Cosmic Ray Trigger
In the new study, the researchers provide additional evidence that cosmic rays trigger gamma ray generation by releasing a "seed electron" from an atom of air.
This begins "runaway breakdown"—a theoretical process in which the liberated electron ionizes nearby air molecules. These molecules become accelerated in the very high electric fields inside a thundercloud, which can reach up to ten million volts in strength.
A group of fast electrons is then formed, which can emit gamma rays as they are gradually slowed down by contact with surrounding air molecules.
"Everybody agrees that in some way this basic process of runaway breakdown is involved, but whether it has to be triggered by a cosmic ray [is uncertain]," said Cummer, who did not participate in the new Japanese research.
"It's appealing on some levels, but others have suggested ways that don't necessarily [require] a cosmic ray coming through."
Cosmic rays are constantly raining down onto Earth, so it is difficult to isolate their impact on any single event, he pointed out.
Lightning Link
Still, studying thundercloud gamma rays could eventually unlock some secrets of Earth's weather, the Japanese researchers say.
"One significance of our result is the prospect that we can use these gamma rays as a good probe to understand how the strong electric field develops in thunderclouds and how lightning discharges occur," study lead author Enoto said.
The electric fields in thunderstorms appear to be too weak to form lightning, so scientists have been puzzled by how the bolts form. Cosmic rays have also been suggested as a trigger for the flashes.
The researchers are now looking into the exact connection—if any—between runaway breakdown and lightning, especially if such a link applies only to certain kinds of lightning or in certain areas of storms.
Thundercloud gamma rays could also shed light on more exotic cosmic processes like solar flares, pulsars, and jets from black holes, Enoto added.
"Our result shows that thunderclouds may be a natural particle accelerator," he said. "Thunderclouds could provide a nearby hidden prototype for other energetic cosmic accelerators."
No Radiation Danger
Powerful gamma rays can be dangerous in some cases, since the radiation can reach energies a hundred to a thousand times higher than x-rays used in medical devices, Enoto said.
But storm clouds are unlikely to produce enough radiation to cause any worry.
"The total number of detected gamma rays is so small that the overall radiation dose is meager—perhaps seven orders of magnitude less than that of a single medical radiography shot," he explained.
"So this event, we presume, has little impact on Earth or human bodies."
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