A critical note on the IAGA-endorsed Polar Cap index procedure: effects of solar wind sector structure and reverse polar convection

摘要

The International Association of Geomagnetism and Aeronomy (IAGA) hasrecently endorsed a new Polar Cap (PC) index version to supersedethe previous seven different versions of the PCN (North) index and the fivedifferent PCS (South) index versions. However, the new PC index has someadverse features which should be known and taken into account by users ofthe index. It uses in its derivation procedure an "effective" quiet daylevel (QDC) composed of a "basic" QDC and an added solar wind sector termrelated to the azimuthal component (B_y) of the interplanetary magnetic field (IMF).The added IMF B_y-related terms may introduce unjustifiedcontributions to the PC index of more than 2 index units (mV m?1).Furthermore, cases of reverse convection during strong northward IMF B_z (NBZ)conditions included in the database for calculation of indexcoefficients can cause unjustified index enhancements of 0.5–1 mV m?1 duringcalm conditions, reduction of index values by more than 20 % duringdisturbed conditions, and inconsistencies between index coefficients andindex values for the northern and southern polar caps. The aim here is tospecify these adverse features and quantify their effects, and to suggestalternative steps for future modifications of the index procedure.