Author: Hugh Hudson at the University of Glasgow

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Introduction

The geomagnetic effect of a solar flare (called an SFE nowadays, but more colorfully “crochet” originally) first appeared on the remarkable recording magnetometer at Kew Gardens in 1859 (Stewart, 1861). This was famously the very beginning of “space weather” and also an early example of “multimessenger astronomy”. We now know that an SFE (Solar Flare Effect) results from XUV radiation impinging on the ionosphere, but since both of these items lay in the future, it was a pure mystery then. We have the 1859 records and would like to use the original SFE to calibrate the Carrington flare, often cited as an example of a “superflare.”

Superflares

A “superflare” seems to be regarded as any really powerful event that is beyond current solar experience – say, with a total energy of order 10³³ erg. It is a useful term for stellar flares, which may certainly be more powerful than solar ones, even on “solar-type” stars (eg, Schaefer et al. 2000). Many such flares have now been systematically observed by space observatories such as Kepler.

In addition the term “superflare” also may apply to the remarkable ¹⁴C radioisotope events discovered by Miyake et al. 2012. Such events, with several more subsequently discovered ranging back through the Holocene era (about 10,000 years), could be explained by huge SEP events associated with huge flares. We need to worry about this, because if this description is correct, Earth could suffer tremendous space-weather trauma at some time in the future.

What does the GOES record show?

The GOES soft X-ray record is about the best and longest quantitative record of solar-flare occurrence, starting in the late 1960s but then systematically reliable from 1975. The GOES spacecraft, now up to GOES-18, are in geosynchronous orbit and thus have excellent duty cycles. Since the beginning of the program, there have normally been redundant spacecraft in orbit, and although they are not calibrated photometrically once in orbit, they have a “daisy-chain” record that establishes a fairly uniform radiometric record. This record defines an occurrence distribution function that largely follows a power law, dN/dE ~ E^-2. The record had been marred only by a dozen extra-powerful events, which saturated the X-ray detector readouts. Recently Hudson et al. 2024 have made reasonable corrections of these saturated events, thus making a complete sample possible. The resulting occurrence distribution function clearly rolls over at about the X10 level, making an extension of the power law untenable (Figure 1). An earlier similar result had been obtained by Nita et al. 2002, using radio bursts.

 

Is the apparent deficit in the most powerful GOES events (topping out at about X40 in the newly revised GOES calibration) really in conflict with existence of the solar radiosotope events or the stellar superflares? Somewhat, it seems, but not so rigorously yet; a simple extrapolation of the data in Figure 1 (always an uncertain thing) is not absolutely inconsistent with the energetics of the “tree ring events”, mainly because of their great uncertainties. But the E^-2 powerlaw does not match the data on the greatest events.

How about the Carrington event?

Our best knowledge of the magnitude of the Carrington event (e.g., Cliver et al. 2022) will eventually come from its SFE, which was measured relatively precisely. Copious modern geomagnetic data exist, with many SFEs measured at 1-s cadence from hundreds of geomagnetic observatories, but a comprehensive study does not exist at the present time. In addition the SFE physics, understood at a basic level, has not yet reached an adequate level of understanding. The Carrington flare may in fact be in the superflare category, but that could not significantly change the appearance of Figure 1. Perhaps this Nugget will inspire more research into this interesting physics and its important implications for human society.

References

• Cliver et al., 2022, (ADS)
• Hudson et al., 2024 (ADS)
• Miyake et al., 2012 (ADS)
• Nita et al., 2002 (ADS)
• Schaefer et al., 2000 (ADS)
• Stewart, 1861 (ADS)