They light up brilliant auroras in mid-latitude skies, but outbursts of ionised gas from the Sun also have a dark side, destroying satellites and knocking out power grids. Now, a technique that uses radio waves to probe solar storms could give us up to three days warning of when the deadliest ones headed our way.
From time to time the relatively steady solar wind is interrupted by a storm, when billions of tonnes of ionised gas, or plasma, are hurled from the Sun鈥檚 outer atmosphere at millions of kilometres an hour. Most of these coronal mass ejections (CMEs) miss the Earth, and others are deflected by our magnetic field, but some are more dangerous.
If the magnetic field of the CME points southwards, it can merge with Earth鈥檚 own field, and the writhing fields can disrupt radio communications and bring down power grids. The CME also offloads its cargo of high-energy particles into the upper atmosphere, which create the auroras, but can also damage satellites and even endanger astronauts.
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鈥淚t is very important to predict or forecast these storms so that we can do something to avoid the damage,鈥� says Ying Liu of the Massachusetts Institute of Technology.
While NASA鈥檚 Sun-watching STEREO spacecraft, positioned a million miles from Earth, can spot an approaching storm early on, determining the direction of the storm鈥檚 magnetic field is tricky. 鈥淭here are currently no remote ways of measuring the magnetic field between the Sun and Earth,鈥� 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 sources emit polarised radio waves; that is they tend to vibrate in a certain direction, and the magnetic field in a CME changes the direction of polarisation, an effect called Faraday rotation.
In theory, astronomers can work out the direction of the CME鈥檚 magnetic field from the polarisation of these radio sources. It is no easy task, because the field within a CME is twisted, but the team have worked out how to do it by looking at several different radio sources through the CME. 鈥淲ith a number of lines of sight, you can disentangle both the direction of field and which way it twists,鈥� says MIT鈥檚 John Belcher.
Today鈥檚 radio telescopes aren鈥檛 up to the job, but the Mileura Wide-Field Array, being built in Australia, is designed to watch many sources across a large area of the sky 鈥� perfect for the new technique. It should be ready in time for the next peak in solar activity around 2010, when the Sun will be spitting out as many as four CMEs a day. A few per month will head for Earth.
鈥淚f we can determine the magnetic field orientation from Faraday rotation measurements, we will have time to shut off spacecraft and power grids to avoid the damage brought about by the storms,鈥� says Liu.
Radio waves could also warn against another dangerous aspect of solar storms. Fast-moving CMEs can generate strong shockwaves in the solar wind, causing unpleasant space weather even if the CME鈥檚 magnetic field isn鈥檛 pointing south.
New research shows that these storms give off a radio 鈥渟cream鈥� as they plunge towards Earth, which can be used to distinguish them from harmless outbursts that appear superficially similar.
This telltale scream was discovered by a team led by Natchimuthuk Gopalswamy of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, US, who presented their results a meeting of the American Astronomical Society in Honolulu, Hawaii, US.
