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Extreme solar particle events (ESPEs) are rare and the most potent known processes of solar eruptive activity. During ESPEs, a vast amount of cosmogenic isotopes (CIs) 10Be, 36Cl, and 14C can be produced in the Earth's atmosphere and deposited in natural stratified archives. Accordingly, CI measurements in these archives allow us to evaluate particle fluxes during ESPEs. In this work, we present a new method of ESPE fluence (integral flux) reconstruction based on state‐of‐the‐art modeling advances, allowing to fit together different CI data within one model. We represent the ESPE fluence as an ensemble of scaled fluence reconstructions for ground‐level enhancement (GLE) events registered by the neutron monitor network since 1956 coupled with satellite and ionospheric measurements data. Reconstructed ESPE fluences appear softer in its spectral shape than earlier estimates, leading to significantly higher estimates of the low‐energy (E < 100 MeV) fluence. This makes ESPEs even more dangerous for modern technological systems than previously believed. Reconstructed ESPE fluences are fitted with a modified Band function, which eases the use of obtained results in different applications. Key Points: Integral fluxes (fluences) of four extreme solar particle events (ESPEs) are reconstructed using a novel multiproxy approachESPE fluences are shown to have a spectral shape similar to the most powerful modern solar particle events but orders of magnitude greaterFor the reconstruction, we used recent cosmogenic isotope measurements combined with state‐of‐the‐art modeling [ABSTRACT FROM AUTHOR]

Cosmogenic isotopes 14C, 10Be and 36Cl measured in datable natural archives provide the only known quantitative proxy for cosmic-ray (CR) and solar-activity variability before the era of direct measurements. Studies of relations between the measured isotope concentrations and CR variability require complicated modeling including the isotope production and transport in the terrestrial system. Here we propose a rough "effective energy" method to make quick estimates of the CR variability directly from the cosmogenic data using an approximate linear scaling between the measured isotope concentrations and the energyintegrated flux of CR above the effective energy. The method is based on the thoroughly computed effective yield function presented here. A simple way to account for the variable geomagnetic field is also provided. The method was developed for both solar energetic particles (SEPs) and galactic cosmic ray (GCR) variability and is shown to provide a robust result within 20% and 1% accuracy, respectively, without an assumption of the specific spectral shape. Applications of the effective-energy method to the known extreme SEP events and the secular GCR variability are discussed. The new method provides a simple and quick tool to assess the CR variability in the past. On the other hand, it does not supersede the full detailed modeling required for precise results. [ABSTRACT FROM AUTHOR]

The short-living cosmogenic isotope 7 Be, which is produced by cosmic rays in the atmosphere, is often used as a tracer for atmospheric dynamics, with precise and high-resolution measurements covering the recent decades. The long-living isotope 10 Be, as measured in polar ice cores with an annual resolution, is a proxy for long-term cosmic-ray variability, whose signal can, however, be distorted by atmospheric transport and deposition that need to be properly modeled to be accounted for. While transport of 7 Be can be modeled with high accuracy using the known meteorological fields, atmospheric transport of 10 Be was typically modeled using case-study-specific simulations or simplified box models based on parameterizations. Thus, there is a need for a realistic model able to simulate atmospheric transport and deposition of beryllium with a focus on polar regions and (inter)annual timescales that is potentially able to operate in a self-consistent mode without the prescribed meteorology. Since measurements of 10 Be are extremely laborious and hence scarce, it is difficult to compare model results directly with measurement data. On the other hand, the two beryllium isotopes are believed to have similar transport and deposition properties, being different only in production and lifetime, and thus the results of 7 Be transport can be generally applied to 10 Be. Here we present a new model, called CCM SOCOL-AERv2-BE, to trace isotopes of 7 Be and 10 Be in the atmosphere based on the chemistry–climate model (CCM) SOCOL (SOlar Climate Ozone Links), which has been improved by including modules for the production, deposition, and transport of 7 Be and 10 Be. Production of the isotopes was modeled for both galactic and solar cosmic rays by applying the CRAC (Cosmic Ray Atmospheric Cascade) model. Transport of 7 Be was modeled without additional gravitational settling due to the submicron size of the background aerosol particles. An interactive deposition scheme was applied including both wet and dry deposition. Modeling was performed using a full nudging to the meteorological fields for the period of 2002–2008 with a spin-up period of 1996–2001. The modeled concentrations of 7 Be in near-ground air were compared with the measured ones at a weekly time resolution in four nearly antipodal high-latitude locations: two in the Northern (Finland and Canada) and two in the Southern (Chile and the Kerguelen Islands) Hemisphere. The model results agree with the measurements in the absolute level within error bars, implying that the production, decay, and lateral deposition are correctly reproduced. The model also correctly reproduces the temporal variability of 7 Be concentrations on annual and sub-annual scales, including the presence and absence of the annual cycle in the Northern and Southern Hemisphere, respectively. We also modeled the production and transport of 7 Be for a major solar energetic particle event (SPE) on 20 January 2005, which appears insufficient to produce a measurable signal but may serve as a reference event for historically known extreme SPEs. Thus, a new full 3D time-dependent model, based on CCM SOCOL, of 7 Be and 10 Be atmospheric production, transport, and deposition has been developed. Comparison with real data on the 7 Be concentration in the near-ground air validates the model and its accuracy. [ABSTRACT FROM AUTHOR]

