Atmospheric Sciences - Deadly Radiation from the Clouds / Joseph R. Dwyer and David M. Smith

Thunderstorms emit intense bursts of gamma rays and x-rays and launch beams of particles, and even antimatter, into outer space. The atmosphere is a stranger place than we thought 

After the space shuttle Atlantis launched a new space telescope into orbit in 1991, Gerald Fishman of NASA's Marshall Space Flight Center realized that something very strange was happening. The Compton Gamma Ray Telescope (CGRO), which was built to detect gamma rays coming from distant astrophysical objects such as neutron stars and supernova remnants, also began to record bright bursts of gamma rays, which lasted several milliseconds and did not come from outer space but from the Earth below.

Astrophysicists already knew then that exotic phenomena, such as solar flares, black holes, and exploding stars accelerate electrons and other particles to extremely high energies, and that these fast charged particles can emit gamma rays: the highest-energy photons in nature. However, in astrophysical events the particles accelerate as they move in an almost free motion in space, which for the purpose can be considered as a vacuum. How then can particles in the earth's atmosphere, which in no way even approaches a vacuum, move in the same way?

At first, the initial data led us, and also other experts besides us, to believe that those "terrestrial gamma ray flashes" come from an altitude of 65 kilometers above the clouds, but now we can determine that they are created at a much lower altitude by electrical discharges within normal storm clouds. Meanwhile, increasingly sophisticated theories formulated to explain these anomalous gamma rays struggled to keep up with the observations: again and again, experiments discovered energies whose presence in the atmosphere had previously been thought impossible. Even antimatter popped in for a surprise visit.
Twenty-one years later, researchers have a pretty good idea of ​​what might create these terrestrial gamma-ray bursts, though uncertainties still remain. The urgency to solve this fascinating puzzle has increased, in view of its possible consequences for public health: if a plane passes too close to the sources of the flashes, the emitted gamma radiation could endanger the passengers inside.

Two birds with one stone?

At first, the scientists wondered if the gamma rays might be related to a different kind of atmospheric wonder that had been discovered just a few years before. Cameras whose lenses were aimed at areas above storm clouds captured short, bright flashes of red light, 80 kilometers above the ground and several kilometers wide, the flashes that looked like giant jellyfish. These impressive electrical discharges are given the imaginative name "sprites". The elves appear almost at the edge of space, so it seems plausible that they might shoot gamma rays that could be picked up by a spacecraft in orbit around the Earth.

Soon came the first attempts of theoretical physicists to explain how sprites can create gamma rays at the boundaries of space. According to conventional wisdom, sprites are side effects of ordinary lightning that occurs in the clouds somewhere below. Lightning is an electrically conductive channel that opens for a short period of time in the air, which in normal conditions is considered electrically isolated. The lightning stroke transfers electrons from one area of ​​the atmosphere to another area, or from the atmosphere to the ground. Lightning is caused by an imbalance of electrostatic charge and is triggered by the resulting electric fields. The potential difference between the fields may exceed 100 million volts.

The raging flow of electrons partially restores the electrostatic equilibrium. And yet, just as patting a bump in a carpet to straighten it often causes another bump to appear elsewhere, so an electrical discharge in a cloud often causes a field to appear elsewhere, including on the ground, where it may later lead to lightning upward, or near the bottom the ionosphere and there an elf may form.

In 1992, Alexander V. Gurevich from the Lebedev Institute of Physics in Moscow and his colleagues calculated that such secondary electric fields near the ionosphere could cause avalanches of energetic electrons, which, following collisions with atoms, would release high-energy photons: X-rays and even more energetic rays, gamma rays , in addition to the red glow characteristic of elves. The mechanism they proposed was derived from a proposal put forward by the Nobel laureate, the Scottish scientist C. T. R. Wilson, in the 20s. At low energies, electrons repelled by an electric field behave like drunken salts, bouncing from molecule to molecule and losing their energy with each and every collision. However, at high energies the electrons move in a straight line, and by the way absorb more and more energy from the electric field, so that any collision that occurs will have even less effect on disrupting their trajectory, and so on in a process that intensifies itself. This gradual process is different from the process we are familiar with from our everyday experience, where the faster we move, the stronger the pulling force we feel, as any cyclist can testify.

It is quite possible that these "racing" electrons could accelerate almost to the speed of light and travel for miles until they stop, instead of the few meters that an electron might normally travel when moving through air. Gurevich's team concluded that when a speeding electron does finally collide with a gas molecule in air, it will be able to release another electron from there, and this new electron can itself start speeding. The result will be similar to a chain reaction: an avalanche of high-energy electrons, which increases exponentially with distance and can extend over the entire length of the electric field. The avalanche effect, Gurevich and his colleagues calculated, could increase the production of X-rays and gamma rays by many orders of magnitude. For a period of time, this image seemed very attractive, because it combined two atmospheric phenomena: gamma-ray flashes and sprites. As we will see later, it turned out that the reality is much more complex.

