The extreme geomagnetic storm on 13–15 May 1921: a study based on hourly means, including observations at Stará Ďala (Hurbanovo)

The magnetic storm that occurred in May 1921 ranks among the most extreme events ever observed by magnetic observatories. Some parts of this storm were also recorded in declination and vertical intensity by the variation station at the Stará Ďala observatory (present-day Hurbanovo in Slovakia). However, the magnetogram on photographic paper for this event not only contained data gaps, it also did not have a marked timeline, and the values of the divisions for the geomagnetic elements were not known. We identified timestamps using global variations observed by other observatories and estimated the values of the divisions based on data from before and after the studied event. Then, the magnetograms were digitized. To interpret the obtained data, we compared them with hourly averages from other observatories in different parts of the globe. Our results seem to confirm the expected assumption that, in the morning hours of 15 May 1921, the equatorward boundary of the auroral oval extended to the European mid-latitude observatories.