Polar Cap Index (North and South) is taken from the Polar Cap Magnetic Index Site.
The PC index is characteristic of the polar cap magnetic activity generated by geoeffective solar wind acting on the magnetosphere. The index is derived by magnetic data of only two stations Thule and Vostok located in the northern (PCN) and southern (PCS) near-pole regions. In 2013 the International Association of Geomagnetism and Aeronomy (IAGA) recommended to international scientific community to use PC index as a proxy for energy that enters into the magnetosphere during solar wind magnetosphere coupling. The procedure adopted in AARI and DTU provides on-line calculation of the PCS and PCN indices corresponding to geoeffective interplanetary electric field EKL value irrespective of time, season and solar cycle.

        IAGA Resolution No. 3 (2013)
        Polar Cap (PC) index
        
        IAGA, noting that polar cap magnetic activity is not yet described by existing IAGA geomagnetic indices, considering that the Polar Cap (PC) index constitutes a quantitative estimate of geomagnetic activity at polar latitudes and serves as a proxy for energy that enters into the magnetosphere during solar wind-magnetosphere coupling, emphasising that the usefulness of such an index is dependent on having a continuous data series, recognising that the PC index is derived in partnership between the Arctic and Antarctic Research Institute (AARI, Russian Federation) and the National Space Institute, Technical University of Denmark (DTU, Denmark) recommends use of the PC index by the international scientific community in its near-real time and definitive forms, and urges that all possible efforts be made to maintain continuous operation of all geomagnetic observatories contributing to the PC index.