The 1859 space weather event revisited: limits of extreme activity

The solar flare on 1 September 1859 and its associated geomagnetic storm remain the standard for an extreme solar-terrestrial event. 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. X35 (±5) for the 4 November 2003 flare) and minimum Dst = 900 (+50, 150) nT (vs. 825 to 900 nT for the great storm of May 1921). We have no direct evidence of an associated solar energetic proton (SEP) event but a correlation between>30 MeV SEP fluence (F30) and flare size based on modern data yields a best guess F30 value of ~1.1 · 1010 pr cm2 (with the ±1r uncertainty spanning a range from ~109–1011 pr cm2) for a composite (multi-flare plus shock) 1859 event. This value is approximately twice that of estimates/measurements – ranging from ~5–7 · 109 pr cm2 – for the larges