The innocence of elves

Over several years, from 1996 onwards, more and more polished versions of the theory were developed, all of which modeled sprites as a manifestation of galloping electron avalanches that produce gamma rays. One of the pieces of evidence that supported the elf model was the energy spectrum of the gamma rays. Gamma rays with higher energies travel a greater distance in air than rays with lower energies, so it is more likely that they will succeed in reaching space. By counting the number of photons of the gamma radiation that reached the spacecraft at each of the energy levels, the scientists can deduce which room was the source that produced them. The first tests of the gamma-ray energies observed by CGRO showed a very high source Rom, consistent with the dwarf Rom.

Then, in 2003, an unexpected turn occurred. While working at a lightning research facility in Florida and measuring the x-ray emissions that reached the ground from lightning created by a missile shot, one of us (Dwyer) along with his colleagues detected a very bright burst of gamma rays that was emitted from the storm cloud above them and washed over the surface of the ground around us [see: "Lightning Strike" by Joseph Dwyer, Scientific American Israel, August-September 2005]. This burst looked to our instruments just like one of the terrestrial gamma-ray bursts that everyone thought had a much higher origin: the rays had the same energies and lasted the same amount of time, about 0.3 milliseconds. At the time, everyone assumed the flashes came from too high up to be seen on the ground. The similarity between the flashes suggested that lightning strikes within storm clouds might be the direct source of the gamma rays that reached CGRO, but at the same time the idea seemed a little crazy: the flash would have to be amazingly bright to be able to send enough gamma rays into space through the entire atmosphere.

But other developments soon came that broke the apparent connection between elves and gamma rays. In 2002, NASA launched the High Energy Solar Spectroscopic Imaging Facility named after Reuven Ramati, or RHESSI for short, to study X-rays and gamma rays coming from the Sun. But RHESSI's large germanium detectors were also admirably suited to measuring gamma rays coming from the atmosphere, even though they also had to pass through the back of the spacecraft when the observatory itself was pointed toward the Sun. One of us (Smith), an astrophysicist and solar physics researcher, was a member of the RHESSI instrumentation team and enlisted Liliana Lopez, then a research student at the University of California, Berkeley, to scan RHESSI's continuous stream of data, spanning several years, for evidence of X-rays. Gamma from below. At the time, terrestrial gamma-ray bursts were considered very rare. Instead, Lopez found a cache: RHESSI detected a flash every few days, a rate about 10 times faster than CGRO's detection rate.

The measurements of the energies of the gamma-ray photons in each flash by RHESSI were infinitely better than any measurement ever made by CGRO. Their spectrum looks exactly like what you would expect from a spectrum created by rushing electrons. But when we compared the results with the simulations we conducted, we concluded that the gamma rays passed through a great deal of air, and therefore their source had to be a bromine that moved approximately between 15 and 20 kilometers: a bromine typical of the tops of thunderstorms, but much lower than the 80 kilometers where the elves live.

Additional independent evidence quickly accumulated in favor of a low-Rum source of gamma rays, and against the dwarf connection. Steven Kamer of Duke University made radio measurements of some of the lightning associated with the RHESSI events, and they showed that these lightning flashes were much weaker than needed to create sprites. Moreover, RHESSI's map of worldwide gamma ray flashes looked very similar to a map of normal lightning, which is concentrated in the tropics, and very different from the map of sprites that sometimes accumulate in higher latitudes, such as the Great Plains of the USA.

However, one argument remains in favor of dwarfs as a source: the energy spectrum from CGRO events seems to point to high bromine sources, which is more suitable for dwarfs than thunderstorms. Many of us have come to think that there may be two types of gamma ray bursts: low rum and high rum. But the final blow to the elf idea came when we realized that terrestrial gamma-ray flashes are much brighter than we previously thought. In fact, working in 2008 with then-research student Brian Griffenstadt, we determined that they were so bright that CGRO was partially blinded by them and could not measure their full intensity. (Saturation that also affected RHESSI, to a lesser extent.) When researchers at the University of Bergen in Norway analyzed the data in 2010, they discovered that if we take into account the devices reaching saturation, the results would be consistent with sources from lower altitudes.
In less than two years, therefore, the estimated radius of the place of formation of gamma-ray flashes has dropped by more than 50 kilometers. It is rare to see a paradigm shift in science that happens so quickly. This change is ironic, because when we started in this field of research ten years ago, elves were the only shining example of how energetic radiation could be generated in our atmosphere. Now, 10 years later, it seems that pretty much anything—thunderstorms, different types of lightning, sparks created in a lab—can create detectable high-energy radiation, but apparently elves can't. Today it is generally accepted that the low energy of the elf radiation dictates that they are not responsible for gamma ray flashes after all.

