Let's first take a look at the magnetic field of the Sun.
The Sun is just like a large bar magnet. At the poles you can see that the magnetic "field lines" don't seem to have an end. We call these " open field lines." The ones which start and end on the Sun's surface are called " closed field lines". Coronal holes are found where there are large regions of these so-called open field lines, that is at the poles of the Sun. Place your mouse pointer over the green words to see the open and closed fields.
By looking at the Sun in different wavelengths (move the cursor over the bar below the image on the left), such as infrared, ultraviolet and X-rays, we can see the temperature at which coronal holes become visible. Coronal holes can be seen in pictures of the Sun which show the hot gas in the corona. They are not visible in the cooler gases from lower in the solar atmosphere. This means the gas in the solar wind is flowing from the solar corona, so we see a hole where the corona is almost empty.
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