Utilizing the polar cap index to explore strong driving of polar cap dynamics

Abstract

The polar cap, defined as a region of open magnetic flux, is an ideal region in which to investigate how properties of the solar wind directly affect the magnetosphere. For such studies, the polar cap (PC) index provides a useful characterization of the state of the polar ionosphere. In this paper, we study how polar cap properties, quantified by the PC index, depend on solar wind parameters and other geomagnetic indices during intervals of exceptionally large (10 mV/m) merging electric field. Using 53 one to two‐day intervals that include such extreme fields, we find that the PC index correlates strongly with the modified electric field (EK‐R) proposed by Kivelson and Ridley (2008). Here, EK‐R is a form representative of several models in which the electric field imposed on the ionosphere by magnetopause reconnection saturates for extreme solar wind driving. However, there are anomalous events during which the auroral oval expanded poleward to the latitude of the PC index station and the index increased because of proximity to auroral zone currents. It is found that nightside magnetospheric processes, as represented by AL, make a significant contribution to the PC index. A linear regression analysis shows that the portion of the PC 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. Neither the solar wind dynamic pressure (Pdyn) nor jumps in Pdyn are found to directly contribute to the PC index. However, there exists some correlation between the PC index and Pdyn, because of the common dependence of EK‐R and Pdyn on the solar wind number density.