Extreme solar particle events of 775 CE, 994 CE, and 660 BCE are nearly two orders of magnitude stronger than those observed instrumentally. Because of the large observational gap between directly measured and historical events, it was unclear whether they can be produced by the Sun "normally" or from an unknown phenomenon. Recent works by Miyake et al. (2021, doi: https://doi.org/10.1029/2021GL093419) and Brehm et al. (2021, https://doi.org/10.1038/s41561-020-00674-0) start filling the gap with weaker yet extreme events approaching the detectability threshold. More such events are expected to be found in the future but the present result, if confirmed, would imply that the extreme solar events likely represent the high‐energy/low‐probability tail of the continuous distribution of solar eruptive events rather than a new unknown type of events. However, more statistic is needed for a solid conclusion. This would lead to better understanding of the processes producing such events that is important for their risk assessments for the modern technology. Plain Language Summary: Hazards related to eruptive solar events such as flares, coronal mass ejections or particle storms are well‐known during the recent decades and are studied by the Space Weather research discipline. However, as we know from historical proxy data, solar particle events (SPEs) can be a factor of 100 stronger than the directly observed ones and can potentially cause dramatic damages to modern technologies. With the huge observational gap between directly measured and historical events, it was not clear whether the latter can be produced by the Sun in a "normal" way or from an unknown phenomenon. A recent work by Miyake et al. (2021, https://doi.org/10.1029/2021gl093419) presents a new candidate for the extreme SPE dated to 5410 BCE discovered using high‐precision measurements of radiocarbon in tree rings. Together with other recent results by Brehm et al. (2021, https://doi.org/10.1038/s41561-020-00674-0), it starts filling the gap. The result suggests that the extreme solar events likely represent the high‐energy/low‐probability tail of the continuous distribution of solar eruptive events. This would lead to a better understanding of the processes producing such events that is crucially important for assessments of the related risks for the modern technological society. Key Points: A new extreme solar particle event (SPE) was found by Miyake et al. (2021) corresponding to 5410 BCE using precise 14C measurementsThis is the third known "weak" extreme SPE filling a huge observational gap between direct and proxy‐based datasetsThis suggests that extreme SPEs likely belong to "normal" solar events and not to an unknown phenomenon, making their detailed study possible [ABSTRACT FROM AUTHOR]

