Speaker
Description
Instrumental observations of extreme solar eruptions are short, restricting their linkages to paleoevents (e.g. Miyake events) derived from cosmogenic $^{14}$C, $^{36}$Cl and $^{10}$Be isotopes of ice cores and tree rings. This constrains estimates of their long-term frequency and probability. The strongest solar proton event (SPE) recorded by ground-level enhancement occurred on 23 February 1956 (GLE #5), during the maximum phase of the strongest solar cycle #19 (1954–1964). Through recent reanalysis of historical data Usoskin et al. (2020) confirmed that SPE1956 ranks among the hardest-energy SPEs ever recorded. The event produced an extraordinary peak in neutron-monitor count rate as 5117–5276% above the Galactic Cosmic Ray (GCR) background sustained for nearly one hour. Despite this intensity, simulated annual production rates of $^{10}$Be and $^{14}$C of ice cores show only ≈5% increase above the GCR background. For comparison, the corresponding increase in the 775 CE Miyake event is about 210-270% (Mekhaldi et al. 2015).
The SPE1956 event occurred during the nuclear testing era, when natural concentration of cosmogenic isotopes, except 10Be, has been severely affected by the thermal neutrons ejected by aboveground nuclear explosions in the late 1950s and early 1960s. These neutrons reacted with atmospheric nitrogen producing anthropogenic $^{14}$C (bomb $^{14}$C). Because bomb-produced $^{14}$C was injected primarily in the Northern Hemisphere, interhemispheric transport and gradient delayed the $^{14}$C peak in the Southern Hemisphere (SH) by 1–2 years. We analyzed tree-ring radiocarbon from the SH Zone 1-2 around the time of SPE1956 to identify the impact of $^{14}$C bomb emissions on the SPE1956 spike signature in tree rings. Annually resolved Δ$^{14}$C series from two tree species: Lenga beech and Chile pine were developed for the interval 1952-1961 from the Tierra del Fuego National Park, Patagonia. The growing season for both trees is about five months, from October to March. The beech series shows a significantly positive offset of 16.6 ‰+0.9‰ (total χ² $_{\nu=9}$ =12.47 or reduced χ² =1.39) relatively to the pine series. According to monthly atmospheric Δ$^{14}$C data for the SH Zone1-2, the increase of bomb $^{14}$C begins gradually in June 1955, only 7 months prior to the SPE1956 event. It then increases rapidly and stabilizes briefly at ~200 ‰ in 1960-1962 before soaring to 680 ‰ by 1969. The tree-ring Δ$^{14}$C changes over 15‰ from 1955 to 1956 closely following the structure of SH zone 1-2 atmospheric Δ$^{14}$C record. We discuss the 14C production rate derived from $^{14}$C tree-ring signature of the SPE1956 event and the seven-month missed opportunity for integrating instrumental observations with the proxy records of GCR intensity.
References:
Usoskin et al. 2020. Revisited reference solar proton event of 23 February 1956: Assessment of the cosmogenic-isotope method sensitivity to extreme solar events. JGR 125, e2020JA027921.
Mekhaldi et al. 2015. Multiradionuclide evidence for the solar origin of the cosmic-ray events of AD 774/5 and 993/4. Nat Commun 6, 8611.