Are TGFs Hazardous to Air Travelers?

Feb 11, 2010 by Dauna Coulter
Lightning might not be the only reason to avoid thunderstorms. TGFs sometimes come blasting out of these clouds, too. Image credit: NOAA.

Instruments scanning outer space for cataclysmic explosions called gamma-ray bursts are detecting intense flashes of gamma-ray energy right here in the friendly skies of Earth. These terrestrial gamma-ray flashes, or TGFs, blast through thunderstorms close to the altitude where commercial airliners fly. In fact, they could be too close for comfort.

In a recent study,* scientists estimated that could be exposed to 400 chest X-rays worth of radiation by being near the origin of a single millisecond blast. Joe Dwyer of the Florida Institute of Technology took part in that research, which used observations from NASA's Reuven Ramaty High Energy Solar Spectroscopic Imager, or RHESSI, to estimate the danger TGFs pose.

"We believe the risk of encountering a TGF in an airplane is very small," says Dwyer. "I wouldn't hesitate to take a flight. Pilots already avoid thunderstorms because of turbulence, hail, and lightning, and we may just have to add TGFs to the list of reasons to steer clear of those storms."

But, he stresses, "it's worth looking into."

NASA's Monitor (GBM) on the Fermi Gamma-ray Telescope will help evaluate the hazards.

"GBM provides the best TGF data we have so far," says Dwyer. "It gets better measurements of their spectra than any previous instrument, giving us a more accurate idea of just how energetic they are."
Although TGFs are quite brief (1-2 milliseconds), they appear to be the most energetic events on Earth. They belch destructive gamma-rays packing over ten million times the energy of visible light photons - enough punch to penetrate several inches of lead.

"It's amazing," says Jerry Fishman, a co-investigator for the Gamma-ray Burst Monitor. "They come blasting right through the whole Fermi spacecraft and light up all of our detectors. Very few cosmic gamma-ray bursts manage to do this!"

A cartoon sketch of electric and magnetic fields in a thunderstorm and some of the phenomena they produce. TGFs may be just one aspect of thunderstorm activity in addition to elves, sprites, blue jets and ordinary lightning. Credit: Stanford University

The origin of TGFs is still a mystery, but researchers know this much: TGFs are associated with thunderstorms and lightning. "We think the electric field in a thunderstorm may get so strong that the storm itself turns into a gamma-ray factory," says Dwyer. "But we don't know exactly how or why or where inside the storm this happens."

So no one yet knows how often, if ever, planes end up in the wrong place at the wrong time.

It's possible that lightning bolts trigger TGFs. Or maybe TGFs trigger lightning bolts. Researchers aren't sure which comes first. GBM's excellent timing accuracy - to within 2 microseconds - will help solve this riddle.

"For some of the TGFs, we've pinpointed the associated lightning," says Dwyer. "This information along with the spectrum should help us figure out how deep in the atmosphere a TGF source is and how many gamma-rays it's emitting. Then we can determine the altitude and location they're coming from in the thunderstorm."

Fishman offers some good news: "If TGFs originate near the tops of thunderstorms and propagate upward from there, airline passengers would be safe."

By looking closely at a TGF's life cycle, that is, how quickly it turns on and off, GBM may also help researchers calculate how large and concentrated the gamma-ray source is. If the gamma-rays are emitted over a large region, the radiation dose would be diluted and much less harmful.

The radiation dose from an ordinary lightning leader vs. the dose from a TGF. Both phenomena are associated with electron beams. Tighter, more compact beams deliver a greater effective dose. Details of this model may be found in an upcoming issue of the Journal of Geophysical Research (Atmospheres). Look for "Estimation of the fluence of high-energy electron bursts produced by thunderclouds and the resulting radiation doses received in aircraft" by J. Dwyer et al. (in press).

"But if the source is compact and the gamma-rays originate close to an aircraft, then that could be a problem," says Fishman.

"Of course the smaller the source the lower the odds of a plane ending up close to it," adds Dwyer.

GBM wasn't designed to look for TGFs, but GBM co-investigator Michael Briggs has greatly enhanced its sensitivity to them by writing new software.

"TGFs have really been an afterthought for missions so far," says Dwyer. RHESSI, for example, points at the sun, but the RHESSI team figured out a way to measure TGFs by detecting gamma-rays coming in through the satellite's backside. "All these instruments have been pointing across the universe, while right over our heads these monsters are going off!"

"Now the whole field of TGFs is on fire," says Fishman. "People are jumping on the bandwagon to try to figure them out."

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More information: *Journal of Geophysical Research (Atmospheres), "Estimation of the fluence of high-energy electron bursts produced by thunderclouds and the resulting radiation doses received in aircraft" by J. Dwyer et al. (in press).

Source: Science@NASA, by Dauna Coulter

4.5 /5 (11 votes)

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axemaster
4 / 5 (1) Feb 11, 2010
""Of course the smaller the source the lower the odds of a plane ending up close to it," adds Dwyer."

That doesn't make any sense. EM waves propagate as 1/(r^2), and the radius of the source makes no difference to the intensity. It shouldn't matter how big or small the source is.

