The hottest part of a fire is near its middle, so you’d expect the same to hold true for big balls of burning gas – like the Sun.
But that’s not the case. The Sun’s outer atmosphere, or corona, is a few million degrees in temperature, 200 to 500 times hotter than the visible surface, which comes in at 5,500°C (10,000°F).
Now, scientists say they are closer to working out why this is the case. Although several factors are thought to be at play, recent observations by a NASA space observatory called the Interface Region Imaging Spectrograph (IRIS) suggest that “heat bombs”, or nanoflares, might be a key factor.
The observations suggest that when the Sun’s magnetic fields cross over each other, they can realign and explode as a miniature solar flare. When this happens, the heat can spread very rapidly over a large region, so the events can be hard to spot – but IRIS managed to catch a glimpse.
“Because IRIS can resolve the transition [between the corona and surface] region 10 times better than previous instruments, we were able to see hot material rushing up and down magnetic fields in the low corona,” said Paola Testa of the Harvard-Smithsonian Center for Astrophysics, lead author of a new paper describing the findings, in a video. “This is compatible with models from the University of Oslo, in which magnetic reconnection sets off heat bombs in the corona.”
It’s still possible that there are other heating mechanisms at work, but scientists are now hoping to figure out how much is being caused by these heat bombs and solve a puzzle that has perplexed astronomers since it was first reported in the 1940s.