Multiradionuclide evidence for the solar origin of the cosmic-ray events of AD 774/5 and 993/4

Abstract

The origin of two large peaks in the atmospheric radiocarbon (14C) concentration at AD 774/5 and 993/4 is still debated. There is consensus, however, that these features can only be explained by an increase in the atmospheric 14C production rate due to an extraterrestrial event. Here we provide evidence that these peaks were most likely produced by extreme solar events, based on several new annually resolved 10Be measurements from both Arctic and Antarctic ice cores. Using ice core 36Cl data in pair with 10Be, we further show that these solar events were characterized by a very hard energy spectrum with high fluxes of solar protons with energy above 100 MeV. These results imply that the larger of the two events (AD 774/5) was at least five times stronger than any instrumentally recorded solar event. Our findings highlight the importance of studying the possibility of severe solar energetic particle events.

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

“…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).…”

“…Since the AD 775 and AD 994 Δ 14 C peak (henceforth M12) was first measured by Miyake et al (2012Miyake et al ( , 2013, several possible production mechanisms for these spike have been suggested, but the work of Mekhaldi et al (2015) shows that a very soft energy spectrum was involved, implying that a strong solar energetic particle (SEP) event (or series of events) was responsible. Here we present Δ 14 C values from AD 721-820 Sequoiadendron giganteum annual tree-ring samples from Sequoia National Park in California, USA, together with Δ 14 C in German oak from 650-670 BC.…”

Large 14 C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene

“…Although such past cosmic ray variations have been studied by measurements of 14 C contents in tree rings with ≥10-y time resolution for the Holocene, there are few annual 14 C data. There is a little understanding about annual 14 C variations in the past, with the exception of a few periods including the AD 774−775 14 C excursion where annual measurements have been performed.…”

“…Although such past cosmic ray variations have been studied by measurements of 14 C contents in tree rings with ≥10-y time resolution for the Holocene, there are few annual 14 C data. There is a little understanding about annual 14 C variations in the past, with the exception of a few periods including the AD 774−775 14 C excursion where annual measurements have been performed. Here, we report the result of 14 C measurements using the bristlecone pine tree rings for the period from 5490 BC to 5411 BC with 1-to 2-y resolution, and a finding of an extraordinarily large 14 C increase (20‰) from 5481 BC to 5471 BC (the 5480 BC event).…”

“…There is a little understanding about annual 14 C variations in the past, with the exception of a few periods including the AD 774−775 14 C excursion where annual measurements have been performed. Here, we report the result of 14 C measurements using the bristlecone pine tree rings for the period from 5490 BC to 5411 BC with 1-to 2-y resolution, and a finding of an extraordinarily large 14 C increase (20‰) from 5481 BC to 5471 BC (the 5480 BC event). The 14 C increase rate of this event is much larger than that of the normal grand solar minima.…”