Earth’s upper atmosphere is a sensitive explosion detector

Scientists have detected the ripple effects of small ground-level explosions 100 kilometers up in ionized layers of the upper atmosphere. The result suggests the remote sensing technique could be used to monitor explosive events—natural or human—hundreds of times smaller than before. "It was a big surprise to me," says Jihye Park, a geodetic scientist at Oregon State University who was not involved in the research. "It's really smart."

The ionized region of the atmosphere, or ionosphere, is most famous as the home of aurorae, which occur when charged particles from the Sun slam into atoms and cause them to light up. But massive explosions burbling up from below can also disturb the ionosphere. In 2022, the Hunga Tonga-Hunga Ha‘apai volcanic eruption in the South Pacific Ocean produced ripples in the ionosphere that were detected thousands of kilometers away. In 1979, an ionospheric disturbance linked to a suspected Israeli–South African nuclear test was detected by Puerto Rico's now-defunct Arecibo radio telescope.

Both explosions set off waves of infrasound, pitched too low for human hearing, which can propagate across large distances and cause vibrations in the ionosphere. Radar beams tuned to bounce off the ionosphere's charged particles detected the vibrating layers.

But the technique has been mostly limited to explosions more powerful than 1 kiloton of TNT. (The nuclear bomb dropped on Hiroshima, Japan, in 1945 was about 15 kilotons.) Now, researchers report that they have successfully detected experimental explosions of just 1 ton of TNT. "Not only can we see those events, but they’re much clearer than I was expecting," says Kenneth Obenberger, a physicist at the Air Force Research Laboratory who led the study. The results were published this month in the journal Earth and Space Science.

Obenberger and his colleagues set out to observe the effects of two 1-ton explosions set off in March 2022 in New Mexico. The team's radar detectors were designed to measure the waves bouncing off the ionosphere's E layer, a region 100 kilometers up. They detected signs of each blast less than 6 minutes after the detonations.

Obenberger says the technique could be used to monitor small human-caused explosions or even remote volcanic eruptions in the Pacific that are otherwise hard to detect. He says the ionosphere's remoteness explains why the technique is only now showing its promise. "You’re sending a shock wave through what we call the ‘ignorosphere,’" he says.

Park says the enhanced resolution of the technique would make it easier to detect ionospheric disturbances associated not only with volcanic eruptions, but also earthquakes, which can trigger tsunamis, landslides, and other disasters. "You could use it for an early warning system, like a tsunami warning system," says Park, who has used global positioning satellites to detect the ionospheric disturbances of North Korean nuclear tests and other events.

Another possible use might be in planetary science. For worlds like Venus, where thick clouds obscure the surface, an ionospheric radar on an orbiting spacecraft could remotely detect unseen eruptions and earthquakes, Obenberger says.

Given the recent discovery of volcanic activity on Venus announced in March, "We might see smaller scale events," says Paul Byrne, a planetary scientist at Washington University in St. Louis. "This is exactly the kind of thing I’m hoping spacecraft engineers will think about incorporating for future missions."

For the time being, Obenberger wants to keep the research grounded on Earth. He is planning to test the approach in different seasons, because the ionosphere shifts over the course of the year. "The other thing I really want to do is set up next to a volcano," he says. "That would be really fun."