Occurrence of extreme solar particle events: Assessment from historical proxy data

The probability of occurrence of extreme solar particle events (SPEs) with the fluence of (>30 MeV) protons F30>10^{10} cm^{-2} is evaluated based on data of cosmogenic isotopes 14C and 10Be in terrestrial archives centennial-millennial time scales. Four potential candidates with F30=(1-1.5)x10^{10} cm^{-2} and no events with F30>2x10^{10} cm^{-2} are identified since 1400 AD in the annually resolved 10Be data. A strong SPE related to the Carrington flare of 1859 AD is not supported by the data. For the last 11400 years, 19 SPE candidates with F30=(1-3)x10^{10} cm^{-2} are found and clearly no event with F30>5x10^{10} cm^{-2} (50-fold the SPE of 23-Feb-1956) occurring. This values serve as an observational upper limit for the strength of SPE on the time scale of tens of millennia. Two events, ca. 780 and 1460 AD, appear in different data series making them strong candidates to extreme SPEs. We built a distribution of the occurrence probability of extreme SPEs, providing a new strict observational constraint. Practical limits can be set as F30~1x, 2-3x, and 5x10^{10} cm^{-2} for the occurrence probability ~10^{-2}, 10^{-3} and 10^{-4} year^{-1}, respectively. Because of uncertainties, our results should be interpreted as a conservative upper limit of the SPE occurrence near Earth. The mean SEP flux is evaluated as ~40 (cm2 sec)^{-1} in agreement with estimates from the lunar rocks. On average, extreme SPEs contribute about 10% to the total SEP fluence.

💡 Research Summary

The authors investigate the frequency and magnitude of extreme solar particle events (SPEs) over centennial to millennial time scales by exploiting cosmogenic isotope records—specifically radiocarbon (^14C) and beryllium‑10 (^10Be)—preserved in terrestrial archives such as tree rings, ice cores, and sediments. Their primary metric is the fluence of >30 MeV protons (F30), with events exceeding 10^10 cm⁻² classified as “extreme.”

Using annually resolved ^10Be data, they examine the period from 1400 AD to the present and identify four candidate events with estimated F30 values in the range (1–1.5) × 10^10 cm⁻². Notably, the historically celebrated Carrington flare of 1859, often invoked as a benchmark extreme event, leaves no discernible signature in the isotope record, suggesting that its particle output was far lower than previously assumed or that the event did not produce a globally detectable SPE.

Extending the analysis to the last 11 400 years, the authors combine long‑term ^14C and ^10Be series to search for larger fluences. They find 19 candidates with F30 between 1 × 10^10 and 3 × 10^10 cm⁻², but no evidence for events exceeding 5 × 10^10 cm⁻²—roughly fifty times the fluence of the well‑documented 23 February 1956 SPE. This absence provides a robust upper limit on the size of SPEs that could have occurred on time scales of tens of millennia.

Statistical modeling of the occurrence probability yields a cumulative distribution that can be approximated by a power‑law tail. The authors propose practical thresholds: an event with F30 ≈ 1 × 10^10 cm⁻² occurs with a probability of ~10⁻² yr⁻¹, events with F30 ≈ 2–3 × 10^10 cm⁻² with ~10⁻³ yr⁻¹, and events with F30 ≈ 5 × 10^10 cm⁻² with ~10⁻⁴ yr⁻¹. They stress that, because of uncertainties in isotope production, transport, and deposition, these figures should be regarded as conservative upper bounds.

The mean solar energetic particle (SEP) flux derived from the isotope data is about 40 (cm² s)⁻¹, in agreement with independent estimates from lunar rock exposure ages. Integrating over the entire SEP fluence spectrum, the authors find that extreme SPEs contribute roughly 10 % of the total fluence, implying that the bulk of the radiation dose at Earth’s orbit is supplied by more frequent, lower‑intensity events.

Two particularly strong candidates, dated to ~780 AD and ~1460 AD, appear in multiple independent isotope series, strengthening their case as genuine extreme SPEs. These events provide valuable calibration points for future modeling of extreme space‑weather scenarios.

Overall, the study demonstrates that cosmogenic isotope archives can set stringent observational constraints on the upper end of SPE fluence distributions. The derived limits have direct implications for spacecraft design, astronaut safety, and the assessment of long‑term solar influence on Earth’s atmosphere and climate. By establishing that SPEs with fluences greater than 5 × 10^10 cm⁻² are exceedingly rare (probability <10⁻⁴ yr⁻¹), the work narrows the range of plausible worst‑case space‑weather events that must be considered in risk assessments.