Sun alive with twisters

STORMS of hot gas as wide as the earth and 1,000 times faster than tornadoes have been observed spiralling from the sun's surface, writes Julia Hinde.

Concentrated around the sun's polar regions, these solar tornadoes, which occur as often as once a week, reach speeds of up to 500,000 kilometres an hour.

United Kingdom scientists have observed the phenomena using instruments aboard SOHO, the Solar and Heliospheric Observatory. They believe that the massive tornadoes generate some of the solar winds that constantly drive charged particles into the earth's atmosphere.

SOHO, a joint European Space Agency/Nasa mission launched in November 1995 that has now been extended to 2003, is shining new light on the workings of the sun, from its internal structure and atmosphere to its constant gas eruptions and solar flares.

From its vantage point 1.5 million kilometres out in space, Soho is helping scientists understand why the sun's atmosphere can reach temperatures of millions of degrees centigrade compared with less than 6,000 degrees at the sun's visible surface.

Eric Priest of St Andrews University told a meeting last week that at least part of the heating appears to come from a clash of magnetic field lines around the sun. Atoms split as they are heated, and the charged particles act like current, setting up shifting magnetic fields. Evidence from SOHO suggests that these fields change completely every 40 hours.

"The magnetic field lines tangle like spaghetti in the solar atmosphere and reconnect, causing thousands of explosions every day that release energy into the atmosphere," Professor Priest explained. This in turn heats the atmosphere.

The extension to SOHO's operations means that, having two years ago observed the sun in its quietest state, it will also see it at its most tumultuous, when the number of sunspots on the star's surface rises to about 2,000. In the most recent sunspot "maximum", solar storms caused power failures in Canada and destroyed some satellites.

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