So Hot Right Now…

SDO/Morton, et al

If you happen to be enjoying a sunny day, thank the bright surface of the Sun, known as the photosphere. At a piping hot temperature of about 5,800 K, the photosphere provides nearly all the sunlight Earth receives. But for all its glorious radiance, the photosphere isn’t the hottest part of the Sun. That award goes to the diffuse outer atmosphere of the Sun known as the corona, which has a temperature of more than a million Kelvin. Parts of the corona can be as hot as 20 million Kelvin, which is hotter than the Sun’s core. Of course, the big mystery is why the corona is so hot.

For most of the Sun, the temperature drops as you move away from the center. This makes sense, since it’s nuclear fusion in the core that heats the Sun. So you would expect the corona to be cooler than the surface, but it isn’t. The corona is superheated by some mechanism we haven’t observed. Until now.¹

Part of the reason the corona is so hot is simply that it’s diffuse. The corona is so diffuse we would consider it a vacuum here on Earth, and that makes it much easier to heat than the dense layers of the photosphere. There is also a heating mechanism known as magnetic recombination. The equatorial region of the Sun rotates a bit faster than the polar regions, so the Sun’s magnetic fields get twisted up over time. Eventually, the field lines snap back into alignment, which we call recombination. It’s like snapping a rubber band, and it releases lots of energy. But when we add up all the energy we can account for by this method, it isn’t enough to explain all the coronal heating. There has to be yet another mechanism.

The most popular candidate is an effect known as Alfvén waves. Alfvén waves are low-frequency waves caused by ions within a magnetic field. Since the ions are charged particles, their oscillation also creates a magnetic oscillation as a bit of a feedback loop. You can think of it as a kind of background hum, similar to that of stereo speakers. The idea is that the Alfvén waves transfer quite a bit of energy to the corona. Previous calculations have suggested that this might account for the missing component, but the key word is might.

This new study uses data gathered by the Cryogenic Near-IR Spectro-Polarimeter (Cryo-NIRSP), which is an instrument on the Inouye Solar Telescope. The device uses polarization to image magnetic fields within the corona, and from their data, the team captured images of Alfvén waves. The images confirm the low-frequency waves we’ve known about, but also show higher frequency waves we didn’t know about. These higher frequency waves don’t transfer nearly as much energy to the corona as low frequency waves, but the fact that they are present means that the Sun’s magnetic fields are efficiently transferring energy to the corona. This shifts the likelihood of the Alfvén wave from might to likely. It will take more observations to confirm the process, but the fact that we can now image coronal Alfvén waves means we can better understand how they behave.

So it looks like the mystery of the corona’s temperature is certainly heating up.