Thunderstorms are some of nature’s most spectacular phenomena. Almost a century ago, it was suggested that the strong electric fields in thunderclouds could accelerate electrons in the atmosphere and induce nuclear reactions1. However, these processes have been difficult to confirm experimentally. On page 481, Enoto et al.2 report the first conclusive observational evidence for thunderstorm-produced nuclear reactions — with implications for our understanding of Earth’s atmosphere and isotopic composition.
The idea that thunderstorms can trigger nuclear reactions was proposed1 by the Scottish physicist and meteorologist Charles Wilson in 1925. However, the state of physics at the time meant that Wilson could not fully substantiate his idea. For instance, it is now known that neutrons are among the possible products of nuclear reactions and therefore that detecting these particles from a thunderstorm would provide evidence for Wilson’s proposal. But neutrons were not discovered3 until 1932.
Thunderstorms occur in the dense lowermost layers of the atmosphere. Electrons in these layers undergo frequent collisions with air molecules and are therefore subject to a strong drag force. Wilson’s proposal requires electrons that have sufficiently high initial energies to overcome this force. It is now known that cosmic rays irradiate the atmosphere and produce such electrons, which multiply in thunderclouds to form an avalanche of high-energy electrons4. However, in the mid-1920s, cosmic rays were extremely mysterious and thought to be of terrestrial origin5.
The first claimed detection of neutrons from a thunderstorm was reported6 in 1985. These observations were carried out in the Himalayas in a region that has extremely high thunderstorm activity (about 30 lightning strokes per day). Since the late 1990s, many other studies have also claimed statistically significant detections of thunderstorm-produced neutrons from all over the world7–10. However, the detectors could not distinguish neutrons from other particles such as electrons and γ-ray photons — all three would produce similar electric-current pulses in the detectors11.
It was initially thought that thunderstorm-induced neutrons were produced in a nuclear reaction in which two nuclei of the hydrogen isotope deuterium fuse in the plasma created by lightning to form a helium nucleus and a neutron. However, it was later shown that the physical conditions in such a plasma do not allow this reaction to occur12.
Instead, the avalanche of high-energy electrons produced in a thundercloud emits X-ray and γ-ray photons. Since the late 1980s, these photons have been detected on the ground, by aircraft flying inside thunderclouds, and by artificial satellites in near space (about 500 kilometres above Earth’s surface)13. The photons have energies of up to hundreds of megaelectronvolts (MeV).
High-energy electrons, and γ-rays that have energies larger than about 10 MeV, can knock out neutrons from atmospheric nitrogen-14 and oxygen-16 nuclei — by electrodisintegration in the case of electrons and photonuclear reactions in the case of γ-rays11,12. Although the ability of thunderstorms to produce neutrons through photonuclear reactions has been demonstrated using computer simulations11,13, direct experimental evidence has been absent.
Rather than focusing on the neutrons, Enoto and colleagues considered the other products of the photonuclear reactions involving nitrogen-14 and oxygen-16: namely, unstable nitrogen-13 and oxygen-15 isotopes (Fig. 1). These isotopes decay after a few minutes into stable carbon-13 and nitrogen-15 nuclei through the emission of a neutrino and a positron — the antiparticle of the electron. Finally, the positron annihilates with an electron of an atmospheric molecule to produce a pair of γ-rays.
Because both positrons and electrons have masses of 0.511 MeV (expressed in energy units), each emitted γ-ray has an energy of 0.511 MeV. Therefore, to confirm the existence of these photonuclear reactions, the authors simply needed to identify a line at this energy in the wide energy spectrum of all γ-rays.
To this end, Enoto et al. carried out ground-based observations of γ-ray emission from low winter thunderclouds above the coast of the Sea of Japan. On 6 February 2017, they detected an intense γ-ray flash that lasted for less than 1 millisecond, which they associated with a lightning stroke. After the initial γ-ray flash, the authors observed a prolonged γ-ray line at an energy of 0.511 MeV that lasted for about a minute (see Fig. 4 in the paper2). This line is a conclusive indication of electron–positron annihilation, and represents unequivocal evidence that photonuclear reactions can be triggered by thunderstorms.
Enoto and colleagues’ discovery is important because it unveils a previously unknown natural source of isotopes in the atmosphere, in addition to the irradiation of Earth by cosmic rays. These isotopes include nitrogen-15, carbon-13 and carbon-14, the last of which is widely used in the dating of archaeological artefacts and artworks. In fact, the contribution of thunderstorms to Earth’s carbon-14 abundance could be comparable in some regions to that of cosmic irradiation14. Future studies should check whether thunderstorms produce other isotopes (such as those of hydrogen, helium and beryllium).
Thunderstorm-induced nuclear reactions could occur in the atmospheres of other planets, such as Jupiter and Venus, and might therefore contribute to the isotopic composition of these atmospheres. However, determining the magnitude of this contribution will require detailed observations of γ-rays and neutrons from thunderstorms on these planets. Another implication of Enoto and colleagues’ discovery is that the neutrons are formed outside the plasma created by lightning. This suggests that these neutrons cannot provide information about the plasma, in contrast to expectations15.
Nature 551, 443-444 (2017)