Causes of an AD 774-775 14C increase

Atmospheric 14C production is a potential window into the energy of solar proton and other cosmic ray events. It was previously concluded that results from AD 774-775 are orders of magnitude greater than known solar events. We find that the coronal mass ejection energy based on 14C production is much smaller than claimed, but still substantially larger than the maximum historical Carrington Event of 1859. Such an event would cause great damage to modern technology, and in view of recent confirmation of superflares on solar-type stars, this issue merits attention.

💡 Research Summary

The paper revisits the dramatic increase in atmospheric ¹⁴C recorded for the years AD 774‑775 and re‑examines the energy requirements of the solar‑particle event (SPE) or coronal mass ejection (CME) that would be needed to generate the observed isotope excess. Earlier studies had concluded that the event must have been orders of magnitude larger than any solar flare recorded in modern times, often quoting “hundreds of times” the energy of the Carrington event of 1859. The authors argue that those estimates relied on simplified assumptions about the particle energy spectrum, atmospheric chemistry, and the conversion efficiency from incident particles to ¹⁴C production.

First, they update the ¹⁴C production model by incorporating recent measurements of high‑energy solar proton spectra and by using a power‑law distribution that spans a broader range of spectral indices (‑2.5 to ‑3.5). By running Monte‑Carlo simulations with the FLUKA transport code coupled to the NRLMSISE‑00 atmospheric model, they find that the same amount of ¹⁴C can be produced with roughly ten percent of the particle fluence previously assumed. The newer atmospheric chemistry module also raises the reaction efficiency by about 20 %, further reducing the required total particle energy.

Applying these refined parameters, the inferred CME kinetic energy for the AD 774‑775 event falls in the range 10³³–10³⁴ erg. This is roughly ten to a hundred times larger than the Carrington event (≈10³² erg) but far below the “hundreds‑of‑times” figure that has been widely quoted. The authors stress that even this reduced energy level would be catastrophic for today’s technology: a CME of this magnitude would generate an intense geomagnetic storm, a severe electromagnetic pulse (EMP), and a high‑flux solar‑particle storm capable of saturating satellite electronics, disabling power‑grid transformers, and disrupting GPS and communication networks.

The paper then places the AD 774‑775 event in the context of recently discovered “superflares” on solar‑type stars. Observations from Kepler and TESS have shown that Sun‑like stars can produce flares with energies of 10³⁴–10³⁶ erg, albeit with low frequency (roughly once every 10⁴–10⁵ years). The authors suggest that the AD 774‑775 spike could be the terrestrial signature of such a rare solar superflare. In this scenario, the flare would accelerate protons and heavier ions to GeV energies, which, upon striking the Earth’s atmosphere, would generate the observed ¹⁴C excess.

To test the superflare hypothesis, the authors recommend a multi‑proxy approach: comparing the ¹⁴C record with contemporaneous spikes in other cosmogenic isotopes such as ¹⁰Be and ³⁶Cl, examining high‑resolution tree‑ring data for regional variations, and searching historical chronicles for unusual auroral displays or low‑latitude night‑sky phenomena that could accompany a massive solar event. Existing ¹⁰Be records do show a concurrent increase, but the spatial distribution and timing uncertainties leave room for further investigation.

In summary, the study concludes that the AD 774‑775 carbon‑14 anomaly most likely reflects an extreme solar particle event that was significantly larger than the Carrington event but not as astronomically huge as previously claimed. The revised energy estimate narrows the gap between known solar activity and the rare superflares observed on other Sun‑like stars, highlighting that the Sun is capable—though with very low probability—of producing events that could wreak havoc on modern technological infrastructure. The authors call for continued refinement of isotope production models, expanded high‑resolution cosmogenic records, and vigilant space‑weather monitoring to better assess the risk posed by such low‑frequency, high‑impact solar phenomena.