A thundercloud over the city of Kanazawa. The glow of gamma rays ended abruptly around position B. Lightning struck between positions A and B. Image: © 2019 Yuuki Wada
Yuuki Wada, a university student at the University of Tokyo, with Japanese colleagues, discovers a connection between lightning and two types of gamma rays in storm clouds. Research suggests that under certain conditions, the weak gamma rays of thunderclouds may precede lightning and their gamma-ray bursts.
In the city of Kanazawa, Ishikawa Prefecture, central Japan, Wada and colleagues work with local schools and businesses to install radiation monitors on buildings. These radiation monitors are not there due to some concern about local radiation levels, though. They form a network, whose purpose is to detect radiation from the sky. It may surprise someone, but it has been known for about 30 years that thunderstorms can carry gamma-ray activity.
"Forever, people saw lightning and heard thunder. These were the ways we could experience this power of nature," Wada said. "With the discovery of electromagnetism, scientists have learned to see lightning with radio receivers. But now we can observe lightning in gamma rays – ionizing radiation. It's like having four eyes to study the phenomenon."
A thundercloud can carry over 1 billion volts of electricity. Image: © 2019 Yuuki Wada
There are two known types of gamma-ray phenomena associated with thunderclouds: gamma-ray glows, weak emissions lasting about a minute and short-lived terrestrial gamma-ray bursts (TGF), which occur as lightning and are much more intense than those with gamma rays. Both occur in regions of storm clouds sandwiched with layers of variable charge. The charged regions accelerate the electrons almost at the speed of light. At these speeds, defined relativistic, the electrons that move very close to the nuclei of the nitrogen atoms in the air slow down a little and emit a gamma ray detector. This is called bremsstrahlung radiation.
"During a winter storm in Kanazawa, our monitors detected a simultaneous TGF and lightning. This is quite common, but interestingly we also saw a gamma-ray glow in the same area at the same time," Wada continued. "Moreover, the glow suddenly disappeared when the lightning struck. We can say conclusively that the events are intimately connected and this is the first time that this connection has been observed."
The mechanism underlying the lightning discharge is highly sought after and this research may offer previously unknown insights. Wada and the team intend to investigate further to explore the possibility that gamma-ray glows not only precede lightning, but could actually cause them. The radiation levels of gamma-ray flashes are quite low, about a tenth of the level that can be received from a typical medical radiograph.
"Our discovery marks a milestone in lightning research and we will soon double our number of radiation sensors from 23 to about 40 or 50. With more sensors, we could dramatically improve predictive models," Wada explained. "It is difficult to say now, but with sufficient sensor data, we may be able to predict lightning within about 10 minutes of their occurrence and within about 2 kilometers of where they occur. I am excited to be part of this ongoing research. "
Probably further investigations will still be conducted in Kanazawa, as the area has rare and ideal weather conditions for this type of work. Most observations of radiation in storms come from air or mountain stations because storm clouds are generally very high. But the winter storms in Kanazawa bring storm clouds surprisingly close to the ground, ideal for studying with low-cost portable monitors developed by the research team.
The researchers created these unique portable radiation monitors in part with technology derived from space satellite observatories designed for astrophysics experiments. This is appropriate as the data of this type of research could be useful for those seeking astrophysics and in particular solar physics in the context of particle acceleration. But there is also a simpler branch.
"The paleontologists who study life over the last 50,000 years use a technique called carbon 14 dating to determine the age of a sample. The technique is based on the knowledge of the levels of two types of carbon, carbon-12 and carbon -14 ", said Wada. "It is commonly thought that carbon-14 is created by cosmic rays at an approximately constant speed, hence the predictive power of the technique. But there is a suggestion that thunderstorms can alter the relationship between carbon-12 and carbon- 14, which may slightly modify the accuracy or calibration required for dating to the carbon-14 work. "
Wada and colleagues will continue to unravel the mysteries of lightning, one of nature's most captivating and iconic phenomena. An upcoming collaboration project based in France will launch a dedicated satellite for lightning-fast observations around the world from space.
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