A new method to reconstruct the high-rigidity part (≥ 1 GV) of the spectral fluence of solar energetic particles (SEP) for GLE events, based on the world-wide neutron monitor (NM) network data, is presented. The method is based on the effective rigidity R eff and scaling factor K eff . In contrast to many other methods based on derivation of the best-fit parameters of a prescribed spectral shape, it provides a true non-parametric (viz. free of a priori assumptions on the exact spectrum) estimate of fluence. We reconstructed the SEP fluences for two recent GLE events, #69 (20 Jan. 2005) and #71 (17 May 2012), using four NM yield functions: (CD00 – Clem and Dorman in Space Sci. Rev.93, 335, 2000), (CM12 – Caballero-Lopez and Moraal in J. Geophys. Res.117, A12103, 2012), (Mi13 – Mishev, Usoskin, and Kovaltsov in J. Geophys. Res.118, 2783, 2013), and (Ma16 – Mangeard et al. in J. Geophys. Res.121, 7435, 2016b). The results were compared with full reconstructions and direct measurements by the PAMELA instrument. While reconstructions based on Mi13 and CM12 yield functions are consistent with the measurements, those based on CD00 and Ma16 ones underestimate the fluence by a factor of 2 – 3. It is also shown that the often used power-law approximation of the high-energy tail of SEP spectrum does not properly describe the GLE spectrum in the NM-energy range. Therefore, the earlier estimates of GLE integral fluences need to be revised. [ABSTRACT FROM AUTHOR]

A usual event, called anisotropic cosmic-ray enhancement (ACRE), was observed as a small increase (≤5%) in the count rates of polar neutron monitors during 12 - 19 UT on 07 June 2015. The enhancement was highly anisotropic, as detected only by neutron monitors with asymptotic directions in the southwest quadrant in geocentric solar ecliptic (GSE) coordinates. The estimated rigidity of the corresponding particles is ≤1 GV. No associated detectable increase was found in the space-borne data from the Geostationary Operational Environmental Satellite (GOES), the Energetic and Relativistic Nuclei and Electron (ERNE) on board the Solar and Heliospheric Observatory (SOHO), or the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) instruments, whose sensitivity was not sufficient to detect the event. No solar energetic particles were present during that time interval. The heliospheric conditions were slightly disturbed, so that the interplanetary magnetic field strength gradually increased during the event, followed by an increase of the solar wind speed after the event. It is proposed that the event was related to a crossing of the boundary layer between two regions with different heliospheric parameters, with a strong gradient of low-rigidity (<1 GV) particles. It was apparently similar to another cosmic-ray enhancement (e.g., on 22 June 2015) that is thought to have been caused by the local anisotropy of Forbush decreases, with the difference that in our case, the interplanetary disturbance was not observed at Earth, but passed by southward for this event. [ABSTRACT FROM AUTHOR]

The “effective” rigidity of a neutron monitor for a ground-level enhancement (GLE) event is defined so that the event-integrated fluence of solar energetic protons with rigidity above it is directly proportional to the integral intensity of the GLE as recorded by a polar neutron monitor, within a wide range of solar energetic-proton spectra. This provides a direct way to assess the integral fluence of a GLE event based solely on neutron-monitor data. The effective rigidity/energy was found to be 1.13 - 1.42 GV (550 - 800 MeV). A small model-dependent, systematic uncertainty in the value of the effective rigidity is caused by uncertainties in the low-energy range of the neutron-monitor yield function, which requires more detailed computations of the latter. [ABSTRACT FROM AUTHOR]

A mysterious increase of radiocarbon 14 C ca. 775 AD in the Earth’s atmosphere has been recently found by Miyake et al. (Miyake, F., Nagaya, K., Masuda, K., Nakamura, T. [2012]. Nature, 486, 240). A possible source of this event has been discussed widely, the most likely being an extreme solar energetic particle event. A new exotic hypothesis has been presented recently by Liu et al. (Liu, Y. [2014]. Sci. Rep., 4, 3728) who proposed that the event was caused by a cometary impact on Earth bringing additional 14 C to the atmosphere. Here we calculated a realistic mass and size of such a comet to show that it would have been huge (≈100 km across and 10 17 – 10 20 g of mass) and would have produced a disastrous geological/biological impact on Earth. The absence of an evidence for such a dramatic event makes this hypothesis invalid. [ABSTRACT FROM AUTHOR]