Ionospheric Signatures of Plasmaspheric Tails Foster, J.C., Coster, A.J., Goldstein, J., Rich, F.J. We present direct comparisons between GPS maps of total electron content (TEC) over the North American continent with Millstone Hill radar observations of storm enhanced density and low and high-altitude satellite measurements of the perturbation of the outer plasmasphere during the March 31, 2001 geomagnetic storm. We find that storm enhanced density (SED) [Foster, J. Geophys. Res., 98, 1675, 1993] and plumes of greatly-elevated TEC result from the erosion of the outer plasmasphere by penetration jet electric fields. Boundaries of the SED/TEC plumes identified at low altitude map directly onto the magnetospheric determination of the boundaries of the plasmapause and plasmaspheric tail determined by EUV imaging from the IMAGE spacecraft. Ground-based GPS observations and radar scans are used to present 2-D snapshots of the ionospheric SED, while DMSP overflights identify the magnetospheric boundaries and mechanisms which contribute to these events. During this event, sunward-convecting plumes of high-TEC plasmaspheric material span the continent from New England to the Canadian Yukon and are responsible for significant ionospheric space weather effects including steep TEC gradients [Vo and Foster, J. Geophys. Res., 106, 21555, 2001] and the occurrence of mid-latitude radio scintillation. Characteristics of the SED/TEC plumes/tails for the March 31, 2001 event are. TEC ~100 TECu; F-region (300 km - 1000 km altitude) sunward velocity ~1000 m/s; sunward flux ~5 x 10^24 ions s^-1. Total transport to dayside magnetopause/merging region (3-hr event) is ~ 5 x 10^28 ions. _______________ Presented at the 2002 Spring A.G.U. Meeting, Washington, D.C., U.S.A., 28-31 May 2002