Statistical Features of Polar Cap North and South Indices in Response to Interplanetary and Terrestrial Conditions: A Revisit

In this study, we investigate statistical features of polar cap north (PCN) and south (PCS) indices in response to various interplanetary conditions (interplanetary magnetic field [IMF] orientation in three-dimensions) and terrestrial conditions (seasonal and magnetic local time [MLT] locations of the index stations). The concurrent PCN-PCS pairs for 1998-2002 and 2004-2018 are divided based on their sign type (positive-positive, negative-negative, negative-positive, and positive-negative PCN-PCS pairs) and time coverage (the times when both index stations are in the dawn/dusk MLT sector during northern summer/winter). Analyzing the IMF orientation dependence on the occurrence probabilities of concurrent indices and on the differences between the indices in various sign types for each time coverage reveals that the statistical features in PCN-PCS pairs obtained in the dawn MLT sector can be largely explained by the effects of the three-component IMF (related to the polar cap convection patterns) combined with season (related to the hemispheric asymmetry in solar illumination-induced ionospheric conductance). However, those obtained in the dusk MLT sector are controlled dominantly by seasonal effects rather than IMF orientation effects. Our findings indicate that PCN-PCS pair data provide local views about the solar wind-magnetosphere-ionosphere (SW-M-I) coupling system with different control efficiencies of IMF orientation and season depending on the MLT location of the stations. Therefore, introducing polar cap indices recorded simultaneously at various locations in both hemispheres and analyzing them are strongly required to infer global views of the coupled SW-M-I system in the open field regions with higher confidence. Polar cap north (PCN) and south (PCS) indices are considered as proxies for the solar wind energy input into the polar cap ionosphere in the Northern and Southern Hemispheres, respectively. Each of these indices is obtained by dividing magnetic variation (relative to the background level) at a single near-pole station in the corresponding hemisphere by the coupling strength between interplanetary parameters and ground magnetic activity caused by the dayside solar wind-magnetosphere interaction. Therefore, investigation of the physical explanations for the similarities/differences in the concurrent PCN and PCS can provide insights into the similarities/differences in the solar wind-magnetosphere-ionosphere (SW-M-I) coupling between the hemispheres. According to our results, the PCN and PCS indices provide local views about the SW-M-I coupling mode (the ionospheric flow pattern above the observation point) with different control efficiencies of interplanetary magnetic field orientation and season depending on the magnetic local time location of the stations. This paper therefore provides evidence for the necessity of polar cap indices obtained at various locations in both hemispheres to infer global views of the SW-M-I system in open field areas with higher confidence. Combined effects of interplanetary magnetic field (IMF) orientation and ionospheric conductivity explain statistical features of polar cap north-polar cap south index pairs (PCN-PCS pairs) in the dawn magnetic local time (MLT) sector For those in the dusk MLT sector, however, ionospheric conductivity effects are dominant over IMF orientation effects PCN-PCS pairs provide local views about the SW-M-I coupling with different efficiencies depending on the MLT location of the stations