It is a scene to watch - the dark shadow move across the land and sea when the moon comes in front of the sun and the earth during a total solar eclipse.

Jianlin Liu of the National Central University in Taiwan and colleagues used Global Positioning System (GPS) technology to confirm a four decade old prediction that this shadow creates a pocket of high pressure air that cuts through the upper atmosphere similar to a boat moving through water.

THE PHENOMENON –IN REAL

To understand this effect, imagine the childhood pastime of watching the ripples created by a paper boat dropped in a water tub. The ripples propagate at a constant speed. Now if the boat travels forward, the front part (bow) pushes the water out of its way and the back part (stern) accommodates that ‘pushed out’ water. Hence, waves are created on both sides. If the boat travels at a speed more than the speed of these waves in water, successive wavefronts are formed and the waves grow until they are unstable and then break. Try it!!

In 1970 George Chimonas and Colin Hines at the University of Toronto predicted that during a total solar eclipse, two pockets of high pressure air are created – at an altitude of 30 km and 80 km. The speed of this air is 3200km/h – more than the speed of sound in air, thereby creating ‘bow and stern’ waves in the upper atmosphere – waves that were christened “acoustic gravity waves” by two Stanford physicists later that year who reported to have found evidence of such waves during a total solar eclipse. The problem was there can be various other reasons for the formation of acoustic gravity waves viz. earthquakes, nuclear explosions and even common phenomena like thunderstorms. Moreover, Chimonas, Hines and the Stanford physicists had used computer models in their predictions. Complex mathematical modelling is required to pinpoint a singular source of a set of acoustic gravity waves. Four decades were to pass before an unambiguous proof (of waves originating due to a total solar eclipse) came to light.

BREAKTHROUGH…FINALLY

On July 22, 2009, while the world was enjoying the longest total solar eclipse of the 21^st century, Liu and his team of researchers were observing its effect on and analysing the data from 13000 ground based GPS receivers – a part of the dense GPS network in the region between Japan and Taiwan.

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. An interval of about 30 minutes between the bow and stern waves was measured, allowing them to calculate that the shadow boats were about 1700 km long.