Since 1975, earth scientists have theorized that the passage of a tsunami would make the ionosphere glow, and now they have observed it 125 miles above the surface, using a camera perched atop Haleakala.
Making it into a practical tsunami detector will take more work, but the demonstration is one of several suggestions for using technology already existing to improve tsunami tracking - and perhaps make it cheaper.
Another is to track the voltages induced in communication cables on the seafloor as the pressure changes as the tsunami passes.
And a third, which grew out of the study of the second, could be to measure magnetic changes in the cables.
Researchers were primed for a real-world test of the airglow theory, said professor J.J. Makela of the Electrical and Computer Engineering Department at the University of Illinois.
A colleague in France, Philippe Lognonne, heard about the big earthquake in Japan on March 11 and quickly alerted Makela to tune the Cornell All-Sky Imager at Science City to look for the glow.
Makela could do that remotely, and the detector, which is at the Haleakala High Altitude Observatory, was calibrated and adjusted. It is not a telescope but a direct-observation instrument.
In the past, mechanical problems or timing had forestalled a test of the theory. Using a ground-based detector, the quake has to happen in the daytime or on a clear night for the detector to make the proper observations.
The theory said that the passage of the wave through the water would compress the air above. Although the open ocean tsunami is measured in inches, as the pressure wave propagates up through the atmosphere, and the density drops, it induces electron changes in oxygen molecules that can be measured with an instrument, although the glow is much too faint to see with the naked eye.
The other needed ingredient is a way of fixing the location of the glow, for which GPS stations will serve.
Some years ago, the very dense GPS network in Japan had been used to locate airglow (not caused by tsunami). Makela said that since Hawaii, where his detector sits, has only about 50 research-quality GPS stations, the trick was to get sufficient accuracy and resolution that would confirm that the glow above was really related to the water movement below.
On March 11, they did it. Airglow is caused by many things, and Makela's research has been primarily intended to help deal with interference with electronic communications from plasma bubbles and other effects in the ionosphere.
Analyzing the graphs, they detected wavefronts that came from different directions and had different strengths, some even perhaps originating from pre-rupture tectonic events.
These, unfortunately, could not be used to give advance warning of the earthquake, because there is a lag of about 40 minutes before the quake events start showing up in the upper atmosphere.
But by matching the airglow with sensors off the Big Island used for tsunami prediction, Makela and his associates were able to confirm that one among the many waves was signaling the advance of the tsunami wave.
It could give a warning about one hour in advance of the arrival of the wave's energy, which when it hits shallow water transforms into crashing, sometimes killing walls of water.
Since land-based detectors like the Cornell All-Sky Imager cannot be distributed over the ocean, and since they work only in daytime or good weather, the Haleakala instrument would not itself be useful in tsunami prediction. But now that it has proved the theory, it opens the prospect of satellite-based detectors. From stationary locations in orbit these could observe big regions and be indifferent to weather.
Advances in tsunami predicting are being made under the sea as well.
In early 2010, Manoj Nair at the University of Colorado and his colleagues proposed that the passage of a tsunami should induce changes on the order of 500 millivolts in the copper wire in seafloor cables.
As a tsunami detector, this would have the advantage, compared with airglow, of being precisely located, but subsequent research has proved that the situation is more complex than Nair's team had thought at first.
Also, he said in a telephone interview last week, it has proved difficult to run a practical test. However, with the cooperation of a cable company, they were able to make a demonstration in the Caribbean. It lasted only 24 hours, "and you cannot expect a tsunami in that short a time," but it did confirm the voltage fluctuations. In fact, it turned up several kinds, some of them stronger than those to be expected from a tsunami passing by.
"It is much more complex that what we thought," he said. But the components probably can be sorted out.
Since then, however, Nair's group successfully observed magnetic changes in seafloor cables induced by the passage of the tsunami from the big Chilean quake of February 2010.
This effect had been predicted even further back than the tsunami-airglow link, but it had not been observed because the flux was so small.
Now magnetometers are so much better - and so cheap, under $2,000 - that they can find the change, which is only one nanotesla. (The very low solar sunspot activity also helped.)
The magnetometer at Easter Island matched the tide gauge there, proving the link to the tsunami.
Like the Maui imager, the magnetometers need islands to sit on, but since they are so cheap, "you could have a lot of them," and even poor countries could afford them, Nair said.
Makela said he was not familiar with the Nair studies, but the more paths of detection, the better. "It's a little extra butter on the bread," he said.
* Harry Eagar can be reached at firstname.lastname@example.org.