From the AGU weekly highlights:
Large solar proton event explains 774-775 CE carbon-14 increase
Tree ring records indicate that in 774-775 CE, atmospheric carbon-14 levels increased substantially. Researchers suggest that a solar proton event may have been the cause. In solar proton events, large numbers of high-energy protons are emitted from the Sun, along with other particles. If these particles reach Earth’s atmosphere, they ionize the atmosphere and induce nuclear reactions that produce higher levels of carbon-14; the particles also cause chemical reactions that result in depletion of ozone in the ozone layer, allowing harmful ultraviolet radiation to reach the ground.
A previous group of researchers suggested that to cause the observed eighth century carbon-14 increase, a solar proton event would have had to be thousands of times larger than any that has been observed from the Sun. However, Thomas et al. believe that group’s calculations were incorrect. They modeled the atmospheric and biologic effects of three solar proton events with different energy spectra and fluences (number of protons per area). They find that an event with about 7 or more times greater fluence (depending on the spectrum) than an observed October 1989 solar flare event could explain the 774-775 CE carbon-14 enhancement. With a hard (high-energy) spectrum, an event with this fluence would result in moderately damaging effects on life but would not cause a mass extinction. They rule out an event with a softer spectrum because such an event would cause severe ozone depletion and mass extinction, which were not observed in the eighth century. The authors estimate that solar proton events of this magnitude occur on average once in a thousand years, and more often if the estimate is based on astronomical observations of flares on Sun-like stars. They note that although that may seem low, such an event would have severely damaging effects on the technology on which society relies.
Geophysical Research Letters, doi:10.1002/grl.50222, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50222/abstract
Terrestrial effects of possible astrophysical sources of an AD 774-775 increase in carbon-14 production
We examine possible sources of a substantial increase in tree ring14C measurements for the years AD 774-775. Contrary to claims regarding a coronal mass ejection (CME), the required CME energy is not several orders of magnitude greater than known solar events. We consider solar proton events (SPEs) with three different fluences and two different spectra. The data may be explained byan event with fluenceabout one order of magnitude beyond the October 1989 SPE.Two hard spectrum cases considered here result in moderate ozone depletion, so no mass extinction is implied, though we do predict increases in erythema and damage to plants from enhanced solar UV.We are able to rule out an event with a very soft spectrum that causes severe ozone depletion and subsequent biological impacts.Nitrate enhancements are consistent with their apparent absence in ice core data. The modern technological implications of such an eventmay beextreme, and considering recent confirmation of superflares on solar-type stars, this issue merits attention.
Brian C. Thomas, Keith R. Arkenberg and Brock R. Snyder II: Department of Physics and Astronomy, Washburn University, Topeka, Kansas, United States;
Adrian L. Melott: Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, United States.