It is true that I have actually wondered about this. So it goes. Phil Plait offers a much more compelling explanation that I might:
This week, astronomers announced they have found the smoking gun. Almost literally.
† † †
The thing is, while the photosphere is hot, roughly 5,500° C, the corona is freaking hot, 2 million degrees on average. That’s weird. Inside the Sun, the temperature drops as you move out from the center, but that trend reverses, viciously, at the corona.
Why is the corona so hot?
It really is a fascinating question, and is the sort of thing that allows us to ponder phrases like, “ten billion one megaton H-bombs”.
Nor should we overlook this detail:
This new breakthrough was made using several different observatories, including SOHO and the orbiting NuSTAR X-ray observatory (usually used to look at distant black holes, but which is also sensitive enough to see small-scale eruptions on the Sun). Interestingly, EUNIS was launched on a sounding rocket, a suborbital flight (basically, up-and-down) that lasted only 15 minutes! It’s amazing to think that in that short a time, such a long-standing mystery was finally solved.
We might call that a pretty darn good show.
Image note: Detail of slideshow from Klimchuk, et al., “Millions of Tiny Explosions Cause the Sun’s Corona”, 28 April 2015, via Southwest Research Institute Planetary Science Directorate.
Plait, Phil. “A Million H-Bombs per Second Heat the Sun’s Corona”. Slate. 29 April 2015.