THE surface of the sun is a pretty cool place. At least when you compare it with the corona, the sun's upper atmosphere, which is nearly 400 times hotter.
This huge temperature difference has long been a mystery, but a newly discovered feature of the sun's magnetic field may help us get to the bottom of things.
The sun's tangled magnetic field includes huge loops that arch from the surface into the corona. We can see the loops because bright surface plasma flows along their curves.
Richard Frazin at the University of Michigan, Ann Arbor, and his team were measuring the temperature of coronal loops during a solar minimum, a quiet phase which sees fewer sunspots and flares. They expected all loops to get hotter with height, since that was the case in measurements taken during more active phases.
Surprisingly, some of the loops nearest the sun's equator got colder near their tops. Frazin thinks these newly found "down loops" exist throughout the solar cycle and may be a symptom of whatever causes coronal heating (The Astrophysical Journal, DOI: 10.1088/0004-637x/755/2/86).
Clare Parnell at the University of St Andrews, UK, agrees that down loops are a new factor that any solar-heating model will need to include.
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