A Single‐Year Cosmic Ray Event at 5410 BCE Registered in ¹⁴C of Tree Rings

Abstract

The cosmic-ray flux into the Earth atmosphere fluctuates over time due to various factors. Solar and geomagnetic modulation of galactic cosmic rays are dominating this process on timescales from a few-days to millions of years, such as Forbush decreases (a shielding of galactic cosmic rays due to a coronal mass ejection), the 11-year Schwabe cycle, multidecadal variation such as Grand Solar Minima, or millennial variations in Earth's magnetic field strength. In addition to these recurring factors, sporadic and intense solar energetic particle (SEP) events can also contribute to a sudden ejection of cosmic rays to the Earth. A large-scale SEP event reaching Earth is known as a ground level enhancement (GLE), which can be detected as a large increase in the count rates of ground-based neutron monitors. Since 1942, 72 GLEs have been recorded (1: GLE database). Prior to direct observations of cosmic rays, signatures of past extreme SEP events can be recorded by cosmogenic nuclides, in particular, 14 C in tree rings, and 10 Be and 36 Cl in polar ice cores, all of which are produced in a particle cascade triggered by interactions of high energy cosmic rays with the constituents of Earth's lower atmosphere (Mekhaldi et al., 2015;Miyake, Usoskin, & Poluianov, 2019).Conceivable SEP-driven cosmic ray events reported so far in multiple studies are the 774/775 CE (Bünt-

Tree rings reveal two strong solar proton events in 7176 and 5259 BCE

“…So far, three strong SEP events that led to an abrupt increase of about 1% or more in atmospheric 14 C concentrations within less than two years have been unambiguously detected over the past 3000 years in tree-rings and confirmed with other cosmogenic radionuclides ( 10 Be, 36 Cl) in ice cores, in the years 993 CE, 775 CE and 660 BCE 13,14,19,20 . More, weaker, yet unconfirmed SEP events have been found in the radionuclide records 17,[21][22][23] . They are currently considered candidate events either because there is insufficient data coverage and precision to clearly distinguish them from normal solar modulation or because they have not yet been confirmed 24 .…”

“…(b) Distribution of the simulated ∆ 14 C increases (blue bars) with a Gaussian fit (dashed line). (c) Distribution of excess 14 C production (blue bars) with Gaussian fit (dashed line).Data Sources: 5410 BCE: Miyake et al 23 , 660 BCE: Sakurai et al 13 , 775 CE and 993 CE: Büntgen et al 48 , 1052 CE and 1279 CE: Brehm et al 17 . Supplementary Fig.…”

Mind the Gap: New Precise 14C Data Indicate the Nature of Extreme Solar Particle Events

“…After the exciting discovery of the extreme SPEs occurring during the recent Sun's past, it is time to study the phenomenon in detail, in particular, collecting statistic and covering long periods with high-quality data in different cosmogenic isotopes, with a special focus on 36 Cl and 10 Be measured in polar ice cores. The recent works by Miyake et al (2021) and Brehm et al (2021) form the cornerstone for the process of paving a way to detect a larger number of smaller events in the proximity of the sensitivity threshold for the cosmogenic isotope method leading to a breakthrough in our understanding of extreme solar eruptive events and their potential impact on the Earth's environment and our society. In particular, the new result by Miyake et al (2021) would imply, if independently confirmed, that the extreme SPEs are not manifestations of unknown phenomena but rather high-energy/low-probability tail of the "regular" SPE distribution, making it possible to study them in greater details.…”

“…factor 25-30 stronger than that of 1956. In the work by Miyake et al (2021), another spike in  14 C has been identified corresponding to the year 5410 BCE. It was about one half of the 775 CE event and a factor of  30 as strong as the one of 1956.…”

“…

A new extreme solar particle event (SPE) was found by Miyake et al (2021) corresponding to 5410 BCE using precise 14 C measurements.• This is the third known 'weak' extreme SPE filling a huge observational gap be-10 tween direct and proxy-based datasets.

11• This suggests that extreme SPEs likely belong to 'normal' solar events and not 12 to an unknown phenomenon, making their detailed study possible.

…”