The AD775 cosmic event revisited: the Sun is to blame

Abstract

Aims. Miyake et al. (2012, Nature, 486, 240, henceforth M12) recently reported, based on 14 C data, an extreme cosmic event in about AD775. Using a simple model, M12 claimed that the event was too strong to be caused by a solar flare within the standard theory. This implied a new paradigm of either an impossibly strong solar flare or a very strong cosmic ray event of unknown origin that occurred around AD775. However, as we show, the strength of the event was significantly overestimated by M12. Several subsequent works have attempted to find a possible exotic source for such an event, including a giant cometary impact upon the Sun or a gamma-ray burst, but they are all based on incorrect estimates by M12. We revisit this event with analysis of new datasets and consistent theoretical modelling. Methods. We verified the experimental result for the AD775 cosmic ray event using independent datasets including 10 Be series and newly measured 14 C annual data. We surveyed available historical chronicles for astronomical observations for the period around the AD770s to identify potential sightings of aurorae borealis and supernovae. We interpreted the 14 C measurements using an appropriate carbon cycle model. Results. We show that: (1) The reality of the AD775 event is confirmed by new measurements of 14 C in German oak; (2) by using an inappropriate carbon cycle model, M12 strongly overestimated the event's strength; (3) the revised magnitude of the event (the global 14 C production Q = (1.1−1.5) × 10 8 atoms/cm 2 ) is consistent with different independent datasets ( 14 C, 10 Be, 36 Cl) and can be associated with a strong, but not inexplicably strong, solar energetic particle event (or a sequence of events), and provides the first definite evidence for an event of this magnitude (the fluence >30 MeV was about 4.5 × 10 10 cm −2 ) in multiple datasets; (4) this interpretation is in agreement with increased auroral activity identified in historical chronicles. Conclusions. The results point to the likely solar origin of the event, which is now identified as the greatest solar event on a multimillennial time scale, placing a strong observational constraint on the theory of explosive energy releases on the Sun and cool stars.

Relationship between solar activity and Δ14C peaks in AD 775, AD 994, and 660 BC

“…The strongest peak of Δ 14 C, corresponding to the event of AD 775 (called henceforth M12), and another peak in AD 993/4 were measured by Miyake (Miyake et al 2012(Miyake et al , 2013) using 14 C in annually resolved Japanese cedar tree-ring sequences, and M12 has been confirmed by several studies (Usoskin et al 2013;Jull et al 2014;Güttler et al 2015;Rakowski et al 2015;Büntgen et al 2016) including three that showed peaks with slightly different starting times. Data from high-latitude Siberian and Altai (Jull et al 2014;Büntgen et al 2016) record the AD 775 peak but show a Δ 14 C rise beginning one year earlier, and Δ 14 C data in New Zealand kauri (Güttler et al 2015) indicate a peak delayed by half a year.…”

“…Furthermore, whereas the maximum SH tropospheric 14 C increase during the bomb spike was just 70% of mid-latitude NH values (Manning et al 1990), reflecting the fact that close to 100% of the input was generated within the NH, the M12 amplitude in the kauri is~90% of the mean of the NH stations ( Table 1), indicating that significant 14 C production occurred in both hemispheres. -17.3 ± 1.8 -6.2 ± 1.4 11.1 ± 2.3 California (sequoia) 36°N -17.6 ± 0.9 -2.1 ± 1.0 15.5 ± 1.4 New Zealand 36°S -23.8 ± 0.8 -9.7 ± 0.7 14.1 ± 1.0 *Average of data from two laboratories (Usoskin et al 2013) Large-scale stratospheric air transport is dominated by the so-called Brewer-Dobson circulation (Butchart 2014) characterized by injection of tropospheric air into the stratosphere in the tropics and subsequently by movement towards the winter pole and subsidence at high latitudes, driven primarily by seasonally varying large-scale eddy motions that act to reduce the mean zonal velocity of air parcels and thus induce a poleward drift (Holton et al 1995). Descending air in the winter polar vortex carrying a high burden of stratospheric tracers is released near the tropopause when the vortex breaks up in the spring, and extratropical injection of stratospheric air into the troposphere peaks in mid-to-late summer (Stohl et al 2003).…”

“…Recently, superflares from Sun-like stars were observed with a thousand times the energy release of the largest flares on our Sun, although the life stage at which Sun-like stars make superflares and differences between the Sun and Sun-like stars are still under debate (Maehara et al 2012(Maehara et al , 2015. This is a popular theory (Miyake et al 2012;Usoskin et al 2013;Jull et al 2014;Mekhaldi et al 2015) but a potential problem is that it requires a SEP fluence at least five times larger than for any event recorded in the era of ground-based or satellite observations (Mekhaldi et al 2015). Therefore, we consider the fact that large SEP events are often also associated with large geomagnetic storms (Li et al 2006), which can have the effect of enlarging the area of the atmosphere that is irradiated by solar energetic particles (Leske et al 2001).…”

On a Solar Origin for the Cosmogenic Nuclide Event of 775 A.D.

“…Melott & Thomas (2012) and Usoskin et al (2013) favor a standard (eruptive flare; Reames 2013) solar source, and Eichler & Mordecai (2012) proposed a non-traditional solar source-a superflare caused by a large comet colliding with the Sun. Recently, Miyake et al (2013) have reported a second rapid 14 C increase from 992-993 a.d. that was ∼0.6 times as intense as the 775 event with a similar time profile and spectrum.…”

The Chinese comet observation in AD 773 January

“…They excluded supernovae as a possible cause due to the lack of any historic observations and of any young nearby supernova remnants, and they also excluded solar super-flares as a cause, because their spectra would not sufficiently explain the 14 C to 10 Be production ratio observed for that time. Then, Usoskin & Kovaltsov (2012), Melott & Thomas (2012), Thomas et al (2013), and Usoskin et al (2013) suggested that a solar super-flare beamed with only ≥ 24 • degree beam size could have caused the event (Melott & Thomas 2012), in particular if four to six times less 14 C was produced than calculated in Miyake et al (2012) due to a different carbon circulation model (Usoskin et al 2013). Hambaryan & Neuhäuser (2013) suggested that a short hard Gamma-Ray-Burst could have caused the event, because all observables including the 14 C to 10 Be production ratio are consistent with such a burst.…”