Give us antimatter
Okay, so if elves don't create gamma ray flashes, what does? And does the process still involve avalanches of rushing electrons? It turns out that the avalanche mechanism according to the model of Gurevich and his colleagues, even though it is too energetic to be related to dwarfs, is not strong enough to create the enormous brightness observed by RHESSI and resulted from the reanalysis of the CGRO data. However, calculations made by Dwyer showed that an enhanced version of the electron avalanche mechanism could release energy at a rate several thousand billion times greater than the predictions made in the past, and that it could occur within a storm cloud. Amazingly, such a mechanism would also involve the production of an abundance of antimatter.

If the electric field inside a storm cloud is strong enough, racing electrons, assuming they are created in some way, will accelerate almost to the speed of light, and when they collide with atomic nuclei in the air molecules, they will be able to emit gamma rays. The photons of the gamma rays, in turn, will be able to react with atomic nuclei and create pairs of particles: electrons and their antimatter twins, positrons. The positrons will also start to race and accumulate energy from the electric field. But while the electrons will move up in the field, the positrons, whose electric charge is opposite, will move down. When the positrons reach the bottom of the electric field, they will collide with air atoms and kick out new electrons that will once again start racing upwards towards the top.

Thus, electrons moving up will produce positrons moving down, which in turn will produce more electrons moving up and so on. When one avalanche creates new avalanches, the discharges spread rapidly over a large area of ​​the storm cloud, up to a width of several kilometers. The numbers predicted by this model, known as the Relativistic Refeed Discharge Model, exactly matched the gamma-ray intensity, duration and energy spectrum as observed by CGRO and RHESSI.

The feedback created by the positrons is analogous to the annoying screeching heard when you hold a microphone next to a speaker. Of course, if we want to make a loud noise, we can just as well just shout into the microphone. That's the logic behind another possible explanation, though it's not yet fully worked out mathematically: Gamma-ray bursts are more energetic versions of the X-ray bursts emitted by lightning as it nears the ground. For several years, researchers at the Florida Institute of Technology, the University of Florida, and the New Mexico Institute of Mining and Technology have measured these X-rays, emitted both by lightning created artificially by missiles and by natural lightning striking the ground. X-ray "videos" taken by a high-speed X-ray camera in Florida show that the bursts originate at the end of the lightning channel, as it moves from the cloud toward the ground. Most scientists believe that the X-rays are created by racing electrons accelerated by strong electric fields in front of the lightning. It is possible that for as yet unsolved reasons, lightning moving through the electric field within a storm cloud is able to more successfully create those racing electrons. If this idea is true, it is possible that the flashes observed by spacecraft from hundreds of kilometers away are nothing more than a version of the X-rays, amplified by an as yet unknown mechanism, produced by lightning and observed by detectors placed on the surface of the ground only a few hundred meters away from the lightning.

Out of the Blue
At the end of 2005, we were already convinced that most terrestrial gamma-ray bursts originated from storm clouds or near their tops, regardless of whether antimatter or enhanced lightning was also involved. But before we could get too comfortable with this new paradigm, our understanding seemed to be shaken again: one of the events picked up by RHESSI took place right in the middle of the Sahara desert, on a sunny day with no storm clouds in sight.

Together with our research students, we spent months struggling to crack this nut. It turned out that storm clouds did form that day, but not where the spacecraft was looking. The storms took place several thousand kilometers to the south, beyond the horizon as far as RHESSI is concerned. Their gamma rays, which like all forms of light move in a straight line, could not reach the spacecraft.

But, charged particles like electrons for example, naturally move in tight spiral orbits around the curved lines of the Earth's magnetic field. The storms occurred exactly at the other end of the magnetic field line that passed through the spacecraft. Electrons that reached a very high RPM circled the planet and hit the RHESSI detectors, creating gamma rays in the process. However, it doesn't seem possible that electrons released inside a storm cloud could pass through miles and miles of atmosphere and reach space where they could catch a ride around the field lines. Again the new observation seems to require a high bromine source.

Moreover, in 2011, the Fermi Gamma-ray Space Telescope observed some more of these surrounding rays and made a sensational discovery: a significant proportion of the rays is composed of positrons. It therefore seems that atmospheric phenomena can also throw antimatter particles into space, and not just electrons and gamma rays. Looking back we can say that the appearance of these positrons should have been expected, given the high energy of the gamma rays. And yet, given that antimatter in nature is such an unusual phenomenon, the Fermi telescope's finding was astonishing.

