BURPS of hot ionised gas from the sun can knock out satellites and power grids when they hit Earth (91av, 21 March, p 31). Till now their arrival has been hard to predict, but the first images of an earthbound burst captured by two satellites simultaneously have shown that we could get warnings 24 hours in advance that trouble is heading our way.
These clouds of plasma, called coronal mass ejections (CMEs), reflect sunlight and can sometimes be seen as expanding haloes around the sun. Unfortunately they are often faint, so CMEs travelling towards Earth are easily confused with material flowing out in other directions.
A demonstration using images from a pair of NASA spacecraft called STEREO () shows the mission can provide early warnings. Launched in 2006, one of the STEREO craft is travelling ahead of Earth in its orbit while the other is lagging behind.
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After a small CME hit Earth on 16 December 2008, Chris Davis of the Rutherford Appleton Laboratory at Harwell, Oxfordshire, UK, and colleagues examined images taken in the preceding few days by wide-angle cameras called Heliospheric Imagers on the STEREO spacecraft.
Though the 16 December CME was too small to cause any damage, it appears clearly in images from each spacecraft (pictured) in the days leading up to its arrival at Earth. “From both spacecraft, we were able to measure an angle which was consistent and showed it was heading towards Earth,” Davis says. With a few upgrades to the imaging software so that relevant data could be analysed in real time, the CME’s impact on Earth could be predicted 24 hours before its arrival, the team says in a paper to be published in Geophysical Research Letters.
“It’s really quite an advance,” says Howard Singer of the Space Weather Prediction Center in Boulder, Colorado, who was not involved in the study. “We’ll be able to use this data and this technique to give forecasts that are far superior to what we’ve had before.”
