Thunderstorm Gamma Rays Linked to Lightning

The formation of gamma rays—the most energetic form of light—in thunderclouds may be linked to lightning production, a new study shows.

Gamma rays are typically produced by cosmic cataclysms like supernovae, but terrestrial thunderstorms can also energize particles enough to create the powerful rays.

Scientists first spotted gamma rays in thunderstorms in the early 1990s. The Compton Gamma Ray Observatory unexpectedly detected radiation originating from the ground while peering at distant supernovae.

"The fact that they are [even] created in something as garden variety as thunderstorms is a surprise," said Steven Cummer, an electrical engineer and lightning researcher at Duke University in Durham, North Carolina.

Lightning Mystery

How thunderclouds produce the rays has not been explained. Neither has the nature of their apparent link to lightning formation, about which many long-standing mysteries remain.

But a new study led by Teruaki Enoto of the University of Tokyo and Harufumi Tsuchiya of RIKEN, Japan's Institute of Physical and Chemical Research, has provided the best look yet at the phenomena.

The team trained an array of gamma ray detectors on two powerful low-pressure air masses that collided over the Sea of Japan on January 6, 2007.

The researchers found that gamma rays were produced some 70 seconds before a lightning strike. They also determined that gamma bursts, which had been previously measured to last less than a second, could occur for almost a minute.

The findings suggest that whatever triggers gamma rays might also be involved in creating lighting—and that their origin might be tied to powerful particles known as cosmic rays that continually rain down from space.

The work will appear in an upcoming issue of the journal Physical Review Letters.

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."