Magnetospheric plasma density profiles: Co-ordinated multi-instrument ground-based
and IMAGE RPI satellite observations of the plasmasphere. 

Zoe C. Dent 1, I. R. Mann 1, F. W. Menk 2, J. Goldstein 3, C. R. Wilford 4, 
M. A. Clilverd 5, L. G. Ozeke 1, B. W. Reinisch 6

1 Department of Physics, University of York, Heslington, York, YO10 5DD, UK
2 Department of Physics, University of Newcastle, Newcastle, NSW, Australia
3 Department of Physics and Astronomy, Rice University, Texas, USA
4 Department of Applied Mathematics, University of Sheffield, Sheffield, UK
5 British Antarctic Survey, Cambridge, UK
6 Environmental, Earth, and Atmospheric Sciences Department, Center for 
  Atmospheric Research, University of Massachusetts Lowell, USA

In-situ electron densities as a function of L-shell, measured using the RPI
instrument aboard the IMAGE satellite, are compared to mass density profiles
derived using the cross-phase technique applied to data from SAMNET (UK
Sub-Auroral Magnetometer NETwork), BGS (British Geological Survey) and IMAGE
(International Monitor for Auroral Geomagnetic Effects) ground-based magnetometer
networks; to electron densities inferred from VLF whistler measurements; and to
mass density profiles from the Sheffield University Plasmasphere-Ionosphere Model
(SUPIM).  Data are presented from the 19th August 2000, a geomagnetically quiet
day (Kp ~ 1), during which the magnetosphere was in a state of recovery following
a geomagnetic storm seven days previously.  In-situ RPI data from two IMAGE orbits
are compared to the ground-based measurements of density profiles. The
plasmasphere is well defined in the ground-based measurements from L = 2.5 Ð 5.85.
 Excellent agreement is obtained between the ground-based and RPI inferred
densities in the plasmasphere, providing important validation of both the ULF
magnetometer and VLF whistler techniques.  The SUPIM model derived plasmaspheric
densities also agree well with both the in-situ and ground-based measurements. 
The density profiles from the first IMAGE orbit at ~ 0300MLT (0615 Ð 0812UT)
suggest a gradual plasmapause, however the second orbit profile at ~ 0300MLT (1928
Ð 2214UT) records a sharper and more well defined plasmapause at L ~ 6.3.  The
difference in the plasmapause structure as observed by IMAGE RPI on consecutive
orbits may be related to an azimuthal asymmetry in plasmapause morphology. IMAGE
EUV imaging of the He+ plasmasphere may provide a framework within which these
features can be understood.  

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Presented at the Magnetospheric Imaging Workshop, 
Yosemite National Park, California, U.S.A., Feb. 5-8, 2002.