Research Article

The 1859 space weather event revisited: limits of extreme activity

¹ Space Vehicles Directorate, Air Force Research Laboratory, Sunspot, NM 88349, USA
² Praxis, Inc., Alexandria, VA 22303, USA

^* Corresponding author: ecliver@nso.edu

Received: 4 April 2013
Accepted: 17 September 2013

Abstract

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.