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

In late March 1940, 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 the year 1868, resulting in space weather hazards that today would have significant societal impacts. The initial solar activity is associated with a short geomagnetic storm and a notable SSC. Afterward, the third flare was reported in the eastern solar quadrant (N12 E37-38) at 11:30–12:30 UT on 23 March, with significant magnetic crochets (up to ≈ |80| nT at Eskdalemuir) during 11:07–11:40 UT. On their basis, we estimate the required energy flux of the source 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 standoff position ≈ 3.4 RE). The time series of the resultant geomagnetic storm is reconstructed using a Dst estimate, which peaked at 20 UT on 24 March at ≈ −389 nT. Around the storm main phase, the equatorial boundary of the auroral oval extended ≤ 46.3° at invariant latitudes. This sequence also caused a solar proton event and Forbush decrease (≈ 3%). These sequences indicate pileups 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.