When is a hole not really a hole…when it is a solar coronal hole!
You may not know that the Sun has a magnetic field. Just about everything that we observe on the Sun is related to this magnetic field. However, unlike the Earth’s dipole magnetic field, the Sun’s field is chaotic and random, because the Sun is not solid at all, but a rotating, turbulent mass of ionized gas AKA plasma.
The magnetic field plays a large part in the variable flow of energy from the Sun’s nuclear fusion in the core outwards. This variability results in varying temperatures. In visible light, cooler temperatures are darker, while higher temperatures are brighter, often bright white. This dark-bright convention is followed when digital solar images captured in non-visible wavelengths are converted into visible colors.
The portion of the Sun that we observe safely via visible light instruments is called the photosphere. The portion of the Sun’s atmosphere outside this is the corona. We can observe the corona ONLY during the totality of a solar eclipse or with the use of instruments that simulate an eclipse or are sensitive to the UV or X-ray wavelengths.
The magnetic field flows between areas on the Sun that have opposite polarity, i.e. positive and negative. This is a ‘closed field.’ However, a magnetic field can be ‘open,’ i.e. extending out into the corona and beyond. This carries large amounts of energy along the field lines away from the Sun. This is termed a coronal hole.
It is not really a hole. Rather, it is an area of the corona that is emitting substantially less UV and X-ray energy than the surrounding area; hence, cooler in those wavelengths. As mentioned before, by convention, images show this as a dark area.
Coronal holes can cause geomagnetic storms on Earth. Interestingly, the impact is the greatest during solar minimum, which should occur sometime in 2030 or 2031.
One instrument for observing coronal holes is the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory satellite. Follow the link in the online KVNF script and hover your mouse over the various wavelengths and decide which wavelength presents coronal holes the best.