Global response of the plasmasphere to a geomagnetic disturbance

[1] 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 greater than or equal to7 hours in duration with a time resolution of 10 min. The two cases presented are 26-27 June 2001, a relatively weak but isolated geomagnetic disturbance, and 9-10 June 2001, a moderate event with a multistaged 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 (less than or equal to0.5 in L and less than or equal to2 hours in local time) crenulations and larger-scale shoulder features, while the nightside boundary remains featureless. In the 26-27 June 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 9-10 June 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.