Soon our team realized that in order to explain the Sahara find it is not necessary to assume that the gamma rays come from a high altitude, but rather that they were created within storm clouds in more abundant quantities than we thought possible at all. A few of them headed for space, collided with a stray air molecule at a height of about 40 kilometers and created secondary electron-positron pairs that hitched a ride on the magnetic field lines surrounding the Earth. The next time you see a storm cloud towering in the sky, stop for a moment and consider that it is capable of shooting high-energy particles into space, and that these may appear on the other side of the Earth.

New exceptions
The appearance of positrons was not to be our last astonishment. Later in 2011, the Italian Space Agency's AGILE observatory discovered that the energy spectrum of terrestrial gamma-ray bursts reaches up to 100 megaelectron volts, a value that would be incredible even if it came from a solar flare. If these observations are correct, they cast a shadow of doubt on our models, as it seems unlikely that the gallop mechanism could generate such energies on its own. In fact, it is not at all clear what could accelerate electrons to such energies within storm clouds. At this point we need more observations to help outline the theory. Happily, teams from the USA, the European Union and Russia are now starting to launch the first space missions designed to detect terrestrial gamma rays.

And in the meantime, to get closer to the heart of the matter, together with our colleagues we built a device to be carried on a plane and designed to measure gamma rays from storm clouds. The fear of the dangers associated with exposure to gamma rays prevents us from flying straight into a storm. But in an early test flight that Dwyer took part in, the plane accidentally took a wrong turn. The feeling of dread soon gave way to elation when suddenly our detectors lit up. Subsequent analysis showed that the area where the plane had flown accelerated racing electrons of the type we expected to produce gamma ray bursts. Fortunately, the emission remained at a low level and no explosive event that could be seen from space developed. With the help of these flights we discovered that normal thunderstorms most of the time emit a continuous and relatively harmless glow of gamma rays.

However, preliminary calculations showed that if a passenger plane were to happen to be directly hit by the gamma rays and energetic electrons in a storm, the passengers and crew members could, without realizing it, absorb within a fraction of a second a dose of radiation that may reach the amount normally absorbed in an entire lifetime. The bright spot is that we don't need to warn pilots to steer clear of thunderstorms, because they do anyway; Thunderstorms are a very dangerous place to be, with or without gamma rays.
In a way, the study of terrestrial gamma-ray flashes complements the work of Benjamin Franklin, who deliberately flew a kite into a thunderstorm to see if it would conduct electricity and thus showed that lightning was an electrical discharge. Amazingly, 250 years after his kite experiment, scientists still do not fully understand, not only how thunderstorms create gamma-ray flashes, but even how they create simple lightning. We both spent much of our careers studying exotic objects far from the solar system, but the lure of that research drew us back to Earth. Perhaps even Franklin himself had no idea that thunderstorms could be so interesting.

About the authors
Joseph R. Dwyer (Dwyer) is an astrophysicist who became interested in lightning after moving to central Florida, the lightning capital of the USA. He serves as a professor at the Florida Institute of Technology.
David M. Smith is an associate professor of physics at the University of California, Santa Cruz. He studies lightning, the Earth's radiation belts and solar flares. He also conducts research observations on X-rays and gamma rays emitted from black holes.
in brief
Storm clouds emit gamma rays in powerful bursts that last a few milliseconds, called terrestrial gamma ray flashes, and which were first detected by observers from space.
These flashes can also create beams of electrons and even beams of antimatter capable of traveling half the circumference of the Earth.
All the proposed explanations for the phenomenon include strong electric fields that release avalanches of electrons within clouds, but none of them provide a full explanation for the enormous energies of the gamma rays.
New dedicated space missions and a new research plane may solve the mystery, and also reveal whether the flashes could put airplane passengers at risk of radiation exposure.
Discharges: a comparative view
Gamma rays vs elves
When space observers began to pick up bursts of gamma rays coming from the atmosphere in the 90s, researchers hypothesized that they originated from high-energy discharges known as sprites. But these terrestrial gamma-ray flashes turned out to be emerging from storm clouds at a much lower altitude. They also produce secondary beams of particles, including antimatter, that can escape into space and fly around the Earth following its magnetic field.
And more on the subject
Discovery of Intense Gamma Ray Flashes of Atmospheric Origin. GJ Fishman et al. in Science, Vol. 264, pages 1313-1316; May 27, 1994.
Runaway Breakdown and the Mysteries of Lightning. Alexander V. Gurevich and Kirill P. Zybin in Physics Today, Vol. 58, no. 5, pages 37-43; May 2005.
Source Mechanisms of Terrestrial Gamma-Ray Flashes. JR Dwyer in Journal of Geophysical Research, Vol. 113, no. D10103; May 20, 2008.
Electron-Positron Beams from Terrestrial Lightning Observed with Fermi GBM. Michael S. Briggs et al. in Geophysical Research Letters, Vol. 38, no. L02808; January 20, 2011.
Watch the video showing how thunderstorms create bursts of gamma rays, on the website www.sciam.co.il