Gamma-ray Emission Predictive of Photonuclear Reactions in Lightning

Lightning sometimes triggers intense gamma-ray flashes, and even photonuclear reactions. However, the condition where the flashes and reactions occur is still unclear. A minute-lasting gamma-ray emission preceding lightning could initiate a lighting bolt accompanied by the flash and reactions.
Gamma-ray Emission Predictive of Photonuclear Reactions in Lightning

In 1990s, an in-orbit satellite serendipitously discovered sub-millisecond gamma-ray flashes from the Earth atmosphere. They are now called “terrestrial gamma-ray flashes (TGFs)”, and known to coincide with lightning discharges. Energy of the gamma-ray photons is enough high (>10 MeV) that atmospheric photonuclear reactions (such as 14N + gamma -> 13N + neutron)  is predicted. In fact, we demonstrated that the nuclear reactions had taken place during a winter thunderstorm in Japan. 

Besides TGFs, thunderclouds are also known as a powerful gamma-ray generator: electrons can be accelerated to a relativistic energy in strong electrostatic fields inside thunderclouds, and emit bremsstrahlung. These emissions, called ”gamma-ray glows”, have been observed by airborne and mountain-top experiments, and even at sea level when thunderclouds get closer to the ground during winter seasons.Although TGFs and gamma-ray glows have been observed separately, the relation between them has yet to be understood.

On January 9th, 2018, two gamma-ray monitors deployed in Kanazawa, Japan, detected a gamma-ray glow during a winter thunderstorm for ~60 sec. The glow moved with ambient wind, and then abruptly ceased with a lightning discharge. Simultaneously, the monitors detected a TGF and subsequent photonuclear reactions. By our low-frequency-radio lightning mapping system, an intensive radio pulse accompanied by the TGF was revealed to be temporally (<10 microseconds) and spatially (<0.5 km) associated with the gamma-ray glow at its disappearance. This is the first direct evidence that the TGF and the glow was related with each other.

From our observational results, we speculate that the gamma-ray glow could facilitate the initiation of the TGF and even the lightning flash. The glow-producing region should have higher electric fields than surrounding regions. Furthermore, the TGF seems to be closely related to the lightning initiation since the TGF-associated radio pulse was emitted in the very beginning of the lightning flash. To verify this speculation, we need further observations to answer whether simultaneous occurrence of TGFs and glows is rare or frequent. 

Our research activity is supported by a new gamma-ray mapping system. We have developed brand-new portable gamma-ray monitors in 2015, and deployed them to multiple places in Kanazawa, where winter thunderstorms frequently take place. The portability and ease of handling allow us to increase the number of detectors, and hence gave us chances to observe unexpected phenomena.

The portable detectors have been deployed in observation sites provided by local high schools, universities and private companies in Kanazawa. At high schools, we give special classes for students to advertise our project. However, this is just a side of our outreach activities: we hope citizens familiar with the unusual thunderstorms in winter will get involved in our project as an open science framework. We have developed another gamma-ray monitoring system much easier to deploy, and will distribute it to citizen supporters to expand our gamma-ray mapping system. A promotion video which we have recently created will help us expand the network and share a fun of high-energy phenomena in lightning with citizens.