THE sun鈥檚 million-degree outer atmosphere is the last place you would expect to find rain, yet a form of it does occur there. The stuff could help explain why the sun鈥檚 outer atmosphere, or corona, is much hotter than closer in.
Coronal rain is made of dense knots thousands of kilometres across consisting of relatively cold gas, at tens or hundreds of thousands of degrees C, which pours down towards the sun鈥檚 visible surface from the outer atmosphere at speeds exceeding 100 kilometres per second. 鈥淭here鈥檚 just this constant rain of these blobs that seem to be coming down from high up,鈥� says Judy Karpen of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland.
Now simulations seem to show that the coronal rain is a result of the process that makes the corona so hot. Two theories have previously been put forward to explain the anomaly. One suggests the corona is heated via small explosions called nanoflares lower in the atmosphere. These would push gas up into the corona, where it radiates away its energy. The other suggests the heat energy is deposited by magnetic waves rippling through the corona.
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When Patrick Antolin and Kazunari Shibata of Kyoto University, Japan, simulated the two processes, they found gas heated from below by nanoflares could cool and condense higher up to make the rain, whereas the magnetic waves kept the high-altitude gas too hot to condense.
鈥淚t鈥檚 a little bit like raindrops condensing,鈥� says Daniel M眉ller, a European Space Agency scientist based at the NASA Goddard Space Flight Center. The gas rises 鈥渓ike steam evaporating from a boiling pot of water,鈥�, he says. 鈥淭hen it cools down and when it gets really dense it forms these blob- like features.鈥�
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