Fermi brings deeper focus to thunderstorm gamma-rays

December 15, 2014 by Francis Reddy, NASA's Goddard Space Flight Center
Merging data on high-energy bursts seen on Earth by NASA's Fermi Gamma-ray Space Telescope with data from ground-based radar and lightning detectors, scientists have completed the most detailed analysis to date of the types of thunderstorms producing terrestrial gamma-ray flashes, or TGFs. Credit: NASA SVS

Each day, thunderstorms around the world produce about a thousand quick bursts of gamma rays, some of the highest-energy light naturally found on Earth. By merging records of events seen by NASA's Fermi Gamma-ray Space Telescope with data from ground-based radar and lightning detectors, scientists have completed the most detailed analysis to date of the types of thunderstorms involved.

"Remarkably, we have found that any thunderstorm can produce , even those that appear to be so weak a meteorologist wouldn't look twice at them," said Themis Chronis, who led the research at the University of Alabama in Huntsville (UAH).

The outbursts, called terrestrial gamma-ray flashes (TGFs), were discovered in 1992 by NASA's Compton Gamma-Ray Observatory, which operated until 2000. TGFs occur unpredictably and fleetingly, with durations less than a thousandth of a second, and remain poorly understood.

In late 2012, Fermi scientists employed new techniques that effectively upgraded the satellite's Gamma-ray Burst Monitor (GBM), making it 10 times more sensitive to TGFs and allowing it to record weak events that were overlooked before.

"As a result of our enhanced discovery rate, we were able to show that most TGFs also generate strong bursts of radio waves like those produced by lightning," said Michael Briggs, assistant director of the Center for Space Plasma and Aeronomic Research at UAH and a member of the GBM team.

Previously, TGF positions could be roughly estimated based on Fermi's location at the time of the event. The GBM can detect flashes within about 500 miles (800 kilometers), but this is too imprecise to definitively associate a TGF with a specific storm.

New research merging Fermi data with information from ground-based radar and lightning networks shows that terrestrial gamma-ray flashes arise from an unexpected diversity of storms and may be more common than currently thought. Credit: NASA's Goddard Space Flight Center

Ground-based lightning networks use radio data to pin down strike locations. The discovery of similar signals from TGFs meant that scientists could use the networks to determine which storms produce gamma-ray flashes, opening the door to a deeper understanding of the meteorology powering these extreme events.

Chronis, Briggs and their colleagues sifted through 2,279 TGFs detected by Fermi's GBM to derive a sample of nearly 900 events accurately located by the Total Lightning Network operated by Earth Networks in Germantown, Maryland, and the World Wide Lightning Location Network, a research collaboration run by the University of Washington in Seattle. These systems can pinpoint the location of lightning discharges—and the corresponding signals from TGFs—to within 6 miles (10 km) anywhere on the globe.

From this group, the team identified 24 TGFs that occurred within areas covered by Next Generation Weather Radar (NEXRAD) sites in Florida, Louisiana, Texas, Puerto Rico and Guam. For eight of these storms, the researchers obtained additional information about atmospheric conditions through sensor data collected by the Department of Atmospheric Science at the University of Wyoming in Laramie.

"All told, this study is our best look yet at TGF-producing storms, and it shows convincingly that storm intensity is not the key," said Chronis, who will present the findings Wed., Dec. 17, in an invited talk at the American Geophysical Union meeting in San Francisco. A paper describing the research has been submitted to the Bulletin of the American Meteorological Society.

Scientists suspect that TGFs arise from strong electric fields near the tops of . Updrafts and downdrafts within the storms force rain, snow and ice to collide and acquire electrical charge. Usually, positive charge accumulates in the upper part of the storm and negative charge accumulates below. When the storm's electrical field becomes so strong it breaks down the insulating properties of air, a lightning discharge occurs.

Under the right conditions, the upper part of an intracloud lightning bolt disrupts the storm's electric field in such a way that an avalanche of electrons surges upward at high speed. When these fast-moving electrons are deflected by air molecules, they emit gamma rays and create a TGF.

About 75 percent of stays within the storm, and about 2,000 of these intracloud discharges occur for each TGF Fermi detects.

