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. |
Researchers
at the University of Illinois have become the first to record an
airglow signature in the upper atmosphere produced by a tsunami using a
camera system based in Maui, Hawaii.
The
signature, caused by the March 11, 2011 earthquake that devastated Japan, was
observed in an airglow layer 250 km above the earth’s surface.
It preceded the tsunami by one hour, suggesting that the technology
could be used as an early-warning system in the future. The findings
were recently published in Geophysical Research
Letters.
The
observation confirms a theory developed in the 1970s that the signature
of tsunamis could be observed in the upper atmosphere, specifically the
ionosphere. But until now, it had only been demonstrated using radio
signals broadcast by satellites.
“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,” says 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—in
this case, the airglow layer. As a tsunami moves across the ocean, it
produces atmospheric gravity waves forced by centimeter-level surface
undulations. The amplitude of the waves can reach several kilometers
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. It was approaching dawn (nearly 2:00
a.m. local time) with no sun, moon or clouds obstructing the view of the
night sky.
Along
with graduate student Thomas Gehrels, Makela analyzed 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, confirming that the pattern
originated from the tsunami. 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. In addition, some areas, such as 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. However, the sun, moon,
and clouds can limit the utility of camera measurements from the ground.
By flying a camera system on a geo-stationary satellite in space,
scientists would be able to avoid these limitations while simultaneously
imaging a much larger region of the earth.
To
create a reliable system, Makela says that scientists would have to
develop algorithms that could analyze 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.
“This
is a reminder of how interconnected our environment it,” Makela says. “This technique provides a powerful new tool to study the coupling of
the ocean and atmosphere and how tsunamis propagate across the open
ocean.”
