The Polar Cap (PC) indices are derived from the magnetic variations generated by the transpolar convection of magnetospheric plasma and embedded magnetic fields driven by the interaction with the solar wind. The PC indices are potentially very useful for Space Weather monitoring and forecasts and for related research. However, the PC index series in the near-real time and final versions endorsed by the International Association for Geomagnetism and Aeronomy (IAGA) are considered unreliable. Both versions include solar wind sector (SWS) effects in the calculation of the reference levels from which magnetic disturbances are measured. The SWS effects are caused by current systems in the dayside Cusp region related to the Y-component, B, of the Interplanetary Magnetic Field (IMF). However, the IAGA-endorsed handling of SWS effects may generate unfounded PC index changes of up to 3 mV/m at the nightside away from the Cusp. For the real-time PCN and PCS indices, the cubic spline-based reference level construction may cause additional unjustified index excursions of more than 3 mV/m with respect to the corresponding final index values. Noting that PC index values above 2 mV/m indicate geomagnetic storm conditions, such unjustified contributions are considered to invalidate the IAGA-endorsed PC index series. Alternative derivation methods are shown to provide more consistent index reference levels for both final and real-time PC indices, to reduce their unfounded excursions, and to significantly increase their reliability.

In the publication Troshichev et al. (2006) on the Polar Cap (PC) indices, PCN and PCS, an error was made by using components of the Interplanetary Magnetic Field (IMF) in their Geocentric Solar Ecliptic (GSE) representation instead of the prescribed Geocentric Solar Magnetospheric (GSM) representation for calculations of index scaling parameters. The mistake has caused a trail of incorrect relations and wrong conclusions extending since 2006 up to now (2020) which should be discontinued, for instance, by issuing a corrigendum note from the authors. The present comment explains the error and discusses in an extended example its consequences for one of the publications that has referenced to the invalid scaling parameter set.

Examination of the contribution from O.A. Troshichev, S. Dolgacheva, N.A. Stepanov, and D.A. Sormakov: “The PC index variations during 23/24 solar cycles: relation to solar wind parameters and magnetic disturbances. https://doi.org/10.1029/2020JA028491 ” has disclosed inconsistencies in the applied methods and serious errors in the calculated values. Some of the discrepancies reported in the present commentary affect directly the illustrations presented in their contribution while other possible errors may not be apparent since the use of relative values in their presentation makes thorough assessments difficult.

Polar Cap PC) indices, PCN North based on magnetic observations from Qaanaaq (THL) in Greenland and PCS South based on magnetic data from Vostok in Antarctica, are very useful indices for studies of solar wind-magnetosphere interactions and for space weather monitoring and forecasts. PCN indices are issued from the Danish Space Research Institute (DTU Space) while PCS indices are issued from the Arctic and Antarctic Research Institute (AARI) in St Petersburg. Unfortunately, series of invalid PCS values have been provided from AARI and used in a number of publications issued since the PC index concept was endorsed by the International Association of Geomagnetism and Aeronomy (IAGA) in 2013. The present contribution defines the invalidating features of the PCS indices in question, conveys examples, and discusses the applications in peer-reviewed publications.

In the publication Troshichev et al. (2006) on the Polar Cap (PC) indices, PCN and PCS, an error was made by using components of the Interplanetary Magnetic Field (IMF) in their Geocentric Solar Ecliptic (GSE) representation instead of the prescribed Geocentric Solar Magnetospheric (GSM) representation for calculations of index scaling parameters. The mistake has caused a trail of incorrect relations and wrong conclusions extending since 2006 up to now (2020) which should be discontinued, for instance, by issuing a corrigendum note from the authors. The present contribution explains the error and discusses in an extended example its consequences for one of the publications that has referred to the invalid scaling parameter set. Further investigations reported here of the PC index versions recommended by the International Association for Geomagnetism and Aeronomy (IAGA) indicate occurrences of similar problems in the present derivation of index scaling parameters.

P. Stauning: Comment on “Troshichev et al. 2020: The PC index variations during 23/24 solar cycles: relation to solar wind parameters and magnetic disturbances.”: Invalid data base. Abstract. The contribution from O.A. Troshichev, S. Dolgacheva, N.A. Stepanov, and D.A. Sormakov: “The PC index variations during 23/24 solar cycles: relation to solar wind parameters and magnetic disturbances”, https://doi.org/10.1029/2020JA028491 , is to a large extent based on data made available at the web portal http://geophys.aari.ru/PCspaceweather. However, an examination of the data presented there has disclosed what appears to be serious errors in the calculated values of polar cap (PC) index values, amplitude span of the quiet day curves (QDC) used for the reference levels, and values of the solar wind merging (or geoeffective) electric field (E KL) used for calibration of polar magnetic data to form PC indices. Parts of the essential data files are reproduced in the Appendix here. Some of the discrepancies reported in the present commentary affect directly the illustrations presented in their contribution while other possible errors may not be apparent since the use of relative values in their presentation makes assessments difficult.

