A comprehensive, time-dependent, high-latitude, one-species F region model has been developed to study the various physical processes which are believed to affect the polar cap plasma density distributions as a function of altitude, latitude, longitude, and local time. These processes include production of ionization by solar extreme ultraviolet radiation and particle precipitation; loss through charge exchange with N2 and O2; and transport by diffusion, neutral winds, and convection E×B drifts. In our initial calculations we have modeled highly structured plasma densities characterized by digisonde observations at Sondrestrom using both a time-dependent global convection pattern and spatially localized regions of transient high-speed flow. We find that the model is very sensitive to the details of the time-dependent convection pattern, and both the time dependence and the high-speed flows contribute to the F-region structure. Further, when we use high-speed flows based on specific radar observations the simulated density structure is in reasonable agreement with that day's digisonde observations.