Scientists have detected the ripple effects of small ground-based explosions at an altitude of 100 kilometers in the ionized layers of the upper atmosphere. The result suggests that 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 clever.
The ionized region of the atmosphere, or ionosphere, is best known as the home of auroras, which occur when charged particles from the Sun collide with atoms and cause them to light up. But massive explosions erupting from below can also disrupt the ionosphere.
In 2022, the Hunga Tonga-Hunga Ha’apai volcanic eruption in the South Pacific Ocean caused ripples in the ionosphere that were detected thousands of kilometers away. In 1979, the now defunct Arecibo radio telescope in Puerto Rico detected an ionospheric disturbance associated with a suspected Israeli-South African nuclear test.
Both explosions set off infrasound waves too low for human hearing that can travel long distances and cause vibrations in the ionosphere. Radar beams tuned to bounce off the charged particles of the ionosphere detected the vibrating layers.
But this technique was mostly limited to explosions stronger 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.
Obenberger and his colleagues set out to observe the effects of two ton explosions set off in March 2022 in New Mexico. The team’s radar detectors were designed to measure waves reflected from the E layer of the ionosphere, a region 100 kilometers away. They noted signs of each explosion less than 6 minutes after the detonations.
Obenberger says the technique could be used to monitor small man-made explosions or even distant volcanic eruptions in the Pacific that are otherwise hard to detect.
Park says the technique’s improved resolution would make it easier to detect ionospheric disturbances associated not only with volcanic eruptions, but also with earthquakes that 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 ionospheric disturbances from North Korea’s nuclear tests and other events.
Another possible use could be in planetary science. On worlds like Venus, where dense clouds obscure the surface, an ionospheric radar on an orbiting spacecraft could detect invisible eruptions and earthquakes from a distance, Obenberger says.
For now, Obenberger wants to keep the research grounded. He plans to test the approach at different times of the year because the ionosphere shifts throughout the year.
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