Also, I would guess that these gamma rays are cyclotron radiation. Every time a bolt of lightning forks or shifts direction, the electrons should be emitting radiation opposite the direction of their acceleration. Given that lightning travels at a significant percentage of the speed of light, and the enormous amperage and voltage, it seems plausible that this could reach the gamma ray level.
PinkElephant
4 / 5 (2) Feb 11, 2010
It shouldn't matter how big or small the source is.

Not quite. It's the difference between standing right next to a nuclear bomb as it goes off, vs. standing 100 miles away. For a smaller source, if you happen to be near it you'll catch (subtend) a greater proportion of its total emitted energy.
axemaster
5 / 5 (1) Feb 11, 2010
It shouldn't matter how big or small the source is.

Not quite. It's the difference between standing right next to a nuclear bomb as it goes off, vs. standing 100 miles away. For a smaller source, if you happen to be near it you'll catch (subtend) a greater proportion of its total emitted energy.


Not true. Assuming that the total output power W is constant, the radius of the source is irrelevant, unless you are actually inside it. This is a basic concept of physics.

Example: The Sun collapses into a black hole. The radius will become extremely small, yet outside Ro the gravitational flux remains constant.

The same applies to EM waves. Regardless of the radius of the gamma burst, you will experience the same radiation flux at a fixed distance in all cases (ignoring the effects of atmospheric attenuation).
PinkElephant
5 / 5 (2) Feb 11, 2010
Assuming that the total output power W is constant, the radius of the source is irrelevant, unless you are actually inside it.

Assume W is evenly distributed over a spherical surface with radius being the distance from you to center of emitter. You subtend a certain fraction of that total surface, and that's the fraction of W that will hit you.

Now, consider a source whose own radius is 1000 m. You are 1 m from its surface. Your distance to source's center is 1001 m, and that determines (via inverse square relationship) the proportion of energy you'll absorb (if we pretend that all energy comes from the center.) On the other hand, consider a source whose radius is 1 m. You're still 1 m from its surface. Now the radius of the sphere is just 2 m, and you subtend a MUCH larger fraction of it. Correspondingly, you'll catch a MUCH greater fraction of the emitted energy.
axemaster
4 / 5 (1) Feb 11, 2010
Oh sure, that's true. My point is in relation to the article, where the source radius is unlikely to be an issue in the manner you describe. A gamma burst due to lightning is unlikely to come from a large volume, due to the intense electric and magnetic fields involved.

"Of course the smaller the source the lower the odds of a plane ending up close to it"

This statement is clearly inaccurate because the source is probably btw 1m-1mm in size. Either way it makes no difference, the aircraft will still experience a 1/r^2 because of the minute difference.
PinkElephant
4.5 / 5 (2) Feb 11, 2010
A gamma burst due to lightning is unlikely to come from a large volume, due to the intense electric and magnetic fields involved.

Well that's the thing: they are not sure what is the source of those gamma bursts. It might not be any sort of ordinary lightning. It might be large-area/large-volume discharges similar to the "sprites", "blue jets", etc.

Cyclotron isn't the only possibility. Let's say electrons get accelerated by large electric fields over long distances, and then finally slam into some hapless nucleus: the near-instantaneous deceleration dumps some of the electron's energy overboard, in the form of a gamma photon...

So then we might be talking about a given gamma flux emanating from 10^6 cubic meters (a cube 100 meters on a side), vs. 10^9 cubic meters (a cubic kilometer), and up...
yyz
5 / 5 (3) Feb 12, 2010
PinkElephant points out(as does the article) that scientists are unsure of the source of the gamma ray bursts, and lightning may or may not be directly involved. Japanese researchers in 2009 reported on GR flashes observed that were not correlated with lightning. From their abstract:

"During thunderstorms on 2008 September 20, a simultaneous detection of gamma rays and electrons was made at a mountain observatory in Japan located 2770 m above sea level. Both emissions, lasting 90 seconds, were associated with thunderclouds rather than lightning. The photon spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung gamma rays arriving from a source which is 60 - 130 m in distance at 90% confidence level. The observed electrons are likely to be dominated by a primary population escaping from an acceleration region in the clouds."

"Observation of an energetic radiation burst from mountain-top thunderclouds" can be found here: http://arxiv.org/...xiv/pdf/
yyz
5 / 5 (1) Feb 12, 2010
Sorry about the link. Try here: http://arxiv.org/...81v1.pdf
CSharpner
3 / 5 (1) Feb 12, 2010
"TGF" is an acronym for "Terrestrial Gama ray Flash". No where in this article does it spell out the acronym... I guess they just assume everyone already knows this acronym? Anyway, in case you were wondering, now you know.
yyz
4 / 5 (1) Feb 13, 2010
"These terrestrial gamma-ray flashes, or TGFs..."

Something I didn't see in the article was any reference to the Firefly satellite due for launch later this year or 2011. This will be the first mission dedicated to the study of TGFs. Firefly mission: http://www.physor...955.html