sun's corona: is the outer a part of the sun's atmosphere.

It extends from the visible surface of the sun to many thousands of kilometers as it slowly converts into the solar wind. That makes it difficult to see without using instruments. 

The corona is regarding ten million times less dense than the sun's surface. this denseness makes the corona a lot of fewer brights than the surface of the sun.

Astronomers are attempting to resolve this mystery for an extended time. The corona is within the outer layer of the sun's atmosphere-far from its surface. Yet, the corona is many times hotter than the sun's surface.