Why Is The Sun’s Outer Layer 200 Times Hotter Than Its Surface?

Space





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A solar prominence erupting into space. NASA/SDO/Steele Hill.

The "coronal heating problem" stumped scientists for more than 70 years. Instinctively, the corona, the outermost layer of the Sun, should be the coolest since it is farthest away from the intense nuclear reactions producing heat in the core. However, this couldn't be farther from the truth. Instead of being the coolest layer, the corona is around 200 times hotter than the layer beneath it, the photosphere.