Low-density regions of the plasmasphere

Bill Sandel (1), Dennis Gallagher (2)

(1) 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

(2) 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

Using global images of the plasmasphere from the IMAGE Extreme Ultraviolet
Imager, we investigate the characteristics of localized low-density regions
of the plasmasphere called "voids" by Sandel et al. (Geophys. Res.  Lett.,
28, 1439, 2001). These structures are of interest from several points of
view. Green et al. (On the Origin of Kilometric Continuum, J. Geophys. Res.,
in press) have identified them as the source region for kilometric continuum
radiation. Furthermore, they are probably related to the troughs or
plasmaspheric cavities that have been known for some time (e.g., Carpenter
et al. J. Geophys.  Res., 105, 23,323, 2000). Our goal is to understand
their relationships with other known features of the plasmasphere and to
elucidate the mechanisms by which they form and dissipate.

Our study to date has encompassed about 30 examples of voids. The voids
are characterized by deep radial indentations in the plasmapause, with a
radial extent of 1-2 Re and an azimuthal extent of 10-20 degrees. The
intensity contrast in an azimuthal profile across the void ranges from 2 to
more than 4. The shape of the low-intensity region suggests reduced density
in a complete flux tube extending from the equator to the ionosphere. The
voids are most often observedduring times of geomagnetic quiet, but also are
sometimes found at times of moderate geomagnetic activity. They can persist
for periods of at least 40 hours. The azimuthal position of the void often
lags the angular velocity of local corotation by about 10%, and the data
suggest that voids form (or at least are observed) preferentially in the
range of magnetic longitude 200-300 degrees.

This analysis attests to the usefulness of global imaging in investigations 
of this type. With global imaging, we can measure the shapes and 
distributions of these macroscopic features, and track specific features
unambiguously and nearly continuously over periods of days.

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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.