One hundred kilometres up in the ionised layers of the upper atmosphere, scientists have discovered the ripple effects of minor ground-level explosions. The finding shows that explosive occurrences, whether natural or man-made, might be monitored using remote sensing technology at scales hundreds of times smaller than before. "It was a big surprise to me," says Jihye Park, an Oregon State University geodetic expert who wasn't involved in the study. It's extremely clever.
The ionised portion of the atmosphere, also known as the ionosphere, is most well-known for being the location of aurorae, which happen when charged particles from the Sun strike atoms and ignite them. However, huge explosions that rumble upward from below can also agitate the ionosphere. Ionosphere swells caused by the Hunga Tonga-Hunga Ha'apai volcanic eruption in the South Pacific Ocean in 2022 might be seen thousands of kilometres away. The now-defunct Arecibo radio observatory in Puerto Rico picked up an ionospheric disturbance in 1979 that was connected to an alleged nuclear test by Israel and South Africa.
Both explosions release infrasound waves that are too low to be audible to humans but can travel great distances and produce ionosphere tremors. Radar beams calibrated to reflect from charged particles in the ionosphere picked up the vibrating layers.
However, the approach has typically only been used for explosions stronger than 1 kiloton of TNT. (In 1945, a nuclear weapon about the weight of 15 kilotons was dropped on Hiroshima, Japan.) Recently, scientists reported that they have discovered experimental explosions of just one tonne 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 and the study's principal investigator. The Earth and Space Science journal released the findings this month.
Obenberger and his team set out to investigate the results of two 1-ton explosives that were detonated in New Mexico in March 2022. The radar detectors used by the researchers were created to track waves reflecting off the E layer of the ionosphere, which is located 100 kilometres above the earth. Less than 6 minutes after the detonations, they discovered indications of each explosion.
According to Obenberger, the method might be used to monitor minor explosions generated by people or even infrequent volcanic eruptions in the Pacific that are otherwise difficult to spot. He claims that the ionosphere's distance explains why the technology has only just begun to show promise. You're propagating a shock wave through the "ignorosphere," he claims.
According to Park, the technique's improved resolution would make it simpler to identify ionospheric disturbances related to earthquakes, which can result in tsunamis, landslides, and other disasters in addition to volcanic eruptions. "You could use it for an early warning system, like a tsunami warning system," claims Park, who has employed global positioning satellites to find the ionospheric disturbances caused by North Korean nuclear testing and other disasters.
Planetary science is yet another potential application. According to Obenberger, ionospheric radar on an orbiting spacecraft could detect hidden volcanoes and earthquakes on planets like Venus where dense clouds conceal the surface.
According to Paul Byrne, a planetary scientist at Washington University in St. Louis, "We might see smaller scale events" in light of the recent discovery of volcanic activity on Venus, which was disclosed in March. This is precisely the kind of thing I'm hoping future spaceship engineers will consider including.
Obenberger wants to keep the research grounded in Earth for the time being. The ionosphere changes throughout the year, therefore he intends to test the method in several seasons. The second thing I really want to do, he says, "is set up near a volcano." That sounds like a lot of fun.
