This project was pursued to better understand a powerful natural phenomenon called terrestrial gamma-ray flashes (TGFs) that occur during thunderstorms, usually together with a lightning flash.  A TGF is a burst of extremely high energy radiation lasting less than a thousandth of a second.  We know that electric fields associated with the thunderstorm and perturbed or enhanced by the lightning flash accelerate electrons to extremely high energies, and that when these electrons collide with air molecules, they make gamma-rays (ultra-high-energy light) that can travel for miles through the air.

What we don't understand yet about TGFs is why, if it's possible for lightning to make them, they happen only rarely -- in maybe 1/1000 lightning flashes.  There is also a lot of disagreement as to why they are so bright -- bright enough to present a significant radiation risk to people if (for example) an aircraft was in exactly the epicenter of the event when it happened.  Whether this does ever happen is another question we can't answer yet.

Most TGFs (thousands of them) have been observed from satellites, which can see up to a million square kilometers of Earth's surface at once.  That's a big advantage, but it only allows one kind of TGF to be observed: TGFs associated with lightning that happens up high in the clouds, pointing upwards.  This is only a very specific kind of lightning (called "positive intracloud"), and not only does it tell us nothing about what other kinds of lightning might make TGFs, this is also not the kind of lightning that people on the ground or in aircraft might be endangered by.

We decided to explore detecting TGFs from the ground, so that we could see whether other kinds of lightning produce TGFs regularly, and which climates, thunderstorm types, and lightning types are most likely to produce them.  We set up gamma-ray detectors at a variety of sites around the world: on mountains in Switzerland, Japan, and the United States (where lightning originates closer to the ground), in a tropical environment in Colombia (where there is a lot of lightning but it tends to originate high up), in a sub-tropical environment in Florida (similar to Colombia but not as dramatic), and in two coastal sites in Japan and Croatia that are particularly susceptible to low-lying winter thunderstorms.   Collaborators around the world share radio data with us.  Radio observations of lightning provide essential information about what kind of flash we are looking at when a TGF happens, and what stage of the lightning process the TGF happens during.

Over the course of the project, we've found that, unlike the single kind of lightning that makes the TGFs seen from space, TGFs seen on the ground can come from most kinds of lightning that are known:  flashes that hit the ground, carrying negative charge from the cloud (these make up most of the spectacular lightning bolts that people photograph); intra-cloud flashes (those that don't reach the ground) that move negative charge both upward and downward; and flashes that start going upward from a tower or other built structure.  While we have confirmed that most lightning does not make a TGF, we've also learned something very tantalizing: if you sort out the very brightest lightning flashes, as defined by their radio emission, that occur near our detectors, virtually all of them can be seen to make a TGF.   Not every TGF accompanies this kind of radio "superbolt," but in at least two of the environments where we've seen the most TGFs -- Mt. Santis in Switzerland and the west coast of Japan in the winter -- all the radio-brightest nearby lightning flashes seem to have made a TGF.   The more confident of this we are, the more we can begin to make large-scale maps of when and where TGFs happen.  This is revolutionary, because good radio measurements can be made from hundreds of kilometers away, while gamma-ray detection is only good out to about 8km distance before too much of the radiation is absorbed by the air.

During this program, we also began to design and build a new kind of gamma-ray detector that will let us measure the amount of radiation in a nearby TGF at very high levels that would saturate or shut down all other existing detector designs.

Over a dozen graduate and undergraduate students gained valuable skills working on this project, by building and testing detectors, deploying them to the field, analyzing data, and running computer simulations of the way radiation interacts in the atmosphere and our instrument.

The illustration shows individual gamma-rays (dots) detected by the four detectors in our instrument, along with a radio signal (black line) showing that the main TGF comes just after the moment when the lightning connects with the ground (t=0), while a smaller gamma-ray signal comes just before.

Last Modified: 12/20/2023
Modified by: David M Smith