Loops in the sun’s plasma may reveal how its corona gets so hot

[Lance]

The mystery of the sun’s super-hot atmosphere may lie in giant loops of plasma that are mostly invisible to today’s 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°C, but the corona, despite being further from the sun’s 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’s surface and fall back in. Since 2010, NASA’s 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.

Missing pieces

Now, Gregory Fleishman at the New Jersey Institute of Technology in Newark and his colleagues have shown that these ions aren’t 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 – meaning it is flowing out of the sun’s surface – 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 Sophie Musset at the University of Minnesota in Minneapolis.

“Maybe there are some loops which do not emit in ultraviolet,” she says. If so, these wouldn’t 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.

Reference: arxiv.org/abs/1803.02851

This article appeared in print under the headline "Plasma loops may explain solar puzzle"

Anil Ananthaswamy

[LeslieDean]

I hate hot Coronas. . They taste so nasty.