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ESA’s Cluster mission has been tuning in to the Earth’s aurora, listening out for an emission that may help the search for alien worlds in the future. The emission may sound a lot like the signature chirping of Star Wars robot R2-D2 (listen here), but it is in fact Auroral Kilometric Emission (AKR), an intense radio outflow 10,000 times more powerful than the strongest military radar signal, that escapes outwards from the Earth’s auroral regions. It is generated high above the Earth by the same flow of solar particles that causes the aurora to light up the sky beneath.
Artist impression of the Cluster 'constellation' flying between 19,000 and 119,000 kilometres above the Earth to study the interaction between solar wind and the Earth's magnetosphere. Image: ESA. For many decades astronomers thought that these radio waves travelled out into space in an ever-widening cone, just like light emitted from a torch. But thanks to the keen ear of Cluster, and the analysis of around 12,000 separate bursts of AKR, they now know that the emission is beamed in a narrow plane, the equivalent of placing a mask over a torch with just a small slit in the middle for light to escape. "We can now determine exactly where the emission is coming from," says Robert Mutel of the University of Iowa, who conducted the three-year study with colleagues. "This result was only possible because of the Cluster mission's four spacecraft."
Graphic showing the location of the four Cluster satellites as the AKR washes over them. Because the satellites are flying in formation, they can be used to triangulate the position of origin of the AKR pulses. Image: ESA/Mutel et al 2003. Cluster’s spacecraft fly in formation, which allows the time of arrival of the AKR at each spacecraft to be precisely determined and triangulated in a similar manner to the way in which GPS operates. The point of origin of each AKR burst was tracked down to a region in the Earth's magnetic field just a few tens of kilometres in size, a few thousand kilometres above where the light of the aurora is formed. And no matter where you are in the Solar System, whenever you have aurora, you have AKR. Aurorae and AKR have been detected at Jupiter and Saturn, but scientists think that it will also be possible to identify similar signatures around extrasolar planets too, although this will require much larger radio telescopes than are currently available. The ‘winking’ on and off of the AKR around these extrasolar planets would tell astronomers how long the planet takes to rotate, as well as helping them to understand the magnetic environment of all stars and planets where aurorae occur. To listen the Earth's chirps and whistles, visit the ESA website.
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