We make direct comparisons between GPS maps of total electron content (TEC) over the North American continent, 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) and plumes of greatly-elevated TEC are associated with the erosion of the outer plasmasphere by strong sub-auroral polarization electric fields. The SED/TEC plumes identified at low altitude map closely onto the magnetospheric determination of the boundaries of the plasmapause and plasmaspheric tail determined by EUV imaging from the IMAGE spacecraft. Characteristics of the SED/TEC plumes/tails for the March 31, 2001 event are: TEC ∼ 100 TECu; F-region sunward velocity ∼1000 m/s; sunward flux ∼5*10²⁴ ions s⁻¹; total transport to dayside magnetopause/merging region (3-hr event) ∼5*10²⁸ ions.