Cosmic rays could reveal secrets of lightning on Earth

Studio Gohde

Despite Benjamin Franklin's best efforts with a kite and a key, the phenomenon of lightning remains a scientific enigma. Now, researchers have developed a new tool that could help them solve some of lightning’s mysteries. By using cosmic rays, space-traveling particles that constantly rain down on our atmosphere, scientists report they can peek inside thunderstorms and measure their electric fields, helping them pinpoint the conditions that cause storms' electrical outbursts. The advance could help researchers predict more precisely when and where lightning is most likely to strike and get people out of harm's way in time.

Lightning is so poorly understood in part because measuring electric fields inside thunderstorms is challenging. Scientists have made measurements by sending balloons or small rockets into the clouds, but such probes can alter the electrical environment, potentially obscuring the natural activity they’re trying to measure. But such measurements fail to explain lightning's origin, as they have yet to find fields strong enough to initiate lightning. It could be that the high field regions are very localized, or it could mean another factor is necessary to set off the light show.

Cosmic rays could help researchers solve that puzzle. When cosmic rays smash into molecules in our atmosphere, the collisions create showers of subatomic particles, including electrons, positrons, and other electrically charged particles. As these particles travel toward the ground, their trajectories are bent by Earth’s magnetic field, causing them to emit radio waves. Scientists watched those patterns of radio waves with the LOFAR radio telescope in the Netherlands and compared their observations to a computer model simulating the radio wave patterns produced by cosmic ray showers. The model was able to reproduce most of the showers the researchers recorded, but things got wonky when the weather took a turn for the worse.

"Sometimes [the shower] looks strange, and the times when it looked strange coincide with times when there are thunderstorms present," says Pim Schellart, an astronomer at Radboud University in Nijmegen, the Netherlands, and the first author of the new study. When a shower passed through a thunderstorm, Schellart and his colleagues found an atypical pattern of radio waves, due to strong electric fields in the storm that wrench charged particles around and change the radio waves' polarization, or the orientation of their electromagnetic wiggles.

By adding electric fields to their model, the scientists were able to calculate what kinds of fields had to be present in the skies above to reproduce the patterns they saw on the ground. For a shower that the scientists studied in detail, they found that the storm was composed of two layers: one between 3 and 8 kilometers above the ground, with an electric field of 50,000 volts per meter, and another, weaker field below that, which pushed particles in the opposite direction of the upper field. Although these results can't yet explain how lightning sparks, the measurement marks the first time a storm has been probed with cosmic rays—the first step in refining a technique that could improve scientists' understanding of storms, the team will report in an upcoming paper in Physical Review Letters.

A cosmic ray shower (diagram shown in red) passes through a thunderstorm over the LOFAR radio telescope. Researchers have used cosmic rays to calculate the electric fields inside of thunderstorms.


"Having a way of remotely measuring the electric fields on the ground … would be very useful," says lightning physicist Joseph Dwyer of the University of New Hampshire in Durham, who was not involved with the research. To improve our predictions of where lightning will strike, Dwyer says, scientists first need to understand the basics of how it happens. With this method, he says, "eventually we may be able to … catch lightning initiation in action," which could help unravel the mystery behind its formation.

Scientists have come up with a few possible explanations for how lightning gets going, and the new technique could eventually discern between the options. One is that chunks of ice and water in a storm concentrate the fields, creating small regions that are strong enough to spark. Another controversial idea is that cosmic rays themselves might initiate lightning, thanks to the charged particles they leave in their wake. But there's no convincing evidence for either option. The new method could finally settle the question, Schellart says.

The technique is still in its early stages, with just a handful of showers characterized so far, so the results aren't reshaping scientists' ideas about thunderstorms yet. But with more data and an improved analysis, it could be a powerful tool. Still, Dwyer says, this is the first time the technique has been demonstrated, "so they certainly deserve a boatload of credit for this."