Rapid Response of the Plasmasphere to Changes in the IMF:  Global
Plasmapause Electric Field Measurements by IMAGE EUV 

Goldstein, J., R. A. Wolf, B. R. Sandel, T. Forrester, D. Gallagher,
and P. H. Reiff

Pictures taken by the extreme ultraviolet imager (EUV) on the Imager
for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite very
often show a well-defined and identifiable plasmapause. For each EUV
time snapshot, this plasmapause can be mapped down to the magnetic
equatorial plane; thus, the EUV data provide a means of obtaining the
time development of the two-dimensional equatorial plasmapause. This
time development can in turn be used to estimate the E-cross-B drift
velocity of the plasmapause boundary.  Assuming knowledge of the
equatorial geomagnetic field, this methodology yields global
equatorial plasmapause electric field measurements. 

For some time, it has been established that the plasmapause moves in
response to changes in the global magnetospheric convection field,
which in turn is driven by solar wind dynamic pressure and
interplanetary magnetic field (IMF) orientation. A handful of time
histories recorded by EUV using the method above show the plasmapause
moving radially inward or outward.  Comparison of the plasmapause
time development with IMF orientation in these cases shows that the
plasmapause responds rapidly (within 25 to 30 minutes) to sudden
changes in the IMF Bz polarity. A change to southward Bz enhances
magnetospheric convection, causing visible inward motion of the
nightside plasmapause. A change to northward Bz seems to trigger an
overshielding condition, producing outward motion of the nightside
plasmapause. Time plots of plasmapause location at given magnetic
local times can be generated. From comparison of these plasmapause
location time plots with corresponding IMF Bz polarity profiles, it
is evident that the plasmapause location is very responsive to
changes in the IMF. 

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