It is hypothesized that the recently observed plasmasphere corotation lag is caused by a corresponding corotation lag in the upper ionosphere. Rotation rates of long-lived plasmaspheric notches are compared to ionospheric drifts observed on DMSP spacecraft in the same longitude sectors. Good agreement between the two observations is found, and the cause of the corotation lag is identified as the ionospheric disturbance dynamo. The observed corotation lag will have to be accounted for in future magnetospheric convection models, which until now have assumed strict plasmasphere corotation.