The sun’s visible surface is around 5,500 ºC – but if you move away, the temperature soars millions of degrees. Why?
Observations by NASA’s Interface Region Imaging Spectrograph (IRIS) spacecraft suggest that “heat bombs” are going off in the sun’s outer atmosphere, helping to explain why the solar corona is so mysteriously hot.
These heat bombs arise when magnetic field lines cross and realign and explode like a miniature solar flare. The heat rapidly spreads over a large area throughout the corona.
Check out the video about for more about the sun and IRIS mission.