By Paul Rincon

Science reporter, BBC News, Belfast

Cannot play media. Sorry you need to have JavaScript enabled on your browser. Advertisement Astronomers have captured the first footage of a solar "tsunami" hurtling through the Sun's atmosphere at over a million kilometres per hour. The event was captured by Nasa's twin Stereo spacecraft designed to make 3D images of our parent star. Naturally, this type of tsunami does not involve water; instead, it is a wave of pressure that travels across the Sun very fast. Details were reported at the UK National Astronomy Meeting in Belfast. In half an hour, we saw the tsunami cover almost the full disc of the Sun

David Long In a solar tsunami, a huge explosion near the Sun, such as a coronal mass ejection or flare, causes a pressure pulse to propagate outwards in a circular pattern. Last year's solar tsunami, which took place on 19 May 2007, lasted for about 35 minutes, reaching peak speeds about 20 minutes after the initial blast. Co-author David Long, from Trinity College Dublin (TCD), Ireland, commented: "The energy released in these explosions is phenomenal; about two billion times the annual world energy consumption in just a fraction of a second. Two Stereo spacecraft are monitoring the Sun's activity "In half an hour, we saw the tsunami cover almost the full disc of the Sun, nearly a million kilometres away from the epicentre." His colleague Dr Peter Gallagher, who is also from TCD, said the shockwave moved out exactly like a tsunami on Earth. "A series of troughs and crests in pressure causes it to propagate outwards. But on the Sun, we have hot gas," he explained. "When Im talking to someone in a room, my voice is carried by pressure waves in the gas that's between us; it's the much the same on the Sun." However, it was not exactly the same, Dr Gallagher added, because on the Sun, magnetic fields also helped the waves along. The phenomenon is therefore known as a magneto-acoustic wave. Theory problem Solar tsunamis were originally discovered by the Soho spacecraft almost a decade ago. However, the observations did not fit at all well with theory: the problem was that the waves were travelling too slowly. After the two Stereo spacecraft launched in 2006, scientists were able to get images of the Sun at a much higher rate than was possible with Soho. And when they observed a solar tsunami again last year, their observations matched theoretical predictions.

Stereo mission guide "We found that the speed was probably twice as fast as we had previously thought," Dr Gallagher told BBC News. "We've seen from this set of observations that if the time interval between images is too long, its easy to underestimate the speed that the waves are moving." With Soho, the researchers were only able to take images in the upper section of the corona - the outer part of the Suns atmosphere. Stereo's Extreme Ultraviolet Imager (EUVI) instruments monitor the Sun at four wavelengths, which allowed astronomers to see how the wave moved through the different layers of the solar atmosphere. "We were able to show for the first time that this wave actually propagates almost all the way from the surface of the Sun to high up in the Sun's atmosphere," said Dr Gallagher. The researchers even saw the pressure wave bouncing off irregular regions of the Suns atmosphere, generating reflections or diffraction patterns - exactly as tsunamis have been observed to do on Earth when they crash against land. Paul.Rincon-INTERNET@bbc.co.uk



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