Tree-Rings Reveal Two Strong Solar Proton Events in 7176 and 5259 BCE

The Sun sporadically produces eruptive events leading to intense fluxes of solar energetic particles (SEPs) that dramatically disrupt the near-Earth radiation environment. Such events have been directly studied for the last decades but little is known about the occurrence and magnitude of rare, extreme SEP events. Presently, a few events that produced measurable signals in cosmogenic radionuclides such as 14C, 10Be and 36Cl have been found. Analyzing annual 14C concentrations in tree-rings from Switzerland, Germany, Ireland, Russia, and the USA we discovered two spikes in atmospheric 14C occurring in 7176 and 5259 BCE. The ~2% increases of atmospheric 14C recorded for both events exceed all previously known 14C peaks but after correction for the geomagnetic field, they are comparable to the largest event of this type discovered so far at 775 CE. These strong events serve as accurate time markers for the synchronization with floating tree-ring and ice core records and provide critical information on the previous occurrence of extreme solar events which may threaten modern infrastructure. © 2022, The Author(s).The Laboratory of Ion Beam Physics is partially funded by its consortium partners PSI, EAWAG, and EMPA. N.B. is funded by the Swiss National Science Foundation (SNSF grant #SNF 197137). The establishment of the BRAMS Facility was jointly funded by the NERC, BBSRC and the University of Bristol and the measurements in this work were partly funded by an ERC Proof of Concept grant awarded to R.P.E. and financing E.C. postdoctoral contract (LipDat H2020 ERC-2018-PoC/812917). We thank Bisserka Gaydarska for sub-sampling the inter-laboratory replicates from M49, M234, Q2729 and Q2750, Cathy Tyers for reviewing the dating of the Irish and German samples, and Alexander Land for assistance in dating sample M49. P.F. received funding from the SNF Sinergia project CALDERA (no. 183571). R.H. is funded by Russian Science Foundation (grant № 21-14-00330). I.U. acknowledges the support from the Academy of Finland (grant 321882 ESPERA). C.L.P.’s and M.W.S.’s work on bristlecone pine was funded by the M.H. Wiener Foundation (ICCP Project). K.N. acknowledges the support provided by the Austrian Science Fund FWF (grant I-1183-N19)

Supplemental information for Mekhaldi et al., JGR: Space Physics 2021: "The signal of solar storms embedded in cosmogenic radionuclides: Detectability and uncertainties ".

Model output of the global average atmospheric production rate of beryllium-10, radiocarbon, and chlorine-36 (10Be, 14C, 36Cl) by solar energetic protons from 59 ground level enhancements and as a function of the dipole moment. The yield functions used are shown in Poluianov et al. (JGR 2016), while the SEP fluence spectra used come from Raukunen et al. (SWSC 2018). See text in the paper (Mekhaldi et al., JGR Space Physics: 2021) for more information on the production model

The Signal of Solar Storms Embedded in Cosmogenic Radionuclides : Detectability and Uncertainties

The threat that solar storms pose to our ever-modernizing society has gathered significant interest in the recent past. This is partly due to the discoveries of large peaks in the content of cosmogenic radionuclides such as radiocarbon (14C) in tree rings and beryllium-10 (10Be) and chlorine-36 (36Cl) in ice cores that were linked to extreme solar storms dated to the past millennia. To better assess the threat that they represent, we need to better quantify the relationship between their energy spectrum and their magnitude with respect to the content of the radionuclides that we measure in environmental archives such as ice cores. Here, we model the global production rate that the 59 largest particle storms coming from the Sun have induced for 10Be, 14C, and 36Cl during the past 70 years. We also consider the deposition flux in 10Be and 36Cl over the high latitudes where all Greenland ice cores are located. Our analysis shows that it is unlikely that any recent solar particle event can be detected in 10Be from ice cores. By relating these values to empirical data from ice cores, we are able to quantify different detection limits and uncertainties for 10Be and 36Cl. Due to different sensitivities to solar energetic particles, we assess that 10Be may only be suitable to detect a limited number of extreme solar storms, while 36Cl is suitable to detect any extreme particle event. This implies that the occurrence-rate estimates of extreme solar storms, based mainly on 14C and 10Be, relate to a small population of potential events

Solar Longitude Distribution of High-energy Proton Flares : Fluences and Spectra

The distribution of the longitudes of solar flares associated with the high-energy proton events called ground level events (GLEs) can be approximated by a Gaussian with a peak at ∼W60, with a full range from ∼E90 to ∼W150. The longitudes of flares associated with the top third (24 of 72) of GLEs in terms of their >430 MeV fluences (F 430) are primarily distributed over E20-W100 with a skew toward disk center. This 120 span in longitude is comparable to the latitudinal spans of powerful coronal mass ejections (CMEs) from limb flares. Only 5 of 24 strong GLEs are located within the W40-80 zone of good magnetic connection to Earth. GLEs with hard spectra, i.e., a spectral index SI30/200(= log(F 30/F 200)) W100) GLEs; (2) quasi-parallel shock acceleration for well-connected (W40-80) GLEs, and (3) proton acceleration/trapping at CME-driven bow shocks from central meridian (E20-W20) that strike the Earth

Measurements of Radionuclides

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The potential for a continuous 10Be record measured on ice chips from a borehole

Ice cores are excellent archives for obtaining long and continuous 10Be records. However, traditional ice core 10Be measurements required a lot of ice (0.5–1kg) and often needed to be connected to a large and costly ice core project. These reasons have been the factors limiting the number and variety of 10Be projects and data. In this paper, we show measurements of 10Be on small samples (∼45g) of continuous auger ice chips from a borehole at Little Dome C (LDC), East Antarctica. The sample preparation method for 10Be accelerator mass spectrometry (AMS) was tested and optimized using test samples (∼50g) including well-mixed surface ice chips from the LDC site, snow collected in Lund (Sweden) and frozen Milli-Q water. The results show that our small ice samples could be processed without ion exchange filtration of the melt water and cleaning the subsequent Be(OH)2 precipitate. In addition, co-precipitating Be with Fe led to more reproducible measurement currents and offer the potential for higher efficiency and precision via longer measurement time. We applied the established preparation method to measure 10Be on 76 samples of the auger ice chips. The resulting 10Be concentration record for the period from 1354 to 1950 CE agrees well with the 10Be concentration in a South Pole ice core and the global 14C production rate and thus reflects well the atmospheric production signal of 10Be. We also observed insignificant mixing among the ice chip samples during the process of drilling and retrieving the ice. Therefore, the new ice chip samples are promising for assessing the long-term changes in 10Be deposition at different ice core sites. A wide application of this novel ice chip samples will increase the variety of 10Be records which will help to improve the assessment of long-term solar and geomagnetic shielding of galactic cosmic rays

Cosmogenic Isotopes as Proxies for Solar Energetic Particles

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