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NASA Goddard - Fermi Helps Study Gamma-Ray Thunderstorms

Even the weakest of thunderstorms found producing the most powerful light flashes in the Universe

Scott Sutherland
Meteorologist/Science Writer

Tuesday, December 23, 2014, 12:57 PM - Each and every day, hundreds of thunderstorms rumble around the world, and a unique pairing of space and ground-based observations has shown that even the weakest of these storms can produce flashes of the highest-energy light known - gamma rays.

In the entire spectrum of light, from the weakest of radio waves, up through the parts of the spectrum we can see with our own eyes, to ultraviolet light, x-rays and beyond, gamma rays rank at the top of that spectrum. As the highest-energy bursts of radiation of all, the weakest of these are roughly 100 times more powerful than a standard medical x-ray, and thus they can penetrate deep into living tissues, possibly damaging DNA, causing cell-death and increasing the risk of cancer.

Credit: NASA/Swift/Stefan Immler, et al.

In nature, they're emitted during radioactive decay of elements, but they're also associated with some of the most energetic events in the universe, such as when a star goes supernova, or when a black hole consumes its latest meal, or during antimatter annihilations. However, in recent years, scientists have discovered that gamma rays are being emitted, naturally, by some of the most common events we know of - lightning flashes.

This was first seen in 1992, by NASA's Compton Gamma-Ray Observatory, and while they were originally thought to result from events like Sprites - which are electrical discharges high above storm clouds - scientists have ruled those out, and have been working to solve the mystery of these 'terrestrial gamma-ray flashes' (TGFs) ever since.

According to NASA, TGFs are thought to occur when a lightning bolt flashing inside the cloud disrupts the powerful electric field that forms at the top of the stormcloud. This causes a shower of fast-moving electrons to blast upwards, where they interact with air molecules and emit gamma rays in the process. Since this occurs so high up in the atmosphere, the gamma rays stand a good chance of escaping up into space, where they can be detected by orbiting telescopes like Compton, and NASA's newer Fermi Gamma-ray Space Telescope.

Now, using a combination of ground-based detection from lightning sensors and radar, along with observations from Fermi, scientists have devised a system where they can pinpoint the location of a gamma-ray emitting lightning flash to within 10 kilometres, anywhere on the planet. What they found from their study is that it doesn't take the most powerful thunderstorms to produce these high-energy flashes of light.

"Remarkably, we have found that any thunderstorm can produce gamma rays, even those that appear to be so weak a meteorologist wouldn't look twice at them," lead researcher Themis Chronis, from the Earth System Science Center at the University of Alabama in Huntsville (UAH), said in a NASA press release.

Three examples of TGF-producing storms - Tropical Storm Andrea and a summer thunderstorm, both in the Gulf of Mexico, and a winter thunderstorm near Havana, Cuba. Credit: NASA's Goddard Space Flight Center/T. Chronis and M. Briggs, UAH

Currently, Fermi is picking up one TGF for every 2,000 or so lightning flashes that are seen in a storm, which comes out to around 1,100 occurring every day. However, it's possible that far more of these flashes are emitting gamma rays, but they're simply not powerful enough for Fermi to see them by the time they reach space.

"We suspect this isn't the full story," said study co-researcher Michael Briggs, who works on team that manages Fermi's Gamma-ray Burst Monitor (GBM). "Lightning often occurs at lower altitudes and TGFs probably do too, but traveling the greater depth of air weakens the gamma rays so much the GBM can't detect them."

No Real Threat to Air Travel

Exposure to gamma rays can pose a danger, of course. However, according to Joseph Dwyer, one of the researchers from the Institute for the Study of Earth, Oceans, and Space at the University of New Hampshire, the threat to airline passengers from TGFs is very low.

"People don’t need to worry about TGFs when they get on an airplane," said he said during the Q&A session after he, Chronis and fellow researcher Pavlo Kochkin, from the Eindhoven University of Technology in the Netherlands presented their research at the 2014 American Geophysical Union Conference (click here to watch).

The main reason for this is that commercial flights avoid thunderstorms, simply due to the dangers posed by air turbulence. Thus, the aircraft wouldn't be in the direct path of the gamma rays. The threat, Dwyer said, is mainly to researchers and pilots who deliberately fly into storms to study these events. In these cases, they are putting themselves in "exactly the wrong place at exactly the wrong time," to be directly hit by the TGF. In that instant, according to the latest research, they would likely receive the equivalent radiation exposure of a full body CT scan.

"This is something that is not necessarily great, but it's not going to kill you," he added.

While this discovery goes a long way towards furthering our understanding of terrestrial gamma ray flashes, the mystery surrounding these events is far from solved. According to NASA, the next stages of the research are to track exactly where the flashes happen in the storm (possibly where weaker updrafts are noted, due to the storm becoming less organized), and relating TGFs to the life cycle of thunderstorms.

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