Moon's shadow creates a wake
Oct 12, 2011 3 comments
During a total solar eclipse the Moon comes directly between the Sun and the Earth, casting a dark shadow that moves across land and sea. Now, researchers in Taiwan and Japan have shown that this shadow creates a pocket of high-pressure air that cuts through the atmosphere much like a boat through water – leaving a discernible wake. As well as confirming a 40-year-old prediction, the discovery could have implications for how nuclear tests are monitored.
Along with plunging a region into darkness, an eclipse also causes a sudden cooling of the atmosphere. The effect this has on atmospheric pressure is complicated and not properly understood. Some places cool faster than others, creating regions where the pressure increases and regions where it decreases.
Jianlin Liu of the National Central University in Taiwan and colleagues have used Global Positioning System (GPS) technology to confirm a 40-year-old prediction that "shadow boats" are created in the atmosphere during an eclipse. These are thought to be pockets of high-pressure air directly under the Moon's shadow that push their way through low-pressure air much like a boat pushing through water.
Bow and stern waves
Indeed, the phenomenon can be understood in terms of a toy boat in a bathtub. If the boat is dropped in the water, ripples will spread out at a fixed speed. If the boat is moved forward, it generates waves at its bow (front) as it pushes water out of the way and at its stern (back) as water rushes in to fill the space behind. If the boat is pushed faster than waves propagate through the water, successive wavefronts will pile up and the waves grow until they become unstable and break.
In 1970 George Chimonas and Colin Hines at the University of Toronto used computer models of the atmosphere to predict that, during a solar eclipse, two pockets of high-pressure air would be created, travelling at over 3200 km/h – one at 30 km above ground level, the other at an altitude of 80 km. Since this is much faster than the speed of sound in air, these "shadow boats" would create bow and stern waves in the atmosphere.
That same year two physicists at Stanford University reported possible evidence of these pressure waves – known as acoustic gravity waves – originating from a solar eclipse. But acoustic gravity waves can be caused by many sources, such as earthquakes, nuclear explosions and even thunderstorms. Therefore a lot of data and sophisticated mathematical modelling are needed to say that a particular set of waves definitely came from a particular source – and over the next four decades scientists had not been able to unambiguously identify acoustic gravity waves from a solar eclipse.
Then, on 22 July 2009, researchers got lucky. Between 10 a.m. and 11 a.m. a total solar eclipse crossed Japan and Taiwan – regions that are covered by dense networks of ground-based GPS receivers. Liu and colleagues were on hand to record the event using about 13,000 GPS receivers and analyse the data collected.
The team used the GPS signals to map fluctuations in the total electron content (TEC) of the ionosphere – the upper part of the atmosphere above about 85 km in altitude. The TEC is related to atmospheric pressure, allowing the researchers to see distinct bow and stern waves from the shadow boats for the first time. They measured an interval of about 30 minutes between the bow and stern waves, which allowed them to calculate that the shadow boats were about 1700 km long.
While bow and stern waves have finally been observed, Liu is cautious about saying that the standard explanation for the effect – pockets of high pressure forcing their way through regions of reduced pressure – is beyond doubt. "Most likely we think it's regions of high pressure but we don't have any exact numbers for that. What we observed are the facts; we still need time and effort to work out what really happened in that atmosphere," he explained.
Jean-Bernard Minster, a geophysicist at the Scripps Institute of Oceanography in California believes that the work's principal importance lies not in what it tells us about solar eclipses but in its advances in monitoring the ionosphere. "From the point of view of monitoring the nuclear test ban treaty, being able to understand what ionospheric disturbances look like and what their sources may be is really important."
The work is described in Geophys. Res. Lett. 38 L17109.
About the author
Timothy Wogan is a science writer based in the UK