One of the key features in the coupling of the solar wind to the magnetosphere and the magnetosphere to the ionosphere coupling is the electric potential drop imposed across the polar cap. This potential drop and its associated potential distribution pattern in the polar ionosphere serves as a proxy measurement of the coupling between the solar wind and the magnetosphere. Recent work has shown that the polar ionosphere behaves differently under the extreme solar wind conditions that produce the very large magnetic storms known as superstorms as compared with the behavior during quiet times and moderate magnetic storms. It appears that the polar cap potential drop saturates at a set level no matter how strong the system is driven by the electric field carried by the solar wind. In addition the size of the polar cap itself appears to stop expanding under these saturated potential conditions. Understanding the behavior of the polar cap potential and the size of the polar cap itself and how they respond to solar wind drivers (the solar wind electric field and the solar wind ram pressure) as well as to the changes in the conductivity in the ionosphere are key to understanding the underlying physics of the phenomenon and in developing accurate and reliable models of the geospace environment. This three-year project will use the database of polar ionospheric parameters from the DMSP spacecraft to explore the polar ionosphere's response to regular storms and to the extreme superstorms. The project will develop an empirical model of the size of the polar cap as a function of the magnitude of the polar cap potential and various solar wind inputs. Periods of medium to high values of the solar wind electric field will be examined to determine the transition region where the saturation of the potential begins to appear. The project will also compare the ionospheric Pedersen conductivity derived from the Hill-Siscoe model with the value derived using the AMIE
