Coronal hole

Coronal holes are regions of the Sun's corona that emit low levels of ultraviolet and X-ray radiation compared to their surroundings.

They are composed of relatively cool and tenuous plasma permeated by magnetic fields that are open to interplanetary space.

This results in decreased temperature and density of the plasma at the site of a coronal hole, as well as an increased speed in the average solar wind measured in interplanetary space.

Their true nature was recognized in the 1970s, when X-ray telescopes in the Skylab mission were flown above the Earth's atmosphere to reveal the structure of the corona.

The coronal holes then increase in size and number, extending farther from the poles as the Sun moves toward a solar minimum again.

When observed in extreme ultraviolet , coronal holes appear as relatively dark patches in the Sun's corona. Here, there is a large coronal hole in the northern hemisphere.
When the Sun's disk is obscured during a total solar eclipse or by a coronagraph (pictured), coronal structures not otherwise visible can be observed above the limb. [ 3 ]
A coronal hole at the Sun's north pole observed in soft X-ray