Every day, some 40 000 thunderstorms roil in Earth’s atmosphere. Yet some of the fundamental physics behind the storms remains unknown, including the process that initiates lightning. The same high-energy electric fields that produce lightning discharges can also generate gamma rays. Glows of relatively low-energy gamma rays can last for tens of seconds. Flashes occur in microseconds and have much higher energies. In 2011, Martino Marisaldi and colleagues found that those terrestrial gamma-ray flashes can reach tens of millions of electron volts (see Physics Today, January 2011, page 16).
Now Marisaldi, Nikolai Østgaard, and Andrey Mezentsev, all with the University of Bergen in Norway, and their colleagues have observed a new kind of atmospheric gamma-ray phenomenon. The so-called flickering gamma-ray flashes suggest that the electric fields inside thunderclouds are more complex than previously thought and could serve as a probe for better understanding thunderstorms.
The new observations—reported in two papers—come from NASA’s July 2023 ALOFT campaign, during which researchers arranged for a former spy plane to fly 20 km above thunderstorms in the Caribbean Sea and Central America. The instrument suite included gamma-ray detectors, photometers, electric field sensors, radars, and radiometers. Various data at 1-second resolution were transmitted to researchers on the ground. That allowed them to identify in real time the clouds with gamma-ray glows, and they could then direct the pilot to adjust the flight path and collect more data from where they thought they could find gamma-ray events.
After the campaign, when analyzing the data at a finer temporal resolution, Østgaard and colleagues saw what looked like multiple flickering pulses of terrestrial gamma-ray flashes. They occurred over coastal regions and lasted for tens to hundreds of milliseconds. But unlike terrestrial gamma-ray flashes, the flickering flashes had no observed accompanying optical or radio emission. The lack of such emissions and the millisecond duration suggest that the flickering flashes could have a similar origin to the gamma-ray glows, which also lack such emissions. Because of the real-time data transmission from the plane, Marisaldi and colleagues collected more information on the spatial and temporal extents of glows than earlier missions.
Entire thunderclouds were previously thought to glow from gamma rays for tens of seconds in a continuous, uniform manner. So the researchers were surprised to learn from the new observations that an entire cloud can glow for hours, dynamically oscillating in intensity between dim flashes and bright ones. The glows can last for hundreds of seconds and are found in strong, narrow convective pockets of large multicell thunderstorms that cover a few thousand square kilometers. The new observations don’t solve the mystery of lightning initiation, but the range of spatial and temporal scales of the gamma-ray phenomena shows that the electric fields that generate the gamma rays and lightning could be more dynamic than previously assumed by models. (M. Marisaldi et al., Nature 634, 57, 2024; N. Østgaard et al., Nature 634, 53, 2024.)
