Terrestrial gamma-ray flashes (TGFs) are the source of the highest-energy nonanthropogenic photons produced on Earth. Associated with thunderstorms—and in fact, with individual lightning discharges—they are presumed to be the bremsstrahlung produced when relativistic electrons, accelerated by the storms’ strong electric fields, collide with air molecules some 10–20 km above sea level. The TGFs last up to a few milliseconds and contain photons with energies on the order of MeV.
Now, Marco Tavani, Martino Marisaldi, Claudo Labandi, Fabio Fuscino, and others working with data from the Italian Space Agency’s AGILE satellite find that TGFs are even more energetic than previously thought, with a significant number of photons having energies of 100 MeV and likely even higher.1 “I think it’s safe to say that all the theorists will be absolutely stumped, at least for a while,” says David Smith of the University of California, Santa Cruz. “We thought that the energy...
