Abstract/Summary

We survey 223,916 auroral records from the Northern Hemisphere observed between January 1650 and July 2024, making full allowance for the secular change in the geomagnetic field. We generate criteria for defining extreme auroral events that are met on 0.015% and 0.023% of nights since 1650 and 1790, respectively. After discussing biases and trends in the data, we compare the event of 10-11 May 2024 with other extreme events and investigate the connections to geomagnetic and sunspot activity. Ranking the events by the lowest geomagnetic latitude from which aurora was observed, the second night of the May 2024 event is shown to be the third most extensive known, the most extensive being 4 February 1872. Allowing for dark adaptation of human vision, we find no evidence that this ranking has been greatly influenced by the increased use of modern digital cameras. We show that the area of the sunspot group from where the causal CME arises (identified by the associated flare) is weakly anticorrelated with the auroral and geomagnetic response; the scatter being large such that, although the February 1872 event arose from a rather small sunspot group, the May 2024 event arose from a large group, as did the ‘Carrington Events’ of August/September 1859 (ranked 2, 4 and 5). We show that the extreme events all occur during Carrington Rotations for which the average open solar flux, FS exceeds 4 × 1014 Wb) but only 3.6% of Carrington Rotations when FS exceeds this value give an extreme event at Earth.