02 June 2007
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Stephen Battersby

THEY give us brilliant auroras in skies far from the poles. But the outbursts of ionised gas from the sun also have a dark side. They can destroy satellites and knock out power grids with little warning. Now, a technique that uses radio waves to probe these solar storms could give us up to three days' warning of the deadliest ones headed our way.

From time to time the relatively steady solar wind is interrupted by a storm, which hurls billions of tonnes of ionised gas, or plasma, from the sun's outer atmosphere at speeds of millions of kilometres an hour. Most of these coronal mass ejections (CMEs) miss the Earth; others are deflected by our magnetic field - but some are positively dangerous.

Sometimes the polarity of the CME's magnetic field is such that it can merge with Earth's own field, and the writhing fields can disrupt radio communications and bring down power grids. The CME also dumps its cargo of high-energy particles into the upper atmosphere, creating auroras away from the poles, but they can also damage satellites and harm spacewalking astronauts. "It is very important to predict these storms so that we can do something to avoid the damage," says Ying Liu of the Massachusetts Institute of Technology.

While NASA's sun-watching STEREO spacecraft, positioned a million miles from Earth, can spot an approaching storm early on, determining the direction of the storm's magnetic field is tricky. "There are currently no remote ways of measuring the magnetic field between the sun and Earth," says Justin Kasper of MIT. The field can be measured directly when the CME sweeps past STEREO, but by then the storm is only about half an hour away.

So Liu and colleagues at MIT and the University of Michigan at Ann Arbor propose using radio waves from distant stars and galaxies to probe the storm. Many of these radio sources emit polarised radio waves, which tend to vibrate in a certain direction. When these radio waves pass through a CME, its magnetic field can change the direction of polarisation, an effect called Faraday rotation.

Astronomers can work out the direction of the CME's magnetic field from the polarisation of these radio sources. It is no easy task because the field is twisted up, but the researchers have worked out that by looking at several radio sources through the CME, they can calculate the direction of its field. "With a number of lines of sight, you can disentangle both the direction of field and which way it twists," says MIT's John Belcher.

While today's radio telescopes aren't up to the job, the Mileura Wide-Field Array, being built in Australia, is designed to watch many sources across a large area of the sky - perfect for the new technique. It should be ready in time for the next peak in solar activity around 2010, when a few CMEs per month will head for Earth. "If we can determine the magnetic field orientation from Faraday rotation measurements, we will have time to shut off spacecraft and power grids to avoid damage," says Liu.

From issue 2606 of New Scientist magazine, 02 June 2007, page 16
http://space.newscientist.com/article/mg19426064.900-radio-waves-warn-of-imminent-storm.html