Plasmaspheric drainage plume observed by the Polar satellite in the prenoon sector and the IMAGE satellite during the magnetic storm of 11 April 2001

During the early main phase of a geomagnetic storm on 11 April 2001, the Polar satellite was inside the magnetosphere in the prenoon sector (similar to 1000-1100 magnetic local times) and experienced a magnetopause crossing at L approximate to 6 because of the high solar wind dynamic pressure and strong southward interplanetary magnetic field (IMF). Just before the magnetopause crossing, Polar observed cold, dense plasma. That is, the cold, dense plasma was immediately adjacent to the compressed magnetopause. Using simultaneous observations by the IMAGE extreme ultraviolet (EUV) imager, we confirm that the cold, dense plasma observed by Polar is a plasmaspheric drainage plume extending outward from the plasmasphere to the magnetopause during the interval of high geomagnetic activity and strong southward IMF. We compare plasmaspheric mass densities determined from ground magnetometer data at L = 2.3 and 2.9 for a magnetically quiet time interval to mass densities determined during the magnetic storm time interval. We find no significant differences in the mass density between both intervals. These observations suggest that the sunward-convecting plasmaspheric plasma observed at Polar is due to erosion of the outer layers of the plasmasphere beyond L = 2.9.