ALFVEN WAVES

The visible surface of the sun, or the photosphere, is around 6,000°C. But a few thousand kilometers above it - a small distance when we consider the radius of the sun is 700,000 km - the solar atmosphere, also called the corona, is hundreds of times hotter, reaching a million degrees celsius or higher. In 1942, the Swedish physicist Hannes Alfvén proposed an explanation based on his fundamental work in magneto-hydrodynamics, something he received the 1970 Nobel Prize in Physics for. This theory was recently empirically verified for the first time.

https://phys.org/news/2021-05-sun-atmosphere-hundreds-hotter-surface.html