Atmospheric Tsunami Airglow Signature

The airglow preceded the tsunami by one hour, suggesting that the technology could be used as an early-warning system in the future. The observation confirms a theory developed in the 1970s that the signature of tsunamis could be observed in the upper atmosphere, specifically the ionosphere. "Imaging the response using the airglow is much more difficult because the window of opportunity for making the observations is so narrow, and had never been achieved before," said Jonathan Makela, an associate professor of electrical and computer engineering and researcher in the Coordinated Science Laboratory. "Our camera happened to be in the right place at the right time."

Tsunamis can generate appreciable wave amplitudes in the upper atmosphere, which in this case was the airglow layer. As a tsunami moves across the ocean, it produces atmospheric gravity waves forced by centimetre-level surface undulations. The amplitude of the waves can reach several kilometres where the neutral atmosphere coexists with the plasma in the ionosphere, causing perturbations that can be imaged.

On the night of the tsunami, conditions above Hawaii for viewing the airglow signature were optimal. Along with graduate student Thomas Gehrels, Makela analysed the images and was able to isolate specific wave periods and orientations. In collaboration with researchers at the Institut de Physique du Globe de Paris, CEA-DAM-DIF in France, Instituto Nacional de Pesquisais Espaciais (INPE) in Brazil, Cornell University in Ithaca, NY, and NOVELTIS in France, the researchers found that the wave properties matched those in the ocean-level tsunami measurements. The team also cross-checked their data against theoretical models and measurements made using GPS receivers.

Makela believes that camera systems could be a significant aid in creating an early warning system for tsunamis. Currently, scientists rely on ocean-based buoys and models to track and predict the path of a tsunami. Previous upper atmospheric measurements of the tsunami signature relied on GPS measurements, which are limited by the number of data points that can be obtained, making it difficult to create an image. It would take more than 1,000 GPS receivers to capture comparable data to that of one camera system. Some areas, like Hawaii, don't have enough landmass to accumulate the number of GPS units it would take to image horizon to horizon.

In contrast, one camera can image the entire sky. Flying a camera system on a geo-stationary satellite in space, scientists would be able to avoid limitations like clouds while simultaneously imaging a much larger region of Earth.

To create a reliable system, Makela says that scientists would have to develop algorithms that could analyse and filter data in real-time. And the best solution would also include a network of ground-based cameras and GPS receivers working with the satellite-based system to combine the individual strengths of each measurement technique.

Image: Airglow waves captured by the Illinois imaging system over Hawaii. The red line represents the location of the ocean-level tsunami at the time of the image. (Image Courtesy: Image courtesy of University of Illinois College of Engineering)