The corona is usaually studied with a coronagraph, whcih creates an artificial eclipse (so only the corona may be seen). However, full-color photographs are now possible, thanks to the SOHO photos. Still, a coronal hole is just as tell-tale as it has always been. Due to the high amount of light released anywhere else on the Sun's surface, a coronal hole appears black. These can also be seen on solar x-rays and He1083 images.
The photo above shows a large coronal hole. It is the black void to the lower right-hand side. The magnetic structure of these regions has been called "indefinate" and "non-circuit", meaning that the field lines simply extend into space infinately. Lack of a circuit back to the photosphere allows mass emission of charged particles from the sun, so much that it actually causes solar wind. Sometimes, plasma will also be released from the Sun, resulting in a Coronal Mass Ejection, or CME.
CME's can cause disruptions in Earth's magnetic field, and have been the source of many magnetic storms as they disrupt our magnetosphere. They may take three days to get here, but when they do, they produce the most stunning propagatory phenomenon of them all: the aurora. Though we usually associate these with the "northern lights," most auroras cannot be seen. The downside of auroral periods, however, is that the magnetic storms caused by the CME disrupt F-layer ionization. You can spot a CME by the increased K- and A-indices, since a large CME can create winds in excess of 700km/h.
Coronal holes and their attached CME's (which are usually accompanied by a solar flare) cause mixed results on earth, especially in terms of radiowave propagation, although are not incredibly common. Most radio operators will rejoice at the sight (especially in the lower years of the cycle).
Images courtesy NASA