In this thesis, I investigate how polar cap dynamics, quantified by the northern polar cap (PCN) index, respond to solar wind direct driving and magnetotail energy unloading during intervals of strong solar wind driving. Using 53 one to two-day intervals with high cross polar cap potential subintervals, I find that, among 11 candidate coupling functions including the electric field of Kan and Lee (1979) and the universal coupling function of Newell et al. (2007), the PCN index correlates most closely with the electric field (EK-R) of Kivelson and Ridley (2008), a form in which the electric field imposed on the ionosphere by low-latitude magnetopause reconnection saturates at high levels of geomagnetic activity. It is found that magnetotail activity, as represented by an unloading AL index (ALU), makes a significant contribution to the PCN index. A linear model is constructed to relate the PCN index to its solar wind and magnetotail drivers. Based on this model, it is estimated that the portion of the PCN index directly driven by the solar wind electric field outweighs the contribution arising from energy release in the magnetotail by roughly a factor of 2. The solar wind dynamic pressure (pdyn) does not play a key role in controlling the PCN index. However, under intense solar wind driving, the number density (n) can influence the solar wind-magnetosphere coupling by changing the solar wind Alfvén conductance, which is incorporated in EK-R. The validity of the linear model is verified by comparing its results with those obtained from a more general, non-linear model, termed additive model. It is found that, except in anomalous events during which the auroral oval expanded poleward to the latitude of the PCN index station and the index increased because of proximity to auroral zone currents, the linear model is a good approximation, since more than 70% of the variation in the PCN index is explained by the linear model. Thus, this linear model provides a useful tool to study the coupling between the solar wind, magnetosphere and ionosphere.
This model is applied day-by-day from 1 February 1998 to 31 December 2009 to investigate the driven and unloading contributions to the PCN index. I find that the relative contributions of driven and unloading components varies with solar cycle with a magnitude ±5%, with the driven-to-unloading ratio highest near solar minimum and lowest slightly after solar maximum, and the driven-to-unloading ratio peaks in summer and decreases in winter with a magnitude as large as ±15%.
Although the theory of Kivelson and Ridley (2008) is successful in predicting the polar cap dynamics from the solar wind input, there is a competing theory of Siscoe et al. (2002). The similarity and difference between these two theories are explored. It is found that, except for some trivial differences, the predictions of the two theories are practically the same in the saturation limit. In addition, the model predictions are compared to the measurements of AMIE, DMSP, the PC index, and SuperDARN. Given the great differences from different measurements, it is impossible to show that one theory outperforms the other.