Regular readers of the IPS Monthly Solar and Geophysical Summary, or those people who receive IPS Warning services, have probably seen the term "coronal hole on the sun" used quite frequently. Not surprising because at this time of the solar cycle, coronal holes are the sources of many of the disturbances to the ionosphere (and HF communications) and to the geomagnetic field. But what is a coronal hole and what does one look like?

Firstly, the solar corona is the outer atmosphere of the sun, extending from the solar "surface" out into space. 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.

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.

Image of Coronal Hole (188K)

Material Prepared by Richard Thompson. © Copyright IPS - Radio and Space Services.