The Global Response of the Plasmasphere to a Geomagnetic Disturbance

Spasojevic, M., J. Goldstein, D. L. Carpenter, U. S. Inan, 
B. R. Sandel, M. B. Moldwin, B. W. Reinisch


Global images of the plasmasphere obtained by the Extreme Ultraviolet 
(EUV) imager on the IMAGE satellite are used to study the evolving 
structure of the plasmasphere during two geomagnetic disturbances. By 
tracking the location of the plasmapause as a function of L shell and 
magnetic local time, quantitative measurements of radial and 
azimuthal motions of the boundary are made for intervals = 7 hours in 
duration with a time resolution of 10 minutes. The two cases 
presented are June 26 - 27, 2001, a relatively weak but isolated 
geomagnetic disturbance, and June 9 - 10, 2001, a moderate event with 
a multi-staged onset and recurring substorm activity after the main 
disturbance.  In both cases the onset of the disturbance, correlated 
with a southward turning of the IMF, is characterized by inward 
motion or erosion of the plasmapause and a smoothing of any existing 
azimuthal variations across the nightside. Over a period of many 
hours, a plasmaspheric plume forms in the afternoon sector as a 
result of sunward flows from dusk and corotational flows across the 
dayside. Azimuthal variations in the plasmapause radius tend to form 
in the local time sector from dawn to the western edge of the plume, 
including mesoscale (<= 0.5 in L and <= 2 hours in local time) 
crenulations and larger scale shoulder features, while the nightside 
boundary remains featureless. In the June 26 - 27, 2001 case, the 
magnetosphere entered a period of deep quiet after the main 
disturbance, and the plasmaspheric plume began to corotate with the 
main plasmasphere from the afternoon sector across the nightside. In 
contrast, the plume in the June 9 - 10, 2001 event became wrapped 
around the main plasmasphere, and a second plume formed in the 
afternoon sector perhaps due to continued substorm activity. In situ 
density data for these events show highly irregular density structure 
within the plumes as measured at geosynchronous orbit whereas a 
measurement by IMAGE RPI suggests that there may be less structure 
near the base of the plume closer to the main plasmasphere.

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J. Geophys. Res., in press, 2003.