Using extreme ultraviolet images and total electron content to study the Earth's plasmasphere

This dissertation employs extreme ultraviolet (EUV) imaging and total electron content (TEC) measurements to study the spatial structure and temporal variability of the Earth's plasmasphere. Photometry of global EUV images yields estimates of diurnal variation in column integrated He+ density, as well as the deviation of the plasmasphere's motion from strict corotation. Case-study comparisons of EUV imagery with simultaneous TEC measurements derived from GPS radio signals yield radial profiles of electron and He+ column abundance along the same lines of sight. Three-dimensional tomographic reconstructions of plasmaspheric electron density using TEC measurements are performed for the first time, and we evaluate the promise and limitations of TEC tomography for 3D imaging of the plasmasphere. We find that He + column abundance along a plasmaspheric flux tube tends to rise in the morning sector, stagnate around local noon, then rise again in the afternoon sector until ∼ 20 MLT, where it peaks at a value 1.5 to 2 times the value at dawn. The mean rotation rate of the plasmasphere is observed to be between 88% and 95% of strict corotation, depending on L-shell, and can be highly variable even at L-shells as low as 2.5. Weak but significant correlations between rotation rate and indices of geomagnetic and auroral activity suggest multiple causes of this variability in rotation, including ionospheric winds and magnetospheric convection. Radial profiles of the column abundance of electrons and He+ suggest that electrons fall off more quickly than He+ at low L, leading to an increasing relative proportion of He+ near L=2.5. Three-dimensional tomography of the plasmasphere is limited by satellite orbit geometry and lack of vertical resolution, but data assimilation of TEC into physics-based models can result in more realistic density reconstructions than can be produced by models alone. This research produces the first global observations of diurnal plasmaspheric refilling, the most comprehensive global investigation of variability in plasmaspheric rotation rate, and the first EUV observations of plasmaspheric superrotation. New techniques are developed for the combined use of EUV and TEC in studying the composition of the plasmasphere, and for 3D imaging of the plasmasphere via TEC tomography.