Coronal holes are usually found at the Sun's north and south poles. Let's find out why. The Sun acts like a giant bar magnet. It is surrounded by a huge magnetic field. The red lines with arrows in these diagrams illustrate the magnetic field around a bar magnetic and show how the magnetic field around the Sun is very similar.
Remember, the Sun's corona is a plasma made up of ions - particles with an electric charge. These are always forced to move in the direction of the magnetic field.
Where the magnetic field loops back, ending up at the south pole, we refer to it as a 'closed' magnetic field. In a closed magnetic field the plasma is trapped and cannot flow into space.
A 'closed' magnetic field
Notice that at both the Sun's magnetic poles, the magnetic field spreads out and doesn't loop back to the other pole. We call this an 'open' magnetic field. In an open magnetic field the plasma can escape to form the solar wind.
An 'open' magnetic field
The above image is from EIT. It shows
a coronal hole at the north pole of the Sun.
Here is an
EIT movie in the UV showing the rotating
Sun at times of low and high activity.
Notice how the coronal
holes (dark areas) on the left image
rotate with the Sun and keep the same shape
for many days.