Our project was an attempt to measure and understand the several ways in which thunderstorms produce gamma radiation.  

One of these ways is called a "terrestrial gamma-ray flash" or TGF.  A TGF is an enormously bright burst of radiation associated with lightning -- but only with a small fraction of lightning flashes, for reasons we don't understand. Thousands of upward TGFs have been observed beaming their gammas into space, seen by satellites in orbit.  But only a very few downward events have ever been seen from the ground.

We pursued TGFs with our custom-made gamma-ray detectors as guests on two aircraft -- the Global Hawk, an unmanned NASA plane, and the Hurricane Hunter's Orion aircraft.  We also built a second detector array that was deployed at two sites on the ground: on the Western coast of Japan, where winter thunderstorms hug the ground so closely that we get a good look at what's going on inside; and on a mountaintop site in Mexico.

With the Hurricane Hunters, we discovered a TGF happening in the eyewall of Hurricane Patricia in 2015, at a moment when that hurricane was the most intense one ever recorded in the Western Hemisphere.  Not only was this the first time a TGF had ever been seen in the very unusual and very violent eyewall environment, there was something else unique about the observation: this was an upward TGF viewed from below.  As had been predicted by theorists but never observed before, the upward TGF -- produced by electrons accelerated upward by the thunderstorm's electric field -- has a backwards "tail" produced by positrons (the electron's antimatter counterpart) accelerating downwards.  This discovery opens up the new possibility of detecting upward TGFs regularly from the ground, since the Orion aircraft was only at 2.5km altitude, a height accessible from mountaintops.   Our calculations show that even lower altitudes, such as the 1.6km altitude of the city of Denver, should still allow these observations to be made.

Another observational first, confirming theoretical predictions, came from our campaign for winter thunderstorms in Japan.  We measured a TGF so extraordinarily bright that it completely paralyzed our instrumentation while the gamma-rays were present; but in the aftermath, we saw a signal in our detectors that could only be produced by a flood of neutrons coming out of the sky in the milliseconds afterwards.  These neutrons are knocked out of atomic nuclei in the air by the gammas themselves, and later absorbed by other atoms, resulting in radioactive nuclei on both ends of the process.  The study of this secondary radiation is an important part of determining the yet-unanswered question of whether there are rare circumstances in which TGF radiation is a hazard to humans.

Finally, as part of this project, we reanalyzed some data from an earlier flight of one of our instruments that took place in 2009.  In this case we were studying a different kind of thunderstorm gamma-ray event, which we call a "gamma-ray glow".  Unlike TGFs, which last for a millisecond or less, glows can last for as long as the electrical charges of a thundercloud are maintained.  The level of radiation in a glow is negligible compared to a TGF, but they may have a different kind of significance: the high-energy electrons that collide with nuclei in air to make the glow gamma-rays also produce a lot of ionization in the air, ripping other electrons from their parent atoms.  This produces electrical currents that start to cancel out the thundercloud's electric charges.  Of course the most dramatic way that thunderstorm charges get canceled out is by lightning; but by calculating the electrical currents associated with our glow observation, we were able to show that it is at least possible that glows may compete with lightning as the dominant way that some thunderclouds lose their charge.

Last Modified: 01/29/2018
Modified by: David M Smith