(Courtesy of the Vacuum Telescope, National Solar Observatory, Kitt Peak)

About the images:

Indirect observations of the corona can be made using the helium 1083.0 nm line. The population of excited chromospheric helium responsible for this line is partially controlled by conditions in the overlying corona. Coronal holes are marginally detectable in this line and are mapped by the National Solar Observatory (NSO) facility at Kitt Peak, AZ. The coronal holes appear as light regions in the He 1083.0 nm image.

What to look for:

Coronal hole regions at moderate to low latitudes produce recurrent magnetic storm activity at the Earth. The coronal holes are sources of high speed solar wind streams. They are somewhat cooler (~500,000 K less) than the surrounding corona and as much as an order of magnitude less dense. The high solar wind streams deplete the matter in these regions and carry away internal energy producing the lower temperatures.