Coronal holes appear as dark regions in the solar corona when observed in Extreme Ultraviolet (EUV) and x-ray wavelengths. They appear dark because they have less material than surrounding areas. The magnetic structure of coronal holes is a unipolar open field topology, which allows the solar wind to escape more readily, and as a result coronal holes are the source of faster-than-normal solar wind. The fast solar wind from coronal holes can cause minor to moderate Geomagnetic Storms.
During solar minimum, the largest and most stable coronal holes occur at or near the solar poles. Coronal holes can also appear at lower latitudes, like the one in the image above. These equatorial coronal holes are the source of geomagnetic storms at Earth. Coronal holes can have lifetimes of months, so they produce recurrent geomagnetic activity as they rotate with the sun's 27-day rotation period.
Coronal holes have a different appearance at different wavelengths of the solar spectrum. They are most easily identified in x-ray images of the Sun, where they appear as dark holes. They are also visible at EUV wavelengths, but the contrast is not as great. Prior to space-based x-ray and EUV observations, space weather forecasters relied on ground-based observations made at the Helium 10830 Angstrom wavelength in the infrared. At these wavelengths, coronal holes appear brighter than the surrounding areas.
Forecasters take note of the development of coronal holes to make Geomagnetic Storm predictions as the holes rotate with the Sun. Because coronal holes can be very large, they can be a source of high-speed wind that buffets Earth for many days and enhances Earth’s radiation belts.
Image courtesy of NOAA GOES SXI.