An overlooked mechanism lets energy from lightning reach the highest layers of the atmosphere, where it could threaten the safety of satellites and astronauts.

When lightning occurs, the energy it carries sometimes gives rise to special electromagnetic waves called whistlers, so named because they can be converted to sound signals. For decades, researchers thought lightning-induced whistlers would remain trapped relatively close to Earth’s surface, below about 1000 kilometres.

Now Vikas Sonwalkar and Amani Reddy at the University of Alaska Fairbanks have discovered that some whistlers can bounce off a layer of the atmosphere called the ionosphere, which is filled with charged particles. This enables the waves, and the energy they carry, to reach distances up to 20,000 kilometres above the planet’s surface. That means they can travel deep into the magnetosphere, the region of space dominated by Earth’s magnetic field.

The researchers found evidence of these reflected whistlers in data from the Van Allen Probes, twin robotic spacecraft that measured the magnetosphere between 2012 and 2019. They also saw signatures of this phenomenon in studies published as early as the 1960s. Old and new data all suggest that it is very frequent and happening constantly, says Reddy.

In fact, lightning could be contributing twice as much energy to this area of space as previous estimates indicated, the team says. And this energy charges and accelerates nearby particles, producing electromagnetic radiation that can damage satellites and harm the health of astronauts.

“Lightning was always believed to be a little bit of a smaller player. We haven’t had this data until a decade ago, and we have certainly not been looking at it with this great level of detail,” says Jacob Bortnik at the University of California, Los Angeles. The new work extends an invitation to other researchers to develop a more accurate picture of the magnetosphere, he says.

Establishing the link between lightning and the magnetosphere is also important because changes in Earth’s climate may be making lightning-heavy storms more common, says Sonwalkar.

The team now wants to analyse data from more satellites. It hopes to learn more about how lightning-based whistlers populate the magnetosphere, and how they may be affected by space weather.