The 1859 Space Weather Event Revisited

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Introduction

After more than 150 years, the space-weather event of 1859 associated with Carrington's name, continues to intrigue and inform us. It was comprehensively observed by many of the most modern tools available at that early time, which of course did not include any of the electronics that we take for granted today. The phenomena observed included the Carrington flare itself, plus various [geomagnetic] processes that (mostly in retrospect) laid the foundation stones for [heliospheric physics] and [space weather]. One of the simplest and most important indices is [Dst], a measure of global geomagnetic variability during a [magnetic storm] such as that associated with the 1859 event.

This "Carrington flare" is doubly intriguing because of its distinction both as the first reported solar flare and, coincidentally, the largest geomagnetic disturbance yet recorded. It is the exemplar and guide for us as regards extreme space-weather events.

Estimates of Event Magnitudes

In a recent paper [Ref. 1], Bill Dietrich and I have reassessed the size of the 1859 event and its consequences. Our best guesses are as follows: a flare magnitude (soft X-ray classification) and total (flare radiation plus CME kinetic) energy of ~X45 and ~5 x 10³² erg, respectively (Figure 1); a >30 MeV proton fluence (F30) of ~10¹⁰ cm^-2; and a minimum Dst index of ~ -900 nT. These estimates and those measured for the closest modern competitors in each category are shown in tabular form here:

Event          GOES      Energy         PFU            Dst 
                         erg            cm-2           [nT]

SOL1859-09-01  X45       1032           1 x 1010       -900 
SOL2003-11-04  X35       1033    
SOL1972-08-04                           5 x 109
SOL1921-05-14                                         -850 

The estimated geomagnetic Dst index for the Carrington magnetic storm, reduced from a reported -1760 nT value based on Colaba (near Mumbai, India) observations, reflects direct (Green and Boardsen, 2006) and indirect (Figure 2) evidence that the Colaba reading included an [auroral] component.

Fig. 1: Greenwich Observatory magnetometer traces (horizontal force on top and declination on the bottom; the two traces are offset by 12 hours) during the time of the solar flare on 1 September 1859. The red arrows indicate the magnetic crochet or solar flare effect (SFE). Comparing the size of the SFE with modern large flares for which the SXR classification is known permits an estimate of the size of the 1859 flare. The writing at the bottom in the red box says "The above movement was nearly coincidental in time with Carrington's observation of a bright eruption on the Sun. Disc[overed] over a sunspot. (H.W.N., 2 Dec 1938)." The annotation "H.W.N." refers to Harold W. Newton, Maunder's successor as the sunspot expert at Greenwich.

In a related paper ([Ref. 2]) we question the recent attribution of the cosmogenic nuclide event of 775 AD to the Sun (Ref. 3]). Such a solar event would imply an F30 value of ~8 x 10¹⁰ cm10^-2 (approximately 10 times the value recorded during the three-month interval of sustained strong SEP activity from August-October 1989), a 1 GV proton fluence 45 times larger than that of the February 1956 ground level event, or a single flare with a GOES classification of X230 (10³⁴ erg) flare.

Fig. 2: Comparison of great magnetic storms observed at Greenwich in October 1847 (top panel) and at Colaba in September 1859 (bottom). The H-component magnetic intensity and time scales are the same in both plots. The peak of the 1847 storm coincided with an aurora observed in bright moonlight in southern England. The similarity of the two time profiles suggests that the 59 event was similarly affected by auroral currents.

Conclusion

The solar flare and terrestrial effects of the Carrington space-weather event of 1859 are still of great current interest, and the work described here is helping to place it in the context of recent remarkable discoveries. There is a caveat regarding the coincidence of first flare and greatest storm in the 1859 event. Hugh Hudson (personal communication, 2013) cautions that the first flare reported was bound to have been a whopper. That said, there is no guarantee that a big flare will produce a big magnetic storm, since their heliographic position is also crucially important for such effects. The 4 August 1972 flare had a CME with a shorter transit time to Earth than that of the 1859 event (14.6 hr vs. 17.5 hr) but the associated 1972 storm (Dst = -125 nT) does not rank in the top 25 of such events.

References

[1] "The 1859 space weather event revisited: limits of extreme activity"

[2] "On a solar origin for the cosmogenic nuclide event of 775 AD"

[3] "Duration and extent of the great auroral storm of 1859"

[4] "The AD775 cosmic event revisited: the Sun is to blame"