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 cm^-2 (with the ±1σ uncertainty spanning a range from ~10⁹-10¹¹ pr cm^-2) for a composite (multi-flare plus shock) 1859 event. This value is approximately twice that of estimates/measurements - ranging from ~5-7 × 10⁹ pr cm^-2 - for the largest SEP episodes (July 1959, November 1960, August 1972) in the modern era.