The Great Storm of May 1921: An Exemplar of a Dangerous Space Weather Event

Abstract We reconstruct the timeline of the extreme space weather event of May 1921, reviewing a wealth of reports from scientific literature, databases, newspaper reports, and reports by historians and astronomers. A series of coronal mass ejections (CMEs) bombarded Earth between 13 and 16 May, as shown by a series of sudden commencements observed across the global network of magnetometers. These CMEs produced three major periods of geomagnetic activity. The first period followed the arrival of two CMEs on 13 May. These may have cleared much density from the inner heliosphere, enabling a subsequent CME to travel quickly to Earth and cause intense activity. Continuing moderate magnetic activity following the first period may also have preconditioned the magnetosphere so it responded strongly to that later CME. This arrived late on 14 May, driving a short period of very intense activity early on 15 May, including technological impacts indicative of strong geoelectric fields. Another CME arrived early on 16 May, driving intense activity similar to that on 13 May. We show how these impacts fit with scientific observations to give a timeline that can be used in worst‐case studies/benchmarks. We also show that some impacts were probably coincidental with the storm, but due to more prosaic faults. This sequence of preconditioning, intense geoelectric fields, and their impacts, plus coincidental faults, makes the 1921 event an excellent basis for building space weather scenarios. Such scenarios are vital scientific input to the development and implementation of policies for mitigation of severe space weather.