X-ray images of the Sun often show the large dark regions that we call coronal holes. They may extend from the Sun's equator to its poles, a few from pole to pole. In the 1960's they were seen in X-ray images taken by sounding rockets and detected with some radio telescopes. They were first seen clearly in images taken by astronauts on board the Skylab space station in 1973 and 1974. Waldmeier had seen coronal holes in the 1950’s with a green line (Fe XIV 5303) coronagraph that allows the corona to be seen along the limb of the Sun but the X-ray images of the disk of the Sun really showed what they were.
The solar corona is the outer atmosphere of the sun, extending from the solar "surface" out into space. It is difficult to observe from the ground, being seen only during solar eclipses or with special equipment. A coronal hole is a large region in the corona that has a lower density than its surroundings but with about the same temperature. It really is a hole in the corona! You can see the surface of the Sun, which is dark because it is cool next to the coronal temperatures of several million Kelvin. Although coronal holes can appear at any time of the solar cycle, they are most common and last the longest during the declining phase of the cycle.
The magnetic field in a coronal hole opens to interplanetary space. This is unlike the active Sun where the solar magnetic fields loop back to the Sun’s surface and form the bright loops and arches seen in X-ray and UV images. The rest of the Sun is covered with the quiet Sun magnetic carpet.
During solar minimum the coronal holes near the poles can last for several years. Holes near the equator may open and close in as little as a day but often remain visible for more than a month. Other magnetic features, such as sunspots and filaments, rarely last more than a rotation of the Sun (27 days). This means coronal holes may be the longest lasting magnetic feature on the Sun.
The presence of open magnetic field lines in coronal holes allows the plasma to escape, meaning that coronal holes have a lower density then the surrounding corona. The escaping particles mean that coronal holes are sources of high-speed solar wind streams. Particles in these streams can move at speeds up to 800 km/s (1.8 million mph). When the particles from these streams hit the Earth they may cause geomagnetic storms.
At times of high solar activity, geomagnetic storms are usually caused by coronal mass ejections (CME's) striking the Earth's magnetosphere. During times of low solar activity, coronal holes are the most common source of geomagnetic storms. Because coronal holes can last for many months, it is often possible to predict the occurrence of this type of geomagnetic disturbance, as the high-speed stream sweeps past the Earth with each solar rotation (like a rotating garden sprinkler). It may be possible to predict solar activity by the size of the coronal holes that form over the poles of the Sun.
In the SDO image from August 14, 2013 shown above the bright regions indicate hotter areas of the solar corona, mainly above active regions. A large dark coronal hole extends across the northern hemisphere. The stringy dark areas are filaments, cooler plasma held above the surface of the Sun by magnetic fields.
This picture shows two sources of geomagnetic storms, coronal holes, a source of high-speed streams, and filaments, whose eruption causes coronal mass ejections, in one picture. The filament in the lower right erupted as a coronal mass ejection a few hours after this image was recorded. Aurora caused by the high-speed stream emitted by the coronal hole were seen last night.
Edit 8/19/2013: It was pointed out on Facebook that the leftmost filament is actually another coronal hole.