The new study confirms previous findings indicating that TGFs tend to occur near the highest parts of a thunderstorm, between about 7 and 9 miles (11 to 14 kilometers) high. "We suspect this isn't the full story," explained Briggs. "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."

Based on current Fermi statistics, scientists estimate that some 1,100 TGFs occur each day, but the number may be much higher if low-altitude flashes are being missed.

While it is too early to draw conclusions, Chronis notes, there are a few hints that gamma-ray flashes may prefer storm areas where updrafts have weakened and the aging storm has become less organized. "Part of our ongoing research is to track these storms with NEXRAD radar to determine if we can relate TGFs to the thunderstorm life cycle," he said.

Explore further: Fermi improves its vision for thunderstorm Gamma-ray flashes

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not rated yet Dec 15, 2014
Great article. One point, however: The radiation flashes are actually X-rays, not gamma rays as indicated in the article. More specifically they are X-rays in the form of Bremsstrahlung (German for 'braking radiation"; which, in this case, occurs when high energy electrons are deflected by atoms. See Wikipedia: http://en.wikiped...trahlung ) I am surprised that the academic authors are unaware of this distinction.
5 / 5 (6) Dec 15, 2014
"The radiation flashes are actually X-rays, not gamma rays as indicated in the article. More specifically they are X-rays in the form of Bremsstrahlung."

Bremsstrahlung can easily produce gamma rays. Electron-electron Bremsstrahlung in air (not metals) is a well-known source of gamma rays:

"Thunderstorms emit terrestrial gamma-ray flashes with photon energies of up to tens of MeV and electron-positron beams that are created by photons with energies above 1.022 MeV. These photons are produced through the bremsstrahlung process when energetic electrons collide with air molecules." http://iopscience...5/252001

I'm confident the authors of the paper know what they are talking about.
5 / 5 (4) Dec 16, 2014
Great article.

Yes, yes it is. I think "terrestrial gamma-ray flash" is a scientific term, not that it isn't all academic.

And another outstanding video from Goddard. If you liked that half as much as I did, you'll thoroughly enjoy the new Stormscapes2
2.1 / 5 (7) Dec 16, 2014
Forget the Wikipedia stuff, the energy produced ranges all the way into gamma. Gamma-spectroscopy is part of my job description. The only reason anybody links to Wikipedia when posting comments on this site is because that person has never been personally engaged in energy science. Quoting Wikipedia is the surest way to detract from your credibility.
Uncle Ira
5 / 5 (5) Dec 16, 2014
Quoting Wikipedia is the surest way to detract from your credibility.

No, it is the 2nd surest way. The surest way is to tell everybody what your job is and that saying that makes peoples believe you are the expert. Bennie-Skippy, everybody here knows you never learned those things you keep saying.

Watch, I will show you what I mean,,,,,,

Differential equations, I know all about them and you never seen one you work out. Trigonometry, you are clueless but I study him for years. Semi-circular universes, you would have to read Einstein-Skippy's book like I have to understand that. I do the spectroscopy everyday and you don't know what it is. E =mc2, that's scientific proof I know about this stuff. Thermodynamics, I learned that when I was three years old.

So since I just proved to you that I am the science expert, you can sit down and shut up. And don't forget to put your silly looking pointy cap back on.
not rated yet Dec 20, 2014
The surest way: be snarky.
1 / 5 (1) Dec 26, 2014
@Benni, here's a simple challenge re your claim

You've so often blurted claim re others NOT able to solve Differential Equation (DE), implying U easily can :-)
Strange Y U provoke & as thermodynamics politely advised U its not as easy as U imagine..!

So Benni, here's a simple challenge for U & Water_Prophet who claimed to graduated as a Physical Chemist (PC) :-)

1. Total Solar Insolation (TSI) has more short wave (SW) energy than long wave (LW) radiance

2. Earth converts SW to LW (SW emission is negligible)
3. LW to space interfered with by absorption/re-radiation of GHG (esp CO2)

Here Benni, re your claim on DEs, offer an estimate of LW radiation resistivity due to CO2 & for Water_Prophet suggest Y it's so much more than the thermal energy contributed by burning fossil fuels ~230,000L petrol/sec (0.1% of TSI) ?

Or google scholar ;-)

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