![]() ![]() In a quirk of history, the first observed solar flare, on 1 September 1859 ( Carrington 1860 Hodgson 1860), was associated with arguably the largest space weather event ever recorded ( Stewart 1861 Loomis 1859, 1860, 1861 (see Shea & Smart 2006) Cliver 2006b). This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ![]() This value is approximately twice that of estimates/measurements – ranging from ~5–7 × 10 9 pr cm −2 – for the largest SEP episodes (July 1959, November 1960, August 1972) in the modern era. We have no direct evidence of an associated solar energetic proton (SEP) event but a correlation between >30 MeV SEP fluence (F 30) and flare size based on modern data yields a best guess F 30 value of ~1.1 × 10 10 pr cm −2 (with the ☑σ uncertainty spanning a range from ~10 9–10 11 pr cm −2) for a composite (multi-flare plus shock) 1859 event. −825 to −900 nT for the great storm of May 1921). The most recent estimates of the flare soft X-ray (SXR) peak intensity and Dst magnetic storm index for this event are: SXR class = X45 (±5) (vs. * Corresponding author: solar flare on 1 September 1859 and its associated geomagnetic storm remain the standard for an extreme solar-terrestrial event. Space Vehicles Directorate, Air Force Research Laboratory, Sunspot, NM 88349, USA
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