Colloquium
Why is the Solar Corona so Hot?

Abstract

This fundamental question has challenged space scientists for decades. At temperatures of several million degrees, the corona is hundreds of times hotter than the solar surface, and heat cannot simply flow upward against the temperature gradient. (The same is true on other stars.) It is widely believed that the energy responsible for the extreme temperatures is extracted from stressed magnetic fields that permeate the corona. This likely occurs in the form of small impulsive energy bursts called nanoflares, but the details of how they work are still a matter of vigorous debate. Understanding these details is crucial, since the basic mechanisms are central to many phenomena--on the Sun, within the heliosphere, and throughout the universe. I will review our current understanding of the coronal heating problem from both the observational and theoretical perspectives.