The mystery of the sun鈥檚 super-hot atmosphere may lie in giant loops of plasma that are mostly invisible to today鈥檚 solar probes.
Normally, the further you go from a heat source, the cooler it gets. Not so with the sun. Its surface is a sizzling 6000掳C, but the corona, despite being further from the sun鈥檚 nuclear core, reaches more than a million degrees.
Physicists aim to solve this mystery by mapping the coronal loops: streams of hot, glowing plasma that follow magnetic field lines that rise from the sun鈥檚 surface and fall back in. Since 2010, NASA鈥檚 Solar Dynamics Observatory has been providing pictures of these loops. The images show ultraviolet radiation that is emitted by ionised elements, such as iron, in the plasma.
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Missing pieces
Now, at the New Jersey Institute of Technology in Newark and his colleagues have shown that these ions aren鈥檛 distributed uniformly throughout a loop. The researchers analysed the properties of the electric current that flows through the plasma along the loops, as well as its effect on the ions.
They found that regions where the electric current is positive 鈥� meaning it is flowing out of the sun鈥檚 surface 鈥� act as ion traps, creating a high density of ions of heavy elements there, while depleting them elsewhere in the loop.
The implication is that the loops may not be emitting UV light from their entire length, but rather only from regions where the ions are found in high concentration. This could mean we are missing part of the picture, says team member at the University of Minnesota in Minneapolis.
鈥淢aybe there are some loops which do not emit in ultraviolet,鈥� she says. If so, these wouldn鈥檛 show up in UV images. This will have to be accounted for in models that try to explain the extreme temperatures of the corona, says Musset.
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Read more: 鈥楥old鈥� solar loops may help solve corona puzzle
This article appeared in print under the headline 鈥淧lasma loops may explain solar puzzle鈥�
