by Sean Treacy Wednesday, December 5, 2012
SAN FRANCISCO: If you’re looking for hints of an underground nuclear explosion, the last place you might think to look is up. But it turns out that the sky is a great place to look for clues, because satellites provide scientists with a way to locate the waves that clandestine weapons tests emit into the upper atmosphere, scientists announced yesterday.
If a nation secretly tests a nuclear bomb, scientists already have a few ways to find out. Seismometers can usually pick up shockwaves through the ground, for example. They can also detect shockwaves underwater or look for a telltale xenon isotope in the atmosphere. The new method — presented at the American Geophysical Union’s 2012 annual meeting in San Francisco — would use GPS satellites to detect disturbances in the ionosphere.
The satellites find these distortions by watching, of all things, the very atmospheric interference that is a big headache for navigation systems. Researchers normally try to remove the disrupting effects of charged particles in the ionosphere, which delay radio signals between the satellites and the Earth’s surface, said Dorota Grejner-Brzezinska, a geoinformation engineer at Ohio State University. But “that noise became our signal for this particular research,” she said.
A nuclear blast creates a pulse that emanates outward from a central point on the ground creating a travelling ionospheric disturbance — or TID. Earthquakes also create TIDs that emanate from their epicenters, and the TIDs from an underground nuclear blast are similar. But TID waves from an underground nuclear bomb are less powerful and travel through the air more slowly than TID waves from a large earthquake, said Ohio State University postdoctoral researcher Jihye Park, which is one way they can be distinguished. Researchers are working on other methods to separate the signals of bomb-generated TIDs from those of smaller earthquakes.
Last year, Park, Grejner-Brzezinska and their colleagues published a study in Geophysical Research Letters that found TIDs in the area above North Korea in 2006 and 2009, when the country tested nuclear bombs underground.
The newest research confirms the method’s effectiveness by using the Very Large Array (VLA), an observatory in New Mexico that also records radio waves, said Joseph Helmboldt, a radio astronomer with the Naval Research Laboratory in Washington, D.C.
Helmboldt said radio astronomers do the same thing scientists working with GPS do: correct for distortions in the ionosphere. In fact, the array picks up particularly high-resolution information about the ionosphere because the array consists of 27 radio antennae that point in the same direction, but through different portions of the ionosphere.
“If there are fluctuations in the ionosphere induced by something like, say, an explosion, that’s going to distort any image we try to make, because each telescope will see a different part of that disturbance,” he said.
The team examined historical VLA data collected during underground nuclear tests in Nevada — code-named Hunter’s Trophy and Divider — in 1992, which occurred before the United States signed the Comprehensive Nuclear-Test Ban Treaty in 1996. The researchers found disturbances in the ionosphere, just as the new GPS method predicted. This allowed them to compare the data from the GPS and VLA.
“GPS gives you a very large coverage area so that you can track the wave as it moves, and the VLA can zoom in and examine the details of the finer scale fluctuations associated with the wavefront itself,” said Helmboldt.
The researchers said the new method could provide a useful alternative to detecting rogue nuclear tests should other methods, such as seismic detection or atmospheric isotope monitoring, fail to pick up evidence of an explosion.
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