Why is the sun’s corona so hot?

The sun is hot. It’s literally blazing hot. Nothing surprising there. Yet, one solar feature, the corona, is much hotter than it should be. For seven decades, physicists have been trying to understand this ‘coronal heating problem’. Thanks to work by Michael Hahn and Daniel Savin from Columbia University, we may have finally cracked it.

The sun’s core clocks in at 15 million degrees Kelvin. As you move outward, the temperature cools down until it’s a mere 6000 degrees at the surface. However, the temperature of the gas in the corona above the solar surface is over a million degrees. This is counterintuitive, to say the least. What could possibly be heating up the corona like that?

There were two leading contenders for the solution, both involving energy transfer via magnetic waves. In one hypothesis, magnetic fields loop across the solar surface, in the other, waves carry energy up from below the surface. Hahn and Savin showed that the latter explanation is most likely correct. Not only do these waves contain enough energy to heat the corona to high temperatures, but they also deposit that energy in the lower reaches of the corona where the solar wind can spread the heat around the entire sun.

There are still mysteries to be solved, including why these waves (known as Alfvén waves) dissipate at such low heights. Hahn and Savin plan to work on that next.

Michael Hahn, & Daniel Savin (2013). Observational Quantification of the Energy Dissipated by Alfvén Waves in a Polar Coronal Hole: Evidence that Waves Drive the Fast Solar Wind The Astrophysical Journal. DOI: 10.1088/0004-637X/776/2/78.