Reliable real-time Polar Cap (PC) indices for space weather monitoring and forecasts

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.