The Polar Cap index: A critical review of methods and a new approach

The Polar Cap (PC) index introduced by Troshichev and Andrezen (1985) is derived from polar magnetic variations and is mainly a measure of the intensity of the transpolar ionospheric currents. These currents relate to the polar cap antisunward ionospheric plasma convection driven by the dawn‐dusk electric fields, which in turn are generated by the interaction of the solar wind with the Earth's magnetosphere. Coefficients to calculate PCN and PCS index values from polar magnetic variations recorded at Thule and Vostok, respectively, have been derived by several different procedures in the past. The PCN index is found in seven different versions while the PCS index is found in five different versions in past publications. Recent publications (in 2011 and 2012) still convey three different PCN and two different PCS indices. The presentation here discusses the principal differences between the various PC index procedures and provides comparisons between coefficient and index values derived using the different procedures. Finally, a new approach to define a simplified PC index procedure is outlined.