Polar cap potential saturation: Observations, theory, and modeling

The polar cap potential (ΦPC) has long been considered an indicator for the amount of energy flowing into and through the magnetosphere-ionosphere (M-I) system. Studies have shown that ΦPC reaches an upper limit, or saturates, during geomagnetic storms where the solar wind electric field becomes increasingly large. Numerous observational, theoretical, and modeling studies of ΦPC have seemingly confirmed that saturation is real, however, many mechanisms for saturation have been suggested with no direct observational evidence confirming any single cause. This paper presents a review of ΦPC saturation from various perspectives and attempts to identify some of the outstanding issues in determining the causal mechanism of this important piece necessary to understanding the coupling between the solar wind and M-I system.