The polar cap magnetic activity PC index is regarded as indicator of the solar wind energy that enters into the magnetosphere during the solar wind‐magnetosphere coupling (Resolutions of XXII IAGA Assembly, 2013). This study presents the results of statistical analysis of relationships between the yearly values of PC index and such indicators as the magnetic activity indices (AE and Dst), the solar wind parameters (velocity Vsw, density Nsw, dynamic pressure Pdyn, vertical interplanetary magnetic field (IMF) component Bz, total IMF field |B|, electric field EKL), and the solar activity parameters (sunspot number, coronal mass ejections, stream interaction regions) in course of 23–24 solar cycles. It is shown that the yearly values of the quiet daily curve magnitude (which serves as a level of reference in estimation of the PC index value) changed during 1998–2019 in remarkable agreement with variation of the Solar UV irradiation (150–200 nm) absorbed in the Earth's ionosphere. The yearly values of PC index perfectly correlate with the yearly values of Vsw and |B| and their product electric field EKL, as well as with the yearly values of AE and Dst indices testifying that PC index serves as the ground‐based indicator of the solar wind energy input into the magnetosphere. The yearly values of EKL, PC, AE, Dst correlate with the total IMF field |B| much better that with the IMF Bz component, testifying that efficiency of the solar wind impact on the magnetosphere is defined by the total IMF value, in contrast with Dungey's concept of reconnection.

The polar cap magnetic activity PC index is calculated by magnetic data from near-pole stations Thule in Greenland (PCN index) and Vostok in Antarctic polar cap south (PCS index). Before 2011 the PCN index was calculated in Danish Meteorological Institute (DMI) either by the official DMI method (Vennerstrom, 1991) or by the solar rotation weighted (SRW) method described later in Stauning (2011). The PCS index was calculated in Arctic and Antarctic Research Institute (AARI) with use of the "unified" method put forward in (Troshichev et al., 2006; thereafter TJS2006) for derivation of both, PCN and PCS, indices. The comprehensive analysis of these competitive methods has been fulfilled by IAGA Division V-DAT in 2009-2010 and the "unified" TJS2006 method has been recommended, as the best, for the IAGA endorsement (McCreadie & Menvielle, 2010). In 2011 the Space Institute of the Danish Technical University (DTU-Space) became responsible for magnetic observations at Thule station and in 2013 the PC index, derived by the "unified" TJS2006 method, was approved by IAGA as a new index characterizing the solar wind energy input into the magnetosphere (IAGA Resolution no. 3, 2013). In line with agreement between the AARI and DTU-Space the PCN and PCS indices for previous years were recalculated in 2014 with application of the "unified" PC derivation method approved by IAGA.According to the IAGA rules, all indices obtained by data of current magnetic observations are considered as "preliminary" indices. They should be recalculated afterward making allowance for all possible faults of observational, technical and computer-assisted origin, to produce the "definitive" indices, which will be valid for ever. This work was fulfilled in 2021 in AARI and DTU Space with use of the restructured and harmonized code (Nielsen & Willer, 2019). Comparison of the provisional and definitive PCN and PCS indices for 22 years has demonstrated perfect conformity between the appropriate PCN and PCS indices. As a result, the definitive PCN and PCS indices were ultimately approved by IAGA and the PC index was recommended for use by international scientific community (IAGA Resolution no. 2, 2021). The definitive PCN and PCS indices are presented at site: http://pcindex.org.

Declaration (Stauning, 2022) on "invalid PCS index" is based on the following arguments: PCS index is calculated with use of incorrect "unified" PC derivation method; PCS index used in analyses is a preliminary index, which was not approved by IAGA and, therefore, it cannot be regarded as a correct index; PCN and PCS indices demonstrate, intermittently, large difference in value, which should be treated as evidence of the PCS index invalidity. The paper presents comments to these arguments. Conclusion is made that criticism of the PCS index, presented in Stauning (2022), is based on groundless arguments.

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The polar cap magnetic activity index (PC) was approved by IAGA as an index characterizing the solar wind energy input into the magnetosphere. Relation of the PC index to solar wind parameters and solar activity during 23/24 solar cycles has been studied in paper (Troshichev et al., 2021, https://doi.org/10.1029/2020JA028491). The paper was commented by Dr. Stauning, who states that results obtained in the study are based on erroneous data presented on web site http://pcindex.org. According to Dr. Stauning, the following invalidities served as a reason for this assertion: incorrect determination of quiet daily curve (QDC), taken as reference level for counting the PC index value, and incorrect QDC amplitudes, incorrect yearly average values of PC index (and EKL field), incorrect methods of the analysis. In our answer we argue the correctness of our analysis and resulted conclusions.