C03_03

Detection of solar events by using radiocarbon in tree-rings

Brehm N¹, Christl M¹, Adolphi F², Casanova E^3,4, Evershed R.P⁴, Muscheler R⁵, Synal H¹, Mekhaldi F⁵, Paleari C⁵, Knowles T⁴, Bayliss A⁶, Nicolussi K⁷, Pearson C⁸, Fonti P⁹, Nievergelt D⁹, Hanterimov R¹⁰, Wacker L¹

¹Eth Zürich, Zürich, Switzerland, ²Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany, ³Muséum National d’Histoire Naturelle, Paris, France ⁴University of Bristol, Bristol, UK, ⁵Lund University, Lund, Sweden, ⁶Historic England, London, UK, ⁷Universität Innsbruck, Innsbruck, Austria, ⁸University of Arizona, Tucson, USA, ⁹Eidgenössische Forschungsanstalt WSL, Birmensdorf, Switzerland, ¹⁰Laboratory of Dendrochronology, Institute of Plant and Animal Ecology, Ekaterinburg, Russia

The Sun irregularly expels large amounts of highly energetic particles into the interplanetary space and towards Earth which can be observed as so-called solar energetic particle (SEP) events. SEP events can potentially cause major damage to satellites and can even disrupt sensitive electronic systems at ground level. While direct observations of SEP events are limited to the last few decades, cosmogenic radionuclides such as ¹⁴C can be used to detect and study them much further back in time.

The production rate of cosmogenic nuclides, such as ¹⁴C, is primarily dependent on the incoming flux of highly energetic galactic cosmic rays. Normally solar particles expelled into the interplanetary space have not sufficient energy to cause radionuclide production in the Earth atmosphere, but SEP events may cause sudden increases in radionuclide production.

Analyzing ¹⁴C concentrations in annual tree-rings from Switzerland, Germany, Ireland, Russia, and the USA, we recently discovered two new spikes in atmospheric ¹⁴C in 7176 and 5259 BCE. The ~2% increases of atmospheric ¹⁴C recorded for both events exceed all previously known ¹⁴C peaks. Here we summarize and characterize all the found ¹⁴C production events that are attributed to SEP events. We find that the two newly found events are comparable to the largest event of this type discovered so far at 775 CE.