Why is the solar corona so hot?

Our Expert Voices conversation about the 2017 solar eclipse.

The solar corona — the outer atmosphere of the Sun that is revealed so spectacularly during a total solar eclipse — is one of the great mysteries in space science. The biggest puzzle of all is its extreme temperature of several million degrees. Why is the corona so hot when the solar surface below is a comparatively paltry 6,000 degrees?

Most solar physicists believe the primary source of the heat is stressed magnetic fields that permeate the corona and impulsively release pent up energy when the fields reach their breaking point. Occasionally this produces giant explosions called solar flares and coronal mass ejections. But much smaller nanoflares are happening all the time — about a million per second across the Sun — each equivalent to the detonation of a 50 megaton hydrogen bomb.

Why it matters: The corona produces large quantities of X-ray and ultra-violet radiation that are absorbed in Earth's upper atmosphere. Variations in radiation affect the propagation of radio signals, which adversely affects communication, navigation, surveillance, and precision weapons guidance systems. The upcoming solar eclipse affords us a rare view of the Sun's atmosphere and a chance to study how it gives rise to nanoflares so that we can improve space weather forecasts.