Published online 14 September 2010 | Nature | doi:10.1038/news.2010.467

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Tsunamis leave ionosphere all shook up

Progress of waves through open sea sends vibrations that magnify with height up the entire atmospheric column.

tsunami damageResearchers hope measuring atmospheric waves will improve early warning of big tsunamis such as the one generated by a February earthquake in Chile.MARCELO HERNANDEZ/dpa/Corbis

The signals of GPS satellites could be used to monitor tsunamis as they sweep across the ocean. In the most detailed study to date of the effect, scientists have shown that even though open ocean tsunami waves are only a few centimetres high, they are powerful enough to create atmospheric vibrations extending all the way to the ionosphere, 300 kilometres up in the atmosphere.

The finding, the researchers hope, could hugely improve tsunami early-warning systems.

In a study published online on 1 September in Geophysical Research Letters1, a team of French geophysicists was able to use these ionospheric effects to trace the progress of three recent tsunamis, including the one triggered by the 27 February earthquake in Chile, which had a magnitude of 8.8. The researchers showed that the strength of the ionospheric effects increased with the height of the wave.

The maximum height of that tsunami, which swept across the Pacific, was only 10 centimetres in mid-ocean, but low-lying tsunami waves can be more than 100 kilometres long. During a tsunami, hundreds of square kilometres of ocean rise and fall, nearly in unison. This produces a rhythmic movement in the atmosphere, generating a vertically propagating wave known as an internal gravity wave. The thinning air causes the wave to spread out vertically and the air movements become larger.

"At around 300–350 kilometres of altitude, the atmospheric wave has been amplified by a factor of 10,000 or more," says Lucie Rolland, a graduate student at the Paris Institute of Geophysics, whose PhD work spearheaded the study. "This means that a 10-centimetre tsunami wave at ocean level will induce atmospheric displacement reaching 1 kilometre."

Fluctuating signals

Rolland says that the collision of this wave with the ionosphere — an upper layer of the atmosphere in which incoming solar radiation has ionized atmospheric gases — compresses it by as much as 10%. That produces a corresponding change in the density of the ionosphere's free electrons. This change is enough to affect the signals from Global Positioning System (GPS) satellites — making it appear on GPS receivers as if surface locations are fluctuating by a few centimetres, in rhythm with the passage of tsunami-induced waves through the upper atmosphere.

By looking at data from numerous GPS stations in Hawaii, the scientists were able to filter out other, localized, fluctuations in the ionosphere. And by looking at signals from GPS satellites in different locations, they were able to map the progress of the ionospheric electron 'wave' as it raced across the sky, about 10 minutes behind the tsunami.

Ultimately, the goal is improved monitoring of tsunamis in transit. Because the technique allows waves to be seen hundreds of kilometres before they hit land, the study concludes that remote sensing "could be particularly decisive for tsunami warning and damage limitations".

Giant leap to applicability

But other scientists are not convinced that the study represents a dramatic advance. It has been suspected since the 1970s that tsunamis can produce ionospheric effects2. And, says Emile Okal, a geophysicist at Northwestern University in Evanston, Illinois, such effects have been observed previously — after the 2004 Sumatran earthquake3.

"I am not trying to be down on the authors, whose work is good and rather unique," says Okal. "But I see neither a spectacular idea nor a definitive observational advance. We have known for a long time that there was potential there, but the giant leap to applicability remains to be achieved."

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Costas Synolakis, director of the Tsunami Research Center at the University of Southern California in Los Angeles, agrees. He applauds the study for producing the "most complete evidence to date" that tsunamis trigger upwards-propagating atmospheric waves. But, he says, "whether remote sensing of the ionosphere can be 'particularly decisive' in tsunami warnings is a leap of faith".

Rolland acknowledges that the ability to produce better warnings still lies in the future. To begin with, she says, the impact of a tsunami is strongly affected by the state of the ionosphere and Earth's overall magnetic field. But that doesn't mean that more precise monitoring isn't on the horizon.

"This is a work in progress," she says. "But we hope that this technique will be used in the near future for better monitoring of the tsunami, and therefore better mitigation of risk." 

  • References

    1. Rolland, L. M., Occhipinti, G., Lognonné, P. & Loevenbruck, A. Geophys. Res. Lett. 37, L17101 (2010). | Article
    2. Peltier, W. R. & Hines, C. O. J. Geophys. Res. 81, 1995-2000 (1976). | Article
    3. Liu, J.-Y. et al. J. Geophys. Res. 111, A05303 (2006). | Article

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  • #60934

    I think you will have to shoot the radio waves into the sky to excite the atoms in the ionosphere causing them to move at a more accelerated rate, and as we learned in physics class with atoms moving at a faster rate they get heated up causing it to rise because hot air rises.

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