The AD 775 14C event : new evidence for a solar origin

• Main Author: Uusitalo, Joonas
• Other Authors: Helsingin yliopisto, Matemaattis-luonnontieteellinen tiedekunta, Fysiikan laitos, University of Helsinki, Faculty of Science, Department of Physics, Helsingfors universitet, Matematisk-naturvetenskapliga fakulteten, Institutionen för fysik
• Format: Master Thesis
• Language: English
• Published: Helsingfors universitet 2016
• Subjects: Astronomy; Tähtitiede; Astronomi; North Magnetic Pole; Lapland
• Online Access: http://hdl.handle.net/10138/230343

In 2012, a sudden and significant increase in the radiocarbon 14C abundance of tree rings at around AD 775 was found by Miyake et al (2012). Since then, various explanations for the cause of the event have been offered. These include a supernova explosion, a short gamma-ray burst, a comet collision with the Sun causing an energetic burst, a comet disintegration in the Earth's atmosphere and an especially energetic solar flare. Even though there have been a lot of studies considering the event, the definite cause is still unclear. Most of the knowledge regarding the event comes from 14C measurements made from trees that grew in various locations. These include Japan, Germany, USA and Russia. To further increase the understanding of the event and its cause, we have measured the event from a subfossil Lapland tree. This measurement is unique, since the other measurements come from locations significantly further from the magnetic pole. Thus, they might be affected by different atmospheric and geomagnetic dynamics. To date, there has not been a considerable effort in trying to quantify possible differences in the various measurements. Only the maximum increase from the background value has been considered. However, it might not be the most robust indicator of the event intensity, since it is susceptible to statistical fluctuations. For this reason, we have adopted a peak fitting method to better quantify the various properties of different measurements. Special interest was put into calculating the area under the curve of the fit to get a more robust indicator of the event intensity. Here we report that the measurement from a Lapland tree shows a significantly stronger 14C signal than what has been found earlier. Furthermore, our peak analysis demonstrates that there is a clear dependency between the latitude, where the trees have grown, and the intensity of the 14C signal, indicating that higher latitude trees have stronger signals. The connection is even more evident when, instead of the latitude, the distance from the North magnetic pole is used. It is known that the production of 14C by charged particles is significantly higher near the polar regions due to geomagnetic effects. Hence, a solar proton event is consistent with the observed latitude effects, whereas a gamma-ray burst or an atmospheric comet disintegration is not. Therefore, a solar origin is strongly implicated. These findings have a societal significance, since a solar storm poses a considerable threat to various infrastructures. We advice that the AD 775 event should be used as a new worst-case scenario when evaluating different risk mitigation strategies.