**Acknowledgements**

The staff at the observatories in Qaanaaq (Thule), Vostok, and Dome-C, and their supporting institutes is gratefully acknowledged for providing high-quality geomagnetic data for this study. The efficient provision of space data from the IMP, ACE, Wind, and Geotail missions by the OMNIweb service, the supply of geomagnetic data from the INTERMAGNET data service center, and the excellent performance of the ISGI and AARI PC index portals are greatly appreciated. The efforts from the many geomagnetic observatories involved in the collection of data for the SuperMag, AE, Dst, SYM, and ASY indices and their data processing team at the University of Maryland, at GFZ data center in Potsdam, and at WDC-C2 in Kyoto are most gratefully acknowledged. The author gratefully acknowledges the collaboration and many rewarding discussions in the past with Dr. O.A. Troshichev and Dr. A.S. Janzhura at the Arctic and Antarctic Research Institute, St. Petersburg, Russia.

### **Data availability**

Geomagnetic data from Qaanaaq, Vostok, and Dome-C observatories were downloaded from the INTERMAGNET data service web portal at http://intermagnet.org. Ring current indices, Dst, SYM-H, and ASY-H, were downloaded from the web portal for World Data Centre WDC-C2 in Kyoto at http://swdcwww.kugi.kyoto-u.ac.jp/dstdir/ index.html. Spacecraft data needed to generate the merging electric field values were downloaded from the OMNIweb service portal http://omniweb.gsfc.nasa.gov. SSC and QD data were downloaded from the ISGI data service portal http://isgi.unistra.fr. The SuperMag indices were downloaded from the web service at http://supermag.jhuapl.edu.

The magnetic observatory in Qaanaaq is managed by the Danish Meteorological Institute, while the magnetometer instruments are operated by DTU Space, Denmark. The Vostok observatory is operated by the Arctic and Antarctic Research Institute in St. Petersburg, Russia. The Dome-C observatory is managed by Ecole et Observatoire des Sc iences de la Terre (France) and Istituto Nazionale di Geofisica e Vulcanologia (Italy).

The "DMI" PC index version is documented in the report SR-16-22 [17] available at the web site: http://www.dmi.dk/fileadmin/user\_upload/Rapporter/TR/2016/SR-16-22-PCindex.pdf.