The relations between transpolar plasma convection intensities recorded by the Polar Cap (PC) indices and magnetospheric ring current intensities recorded by the asymmetric ASY-H indices and the symmetric Dst and SYM-H indices are examined. The present work believed to be the first of its kind examines the validity of previously derived relations between polar cap and ring current indices when used in real time applications. Polar cap (PC) indices are here derived in simulated real-time versions by using past data only from -40 days up to current time in the construction of the quiet reference levels (QDCs) for the magnetic data. From analyses spanning a decade (2009-2018), equivalent ASY-H index values were derived from a linear relation with simulated real-time PCN (North) and PCS (South) indices combined to form the non-negative PCC indices. For cases of strong magnetic storms (Dst(peak)<-100 nT, the equivalent ASY-H indices were found to agree well with reported (real) ASY-H index values. The simulated real-time PCC indices, furthermore, have been used in a PC-based source function to derive equivalent values of the total ring current indices Dst (or SYM-H) up to one hour ahead of time. With integration of the source function throughout a decade (2009-2018) with no attachment to reported Dst values, the simulated real-time equivalent Dst indices displayed close agreement with real Dst index values. The applied method could be used without modifications to generate PC index values and derived ASY-H and Dst (or SYM-H) index values in real-time space weather applications.

The non-negative Polar Cap PCC index built from PCN (North) and PCS (South) indices correlates better with the solar wind merging electric field and is more representative for the total energy input from the solar wind to the magnetosphere and for the development of geomagnetic disturbances represented by the Kp index and ring current indices than either of the hemispheric indices. The present work shows that the ring current index, Dst, to a high degree of accuracy can be derived from a source function built from PCC indices. The integration of the PCC-based source function throughout the interval from 1992 to 2018 without attachment to the real Dst indices based on low latitude magnetic observations has generated equivalent Dst values that correlate very well (R=0.86) with the real Dst index values, which are represented with a mean deviation less than 1 nT and an overall rms deviation less than 13 nT. The precise correlation between the real and equivalent Dst values has been used to correct the PCC indices for saturation effects at high intensity disturbance conditions where the Dst index may take values beyond-100 nT. The relations between PCC and the ring current indices, Dst and ASY-H have been used, in addition, to derive the precise timing between polar cap convection processes reflected in the polar cap indices and the formation of the partial and total ring current systems. Building the ring current is considered to represent the energy input from the solar wind, which also powers auroral disturbance processes such as substorms and upper atmosphere heating. With current available PC indices, detailed and accurate SYM-H or Dst index values could be derived up to nearly one hour ahead of actual time by integration of the PCC-based source function from any previous quiet state. Thus, the PCC indices enabling accurate estimates of the energy input from the solar wind are powerful tools for space weather monitoring and for solar-terrestrial research. 1. Introduction. In the early Space Age, Dungey (1961) formulated the concept of magnetic merging processes taking place at the front of the magnetosphere between the Interplanetary Magnetic Field (IMF), when southward oriented, and the geomagnetic field, followed by the draping of the combined field over the pole and reconnection processes in the tail region, where the solar wind magnetic fields as well as the geomagnetic fields were restored. The model implies a two-cell convection system, where the high-latitude antisunward ionospheric and magnetospheric plasma drift across the polar cap and the return flow in a sunward motion along auroral latitudes generate the two-cell “forward convection” patterns, now termed DP2. Later, Dungey (1963) extended his model to include cases where IMF is northward (NBZ conditions),

The standard Polar Cap (PC) indices, PCN (North) based on magnetic data from Qaanaaq in Greenland and PCS (South) based on data from Vostok in Antarctica, have been submitted from the Arctic and Antarctic Research Institute (AARI) in St. Petersburg, Russia, the Danish Meteorological Institute (DMI), and the Danish Space Research Institute (DTU Space) in different versions. In order to consolidate PCS indices based on Vostok data or replace poor or missing index data, derivation procedures have been developed to generate alternative PCS index values based on data from Dome Concordia (Dome-C) magnetic observations from epoch 2009-2020 of solar cycle 24. The reference levels and calibration parameters needed for calculations of Dome-C-based PCS values in post-event and real-time versions are defined and explained in the present work. Assessment of the new PCS index has shown its unprecedented high relevance. Part of the methods used here such as the quiet reference level construction and the correlation and regression procedures used for calculations of scaling parameters deviate from corresponding features considered inadequate of the IAGA-endorsed PC index derivation methods