Energetic gamma rays on Earth

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. According to the prevailing theoretical model of that process, and to some previous observations, the TGF energy spectrum should follow a power law at low energies but decay exponentially at energies above about 7 MeV, as shown by the red line in the figure. Now, researchers working with data from the Italian Space Agency’s AGILE satellite find a rather different and theoretically challenging spectrum. Based on data from 130 TGFs collected over a 20-month period, their observed spectrum (black dots) is well fitted at the high-energy end by a second power law (blue line) that extends to at least 100 MeV with no sign of an exponential cutoff. According to the theory, the highest-energy electrons must have had a long history of flying through the electric field, colliding with air molecules, and releasing electrons with relatively low energies. But so many low-energy electrons would produce many more low-energy photons than the AGILE team observed. (M. Tavani et al., Phys. Rev. Lett., in press.)—Johanna Miller