Question

Why is the corona of the sun rarely visible?

Answer 1

The corona is only a millionth of the brightness of the photosphere of the Sun.

Explanation:

Although the corona is very large and hot, it is only a millionth of the brightness of the photosphere of the Sun. In order to see the corona, the photosphere needs to be covered as is the case of a solar eclipse.

There are devices called coronagraphs which simulate an eclipse to make the corona visible. These don't work too well in the atmosphere as the light from the sun gets scattered and obscures the corona. They work better in space and on telescopes on high mountain peaks.