Departures from corotation in the inner magnetosphere Bill R. Sandel, William T. Forrester, Robert A. King Using maps of the distribution of singly-ionized helium in the plasmasphere made by the IMAGE Extreme Ultraviolet Imager, we investigate departures from corotation in the inner magnetosphere. We deduce the angular velocity of the plasma by tracking discrete,longlived features within the plasmasphere. On the basis of first-order theory, we would expect the cold plasma that we observe (energy < 1 eV) to be locked by the Lorentz force to Earth«s magnetic field and to corotate with it. In contrast, our analysis shows that the plasma in the range 2 < L < 4 most frequently rotates at a rate that is roughly85-90% of the corotation rate. A particular feature may move at a constant or variable rate during its lifetime. It may move at the corotation rate for a time, and then begin to lag. Our analysis is based on features that persisted for times in the range 15-60 hours. We find a range of angular velocities (averaged over the lifetime of the features) that is 77% to 93% of the corotation velocity. Structures having large radial extent frequently maintain their shapes over the period of observation. In these cases, measurable shearing by an L-dependent corotation lag must therefore be absent. We investigate the implications of these measurements for our understanding of electric fields and plasma motions in the inner magnetosphere. _______________ Presented at the IUGG/IAGA meeting, Sapporo, Japan 30 June - 11 July 2003