Tsunami lit up the heavens
Japan’s recent monster earthquake did more than jolt the island nation and send a tsunami racing across the Pacific Ocean. Hundreds of kilometers overhead, that tsunami also lit up the atmosphere in celestial glowing ripples.
In the first picture of its kind, scientists photographed these “airglow” ripples as they washed over Hawaii hours after the quake. The report will appear in an upcoming Geophysical Research Letters.
“It’s just total serendipity that we got this measurement,” says team leader Jonathan Makela, a space scientist at the University of Illinois at Urbana-Champaign. “It’s a really neat example of how the environment is coupled together.”
When the magnitude-9.0 quake ruptured the seafloor off eastern Japan on March 11, it displaced water that started rushing outward as a tsunami. Over the open ocean those waves were just centimeters high, but that small shift was enough to displace the air above the water’s surface. The result: dense waves of atmospheric particles propagating upward.
Roughly 250 kilometers up in the layer known as the ionosphere, those waves encountered charged and neutral particles, slamming them together to combine and release a bit of energy.
On an ordinary night, skywatchers can see airglow from the odd particle collision in the ionosphere. But the passing tsunami caused more particle collisions than usual, which appeared as undulating waves moving in lockstep with the oceanic tsunami below.
Makela and his colleagues caught the event because their wide-angle camera, which sits atop the Haleakala volcano in Maui, photographs the sky every night looking for ionospheric disturbances.
Other scientists have measured ionospheric ripples from past tsunamis, including the deadly one in the Indian Ocean in 2004. But those studies aren’t true pictures; they measured the time it took global-positioning signals to travel through the ionosphere, and from that deduced whether something interesting was happening.
The Japan quake has given scientists some of the best such measurements yet, because the country is covered so densely with GPS receivers, says Lucie Rolland, a geophysicist at the Institut de Physique du Globe de Paris in France. In a paper to appear in Earth, Planets and Space, she and her colleagues describe seeing ionospheric signals as soon as 10 minutes after the quake.
Cameras might provide a heads-up that a tsunami is on the way, Makela says. Because the camera can photograph airglow ripples about 1,000 kilometers away, it might be able to flag an incoming tsunami even as automated computer programs spend precious minutes crunching data from ocean buoys. Launching a satellite dedicated to airglow measurements could thus complement traditional tsunami warning systems, Makela says.
Such satellites are expensive and aren’t likely to launch anytime soon, says Michael Hickey, an atmospheric scientist at Embry-Riddle Aeronautical University in Daytona Beach, Fla. But the camera images can help scientists better understand tsunamis’ effects on the atmosphere, says Hickey, who last year published a paper describing what the tsunami-generated airglow might look like.
Makela’s team is now considering putting more skyward-looking cameras in tsunami-prone places such as the Indian Ocean.