Gamma rays made on Earth have unexpectedly high energies

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 spectrum was the one thing we understood and could explain well.”

Relativistic runaway

A free atmospheric electron starting from rest would have a tough time accelerating to relativistic speed. Even in the electric field of a thunderstorm, which can reach hundreds of kilovolts per meter, collisions with air molecules would decrease its energy faster than the field could increase it. But if an electron is already traveling very fast, it sees the passing molecules with much smaller scattering cross sections, so it builds up even more speed as it zips through the field. When it does collide with air molecules, it releases additional electrons, a few of which have enough kinetic energy to be accelerated by the field as well, so the number of fast-moving electrons increases exponentially.

That process, called a relativistic runaway electron avalanche (RREA), is the mechanism attributed to lightning discharges. (See the article by Alexander Gurevich and Kirill Zybin, PHYSICS TODAY, May 2005, page 37 .) The fast seed particle that starts it all may be a cosmic ray. Ordinary plasma discharges, of the kind that you feel when you touch a doorknob on a dry day, proceed by a different mechanism, which requires a field much stronger than is present in a storm.

The same RREA mechanism is likely to be involved in producing TGFs. Monte Carlo simulations ² of RREAs in air yield spectra like the red line in the figure: a power-law decline at the low-energy end, interrupted by an exponential cutoff somewhere around 7 MeV.

High-energy tail

Launched in 2007, AGILE was designed for astrophysical research. Its onboard hardware and software were tailored for the observation of cosmic gamma-ray bursts. But its sensitivity to fast time scales and high photon energies make it ideal for TGF viewing as well.

Based on data from 130 TGFs collected over a 20-month period, the observed spectrum (black dots in the figure on page 17) is well fitted at the high-energy end by a second power law (blue line) that extends at least to 100 MeV with no sign of an exponential cutoff. But in the RREA model, 100-MeV electrons—which are required to produce 100-MeV photons—must have had a long history of flying through the field, colliding with air molecules, and releasing electrons with energies at the low end of the spectrum. “What’s impossible to explain in the current model,” says Smith, “is that there are so many high-energy photons without a lot more low-energy photons than appear to be there.”