It is a region which is difficult to observe, being seen only during solar eclipses or with special equipment. A coronal hole is a large region in the corona which is less dense and is cooler than its surrounds. Such holes may appear at any time of the solar cycle but they are most common during the declining phase of the cycle. These images are not the normal visible light but instead receive X-rays, a wavelength range at which the corona normally produces radiation.
Coronal holes appear as large dark regions over the brighter image of the solar corona in X-rays.
In the Yohkoh image below the bright regions indicate hotter areas of the solar corona, mainly above the sunspot regions. A large dark region extends from one pole right across the solar equator and well into the other hemisphere. It is from coronal holes like this one that IPS makes forecasts of disturbances to HF communications and other earth and space-based technology. All rights reserved.