Abstract
The Polar Cap (PC) indices were approved by the International Association for Geomagnetism and Aeromony (IAGA) by Resolution No. 3 (2013) noting that “IAGA … recommends use of the PC index by the international scientific community in its near-real time and definitive forms”. PC indices were made available in 2014 at the web portal http://pcindex.org holding near-real time as well as final index values. The near-real time PC index values are not permanently available. However, analyses of indices on basis of occasional downloads have detected differences between near-real time and final PC indices of up to 3.65 mV/m (Stauning, 2018b, Ann Geophys, 36, 621–631). At such differences, one or the other index may indicate (or hide) strong geomagnetic activity without justification in the actual conditions. The present work has disclosed the cause of observed large differences between real-time and final PC index values in the IAGA-endorsed versions. In addition, anticipated differences are derived on a general basis from the available basic magnetic data by using the index calculation procedures and calibration constants provided by the PC index suppliers. It is shown that corresponding or even larger anomalies are expected to be common during moderate to strong magnetic activity where the near-real time PC indices might otherwise prove very useful for space weather monitoring, e.g., for power grid protection. An alternative real-time PC index derivation scheme described here reduces the excessive differences between real-time and final PC index values by an order of magnitude.
Highlights
The Polar Cap (PC) indices, PCN (North) based on magnetic data from Qaanaaq (Thule) and PCS (South) based on Vostok data, reflect the transpolar convection of plasma and magnetic fields driven by the interaction of the solar wind with the magnetosphere
The direct inclusion of the median-based solar wind sector term, FSWS, into the reference level from which the disturbances involved in the calculation of PC index values are measured, generates an effective QL level that may not represent the quiet level found at corresponding local time, seasonal, and IMF BY conditions (Stauning, 2013a, 2015)
Comparisons of PC index values downloaded from the web server http:// pcindex.org at different times may disclose features of the derivation procedures that are not illustrated in the available reference publications or in other documentation provided for the International Association for Geomagnetism and Aeromony (IAGA) endorsement
Summary
The Polar Cap (PC) indices, PCN (North) based on magnetic data from Qaanaaq (Thule) and PCS (South) based on Vostok data, reflect the transpolar convection of plasma and magnetic fields driven by the interaction of the solar wind with the magnetosphere. It has been demonstrated (Stauning, 2018a) that monitoring PC indices could have given 6 h of early warning (‘‘alert’’), of which the last two hours were ‘‘red alert’’ (PCN > 15 mV/m), up to the onset of the strong substorm event on 13 March 1989 that caused the major power outage in Quebec It is imperative for using the real-time PC indices in space weather monitoring or forecast services that they are reliable and robust to possible irregularities like temporary disruptions of the supply of polar geomagnetic data. It has been found that the method can be expected to generate unjustified excursions of several mV/m in the near-real time PC index values compared to their final values during many cases of moderate to strong magnetic activity. The PCN values available since 1975 and the PCS index values available since 1994 are presently termed ‘‘provisional’’ by the International Service of Geomagnetic Indices (ISGI) as reported at their web site, http://isgi.unistra.fr
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