The extreme solar and geomagnetic storms on 1940 March 20-25

In late 1940 March, at least five significant solar flares were reported. They likely launched interplanetary coronal mass ejections (ICMEs), and were associated with one of the largest storm sudden commencements (SSCs) since 1868, resulting in space weather hazards that would have significant societal impacts should it occur today. The initial solar activity is associated with a solar proton event. Afterwards, another flare was reported in the eastern solar quadrant (N12 E37-38) at 11:30-12:30 ut on March 23, with significant magnetic crochets (up to approximate to |80| nT at Eskdalemuir) during 11:07-11:40 ut. On their basis, we conservatively estimate the required energy flux of the source solar flare as X35 +/- 1 in soft X-ray class. The resultant ICMEs caused enormous SSCs (up to >425 nT recorded at Tucson) and allowed us to estimate an extremely inward magnetopause position (estimated magnetopause stand-off position approximate to 3.4 R-E). The time series of the resultant geomagnetic storm is reconstructed using a Dst estimate, which peaked at 20 ut on March 24 at approximate to -389 nT. Around the storm main phase, the equatorward boundary of the auroral oval extended <= 46.3 degrees in invariant latitudes. This sequence also caused a solar proton event and Forbush decrease (approximate to 3 per cent). These sequences indicate pile-up of multiple ICMEs, which even achieved a record value of inward magnetopause position. Our analyses of this historical pioneer event bring more insights into possible serious space weather hazards and provide a quantitative basis for future analyses and predictions.