The highly variable nature of the plasmasphere may, in fact, limit the
fidelity that can be achieved in any statistical characterization.

In contrast to that pessimistic suggestion, the IMAGE Mission now affords
the opportunity to use global observations of thermal plasma density
distributions in our empirical modeling efforts.  What has previously
required years of in situ measurement to "paint" a picture of plasma
densities over a broad spatial extent can now be acquired in a single
orbital pass from the Radio Plasma Imager (RPI) (Reinisch, et al., Space
Science Reviews, 91, 319, 2000) or a single 10-minute image from the Extreme
Ultraviolet Imager (EUV) (Sandel et al., Space Science Reviews, 91, 197,
2000).  Extended IMAGE observations may enable empirical modeling of thermal
plasmas to finally resolve spatial and temporal ambiguities that have
hampered many decades of study.  Plasmaspheric erosion and the development
of small and large-scale density structures can be statistically
characterized in the context of storm onset and recovery.  The refilling of
magnetic flux tubes as a function of L-shell and local time can be mapped.
The statistical characterization of plasmaspheric properties can be compared
and used to improve existing empirical models and theoretical predictions.

This paper will present early empirical modeling results from the inversion
of IMAGE/EUV global intensity images to density distributions.  Densities
are obtained in this initial study through use of forward image modeling
with a simple 3-parameter plasmaspheric and plasmapause mathematical model.
Individual interior plasmaspheric density profiles and plasmapause locations
are obtained every 10 degrees in magnetic local time for each EUV image
analyzed.  Derived profile parameters are statistically characterized in the
context of storm magnitude and evolution.  Identified patterns in the
appearance of plasmaspheric structures, plasmapause erosion, and refilling
will be presented.  Comparisons to existing empirical plasmaspheric models
and the implications for new modeling will be presented.

_______________
To be presented at the 27th General Assembly of International Union of 
Radio Science (URSI), Maastricht, The Netherlands, August 17-24, 2002.