Thunderstorms can generate powerful radiation

SAN FRANCISCO — If you’re close enough to a thunderstorm, you’ll hear lightning crackle and roar as it streaks across the sky. Yet some storms offer an even more stunning spectacle — a type of high-energy fireworks. Although invisible to the human eye, some cameras can spy these high-altitude light shows. The radiation can be bright enough to see from hundreds of miles away in outer space. And scientists have now uncovered new data on the source and frequency of this unusual phenomenon.

Thunderclouds can unleash many types of radiation. One type is X-rays. It’s the same type of radiation that doctors use to peer inside the body in search of a broken bone, say, or evidence of cancer. A related form, gamma radiation, can be hundreds of times or more energetic than X-rays. Most gamma rays come from solar flares, black holes or the explosive death of stars. But occasionally, earthly thunderstorms unleash a monster explosion of gamma rays.

The discovery of these terrestrial gamma-ray flashes — or TGFs — initially caught scientists by surprise. Earth-orbiting spacecraft accidentally spotted some of these flashes in the 1990s. At the time, those satellites had been looking for gamma rays from traditional sources outside the solar system.

It was “really strange that a thunderstorm — a big, puffy cloud — would be emitting gamma rays,” recalls Joseph Dwyer. It’s what physicists expected astrophysical objects like black holes or supernovas would emit, he told news reporters on December 15. Dwyer is a space physicist at the University of New Hampshire in Durham. But he and others presented new data on Earthly sources of these gamma rays here at the fall meeting of the American Geophysical Union.

Their findings shed light on the frequency and origins of these high-energy fireworks. And their new data may offer clues to more fundamental questions about storms. These include what happens deep inside thunderclouds and even what triggers lightning in the first place.

Garden-variety storms

TGFs flash roughly 1,100 times a day, according to recent estimates. Each lasts a mere fraction of a millisecond. But the energy released in that short flash is huge. It’s equal to the power needed to light more than 10 million 100-watt incandescent light bulbs, Dwyer notes.

Indeed, gamma-ray flashes are so big and spectacular that scientists had assumed only certain types of thunderstorms could trigger them. But at the meeting, researchers from the University of Alabama, in Huntsville, showed data that seem to disprove this.

They used a gamma-ray space telescope along with ground-based lightning detectors. With this duo, the researchers identified 24 coastal thunderstorms that produced TGFs. These storms had rumbled through Florida, Louisiana, Texas, Puerto Rico and Guam. With help from weather radars, the scientists studied features of each storm as it developed.

Surprisingly, a storm’s intensity didn’t seem to matter. All kinds of storms gave rise to TGFs. Even thunderstorms so weak that a weather forecaster wouldn’t look twice at them triggered gamma rays, team leader Themis Chronis reported. It appears that “the secret lies elsewhere,” he concludes. Chronis is an atmospheric scientist at the University of Alabama in Huntsville.

Another intriguing observation: Although TGFs originated from a variety of thunderstorms, all emerged from the highest region of the storm. They came from an altitude of 11 to 14 kilometers (7 to 9 miles).

Tiny TGFs?

Could TGFs occur at lower altitudes but have escaped detection by space telescopes? David Smith and his co-workers at the University of California, Santa Cruz, looked into that.

Small gamma flashes are individually too dim for space telescopes to record. Still, the scientists reasoned, an analysis that tracked enough events should be able to measure their collective signal.

So the team used this approach to estimate the average number of TGFs triggered per lightning flash. The analyses now suggest that “less than 1 percent of lightning strikes produce TGFs, even the ones too small to see individually,” Smith reports.

“When all was said and done, I was a little surprised we saw so few of them,” he told Science News for Students.

Considering that lightning strikes about four million times each day, though, even if only 1 in 100 produced gamma rays, “that’s still a lot of TGFs,” Dwyer notes. Still, there’s still much that scientists don’t understand about these outbursts and what generates them.

To study them properly, “you want to be where the action is,” Dwyer says. “You want to detect them near the thunderstorm — or even better, in the storm.”

Flying through thunderstorms

And that’s exactly what another research team did earlier this spring.

“We took an aircraft — a real one — equipped it with many different sensors and sent it right into a thunderstorm,” Pavlo Kochkin reported at the meeting. Kochkin just earned his PhD in electrical engineering at Eindhoven University of Technology in the Netherlands.

Working with the Dutch national aerospace laboratory, Kochkin and his coworkers developed a special monitoring system. They installed it on an Airbus A380. That’s the world’s largest passenger plane. The system detected lightning strikes. It also assessed the strikes’ impacts on the aircraft as it rumbled through the skies. Other detectors and sensors aboard this plane measured X-rays and gamma rays.

The plane’s daring crew endured more than five hours of severe turbulence on one flight as it steered the aircraft from southern France to northern Italy. Twenty lightning bolts struck the plane in this first-of-its-kind experiment. Another 42 bolts struck during four additional days of stormy travel.

The Airbus didn’t catch any TGFs. However, this study was the first to measure X-rays in thunderclouds — “in the heart of where they originate,” Kochkin says.

Had a TGF been detected during the Airbus study, seeing it that close would have helped settle a longstanding debate over how the flashes arise, Smith says. Some scientists have proposed that TGFs are just a souped-up version of the X-rays generated by lightning. Another theory says that TGFs may have nothing to do with a storm’s lightning. So whether TGFs relate to a storm’s X-rays is still a matter of debate.

“We don’t understand very well how thunderstorms and lightning work,” Dwyer says. That’s why studying TGFs “is a way of probing what’s happening inside the thunderstorm and what’s going on with lightning.”

Power Words

astrophysics   An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

atmosphere   The envelope of gases surrounding Earth or another planet.

black hole  A region of space having a gravitational field so intense that no matter nor radiation (including light) can escape.

cancer  Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.

gamma rays  High-energy radiation often generated by processes in and around exploding stars. Gamma rays are the most energetic form of light.

geophysics    The study of matter and energy on Earth and how they interact.

incandescent light     The old-style lighting technology that relied on a glass bulb. Electricity passing through the bulb heated a thread-like tungsten filament, making it glow white hot. Thomas Edison commercialized this technology in 1879. By that time, the technology was already about 50 years old. Incandescent lights used to illuminate everything from tiny flashlights to whole rooms. Many governments have moved to ban these bulbs because they waste so much of their energy as heat.

lightning  A flash of light triggered by the discharge of electricity that occurs between clouds or between a cloud and something on Earth’s surface. The electrical current can cause a flash heating of the air, which can create a sharp crack of thunder.

millisecond   Thousandth of a second.

PhD  (also known as a doctorate) A type of advanced degree offered by universities — typically after five or six years of study — for work that creates new knowledge. People qualify to begin this type of graduate study only after having first completed a college degree (a program that typically takes four years of study).

physicist  A scientist who studies the nature and properties of matter and energy.

radiation  Energy, emitted by a source, that travels through space in waves or as moving subatomic particles. Examples include visible light, infrared energy and microwaves.  

solar system  The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star  Thebasic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

supernova  (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.

telescope    Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

terrestrial    An adjective for things of or relating to Earth.

terrestrial gamma-ray flashA brief burst of gamma rays, a high-energy type of radiation, that can be produced in Earth’s atmosphere (usually due to thunderstorms). When equipped with the right cameras or radiation detectors, satellites in low-Earth orbit can record these fleeting flashes.

X-ray     A type of radiation analogous to gamma rays, but of somewhat lower energy.