The model of Pontius et al. (1986) for plasma transport in a corotation-dominated magnetosphere is modified to satisfy observational constraints introduced by Richardson and McNutt (1987). As in the earlier model, small discrete flux tubes whose plasma content differs significantly from the longitudinally averaged value (transient flux tubes) move steadily under the influence of the centrifugal force. We consider the electric fields surrounding a transient flux tube of elliptical cross section and derive expressions for the translational velocity and cross-sectional distortion of such flux tubes. To account for the observed radial decrease of flux shell content at Jupiter, we assume that plasma can pass between transient flux tubes and the background by means of a single-particle microdiffusion. The background plasma distribution is affected by the passage of a transient in two ways: direct mass loading through microdiffusion, and radial displacement to conserve magnetic flux. Upon adopting a simple form to describe the microdiffusion process, we find that there exists a particular slope of flux shell plasma content for which these effects cancel and the background maintains a steady state distribution. ¿ American Geophysical Union 1989