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Doug Rowland, principal investigator for Firefly stands next to the a life-sized model of the tiny satellite

January 29, 2010: High-energy bursts of gamma rays typically occur far out in space, perhaps near black holes or other high-energy cosmic phenomena. So imagine scientists' surprise in the mid-1990s when they found these powerful gamma ray flashes happening right here on Earth, in the skies overhead.


Gamma Ray Lightning from planet Earth

They're called Terrestrial Gamma-ray Flashes, or TGFs, and very little is known about them. They seem to have a connection with lightning, but TGFs themselves are something entirely different.

Right: An artist's concept of TGFs. Credit: NASA/Robert Kilgore [more]

"In fact," says Doug Rowland of NASA's Goddard Space Flight Center, "before the 1990s nobody knew they even existed. And yet they're the most potent natural particle accelerators on Earth."

Individual particles in a TGF acquire a huge amount of energy, sometimes in excess of 20 mega-electron volts (MeV). In contrast, the colorful auroras that light up the skies at high latitudes are powered by particles with less than one thousandth as much energy.

At this stage, there are more questions about TGFs than answers. What causes these high-energy flashes? Do they help trigger lightning--or does lightning trigger them? Could they be responsible for some of the high-energy particles in the Van Allen radiation belts, which can damage satellites?

To investigate, Rowland and his colleagues at GSFC, Siena College, Universities Space Research Association, and the Hawk Institute for Space Sciences are planning to launch a tiny, football-sized satellite called Firefly in 2010 or 2011. Because of its small size, Firefly will cost less than $1 million — about 100 times cheaper than what satellite missions normally cost. Part of the cost savings comes from launching Firefly under the National Science Foundation's CubeSat program, which launches small satellites as "stowaways" aboard rockets carrying larger satellites into space, rather than requiring dedicated rocket launches.

If successful, Firefly will return the first simultaneous measurements of TGFs and lightning. Most of what's known about TGFs to date has been learned from missions meant to observe gamma rays coming from deep space, such as NASA's Compton Gamma Ray Observatory, which discovered TGFs in 1994. As it stared out into space, Compton caught fleeting glimpses of gamma rays out of the corner of its eye, so to speak. The powerful flashes were coming--surprise!--from Earth's atmosphere.

Subsequent data from Compton and other space telescopes have provided a tantalizingly incomplete picture of how TGFs occur:

In the skies above a thunderstorm, powerful electric fields generated by the storm stretch upward for many miles into the upper atmosphere. These electric fields accelerate free electrons, whisking them to speeds approaching the speed of light. When these ultra-high speed electrons collide with molecules in the air, the collisions release high-energy gamma rays as well as more electrons, setting up a cascade of collisions and perhaps more TGFs.

To the eye, a TGF probably wouldn't look like much. Unlike lightning, most of a TGF's energy is released as invisible gamma rays, not visible light. They don't produce colorful bursts of light like sprites and other lightning-related phenomena. Nevertheless, these unseen eruptions could help explain why brilliant lightning strikes occur.


Elf and Sprite

A longstanding mystery about lightning is how a strike gets started. Scientists know that the turbulence inside a thundercloud separates electric charge, building up enormous voltages. But the voltage needed to ionize air and generate a spark is about 10 times greater than the voltage typically found inside storm clouds.

"We know how the clouds charge up," Rowland says, "we just don't know how they discharge. That is the mystery."

TGFs could provide that spark. By generating a quick burst of electron flow, TGFs might help lightning strikes get started, Rowland suggests. "Perhaps this phenomenon is why we have lightning," he says.

If so, there ought to be many more TGFs each day than currently known. Observations by Compton and other space telescopes indicate that there may be fewer than 100 TGFs worldwide each day. Lightning strikes millions of times per day worldwide. That's quite a gap.



Then again, Compton and other space telescopes before Firefly weren't actually looking for TGFs. So perhaps it's not surprising that they didn't find many. Firefly will specifically look for gamma ray flashes coming from the atmosphere, not space, conducting the first focused survey of TGF activity. Firefly's sensors will even be able to detect flashes that are mostly obscured by the intervening air, which is a strong absorber of gamma rays (a fact that protects people on the ground from the energy in these flashes). Firefly's survey will give scientists much better estimates of the number of TGFs worldwide and help determine if the link to lightning is real.

Stay tuned to Science@NASA for updates.

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Originally posted by FloralBunny:
Came here to post the article, but you beat me to it.
I think Serenity will look better on my key chain than a model of this.


bun

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Originally posted by FloralBunny:

Came here to post the article, but you beat me to it.

I think Serenity will look better on my key chain than a model of this.

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Originally posted by KingEICHOLZ:

I am an expert on this i have been working with NASA on this this is my study on it

Various electrodynamic processes above thunderstorms in the middle atmosphere give rise to a variety of phenomena, such as red sprites, elves, blue jets and terrestrial gamma ray flashes (TGF). We investigate the role of avalanching relativistic runaway electrons in the production of red sprites and terrestrial gamma ray flashes. The Red Sprites are optical emissions primarily in the red region of spectrum, occuring at altitudes of 50-90 km and associated with positive cloud-to-ground discharges. The terrestrial gamma ray flashes were observed by Burst and Transient Source Experiment (BATSE) detectors, located on the Compton Gamma Ray Observatory (CGRO), and described by Fishman et al. [1994]. They last around 1 ms, and the observed photon energies are 20 keV-2 MeV.
The runaway electron breakdown is modelled using Monte Carlo technique [Lehtinen et al., 1999a] to find the avalanche rates and the direction of the electron beam. We also study the nonuniform properties of the avalanche [Lehtinen et al., 1999b] using the same model.

The Monte Carlo model is applied to the Earth's middle and upper atmosphere above thunderstorms, where the electric fields are strong enough to accelerate the electrons upward. The atmosphere can be described as cylindrically symmetric with a vertical axis [Lehtinen et al., 1997] or translationallly symmetric in horizontal direction [Lehtinen et al., 1999a]. Note that the former allows a description only of vertical geomagnetic field, whereas the latter can describe magnetic field at any dip angle.

We investigate the role of avalanching runaway electrons in the production of red sprites and terrestrial gamma ray flashes. The calculated optical emissions in red sprites associated with the runaways are negligible compared to the emission from thermal electrons in the conventional type of breakdown. This result is obtained for all values of the lightning discharge which causes the runaway breakdown in the middle atmosphere. However, the calculated gamma ray flux, due to bremsstrahlung emissions from relativistic electrons, is of the same order as the terrestrial gamma ray flashes observed by the BATSE detector on the Compton Gamma Ray Observatory.

The energetic electrons leaving the atmosphere enter the radiation belts [Lehtinen et al., 1998]. The electron beam interacts with plasma waves in the ionosphere and magnetosphere and precipitates at the geomagnetically conjugate point. Part of the energetic electrons is trapped in the radiation belt, forming an electron "curtain" as the electrons drift in the longitudinal direction.

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Originally posted by Bytemite:

And wow, that circular spider lightning in that one picture is something else.

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