Constraints on the Parameters of Solar Superflares Based on Cosmogenic Radiocarbon Data in the Lunar Regolith

Samples with a short-term (less than a year) increase in the content of the radioactive isotope 14C were recently discovered in tree rings, in four cases accompanied by concentration growth of 10Be and 36Cl in other natural archives. Most publications suggest that this increase is due to a sharp increase in the flux of solar cosmic rays (SCR) at the boundary of the Earth’s atmosphere caused by solar superflares. Other reasons may be connected with the flux rise of the galactic cosmic rays (GCR) as the Solar System passes through a dense interstellar cloud, or a galactic gamma-ray burst. To reconcile the amount of 14C with cosmogenic isotopes 10Be and 36Cl formed in the atmosphere, it is necessary to assume that the proton spectra in such superflares should be harder than most modern experimentally recorded ones. Measurements of the 14C content in lunar regolith cores returned by the Apollo 15 expedition showed a significant drop in radiocarbon concentration to a depth of 5 g/cm2, followed by an increase to maximum values at about 50 g/cm2 then a decrease. At shallow depths, the contribution from low-energy SCRs predominates, and at large depths, the contribution from high-energy GCRs prevails. Analysis of the depth profile of the 14Cconcentration makes it possible to establish SCR fluxes and spectra over several radiocarbon half-lives (10 000–20 000 years) and highlight the possible contribution of hypothetical superflares. Our analysis shows that the hypothesis of solar superflares worsens the agreement with the observed depth variations of 14C in the lunar regolith.