Coronal holes (Altschuler et al., 1972; Munro and Withbroe, 1972) are "dark" coronal regions of the Sun with open magnetic field structure. During the minimum years of the solar cycle they are confined in the Sun's polar regions, while at solar maximum they can be found at all latitudes.
- Altschuler, M. D., D. E. Trotter, and F. Q. Orrall, Coronal holes, Sol. Phys., 26, 354-365, 1972.
- Krieger, A. S., A. F. Timothy, and E. C. Roelof, A coronal hole and its identification as the source of a high velocity solar wind stream, Sol. Phys., 29, 505-525, 1973.
- Munro, R. H., and G. L. Withbroe, Properties of a coronal "hole" derived from extreme-ultraviolet observations, Astrophys. J., 176, 511-520, 1972.
See also Wikipedia on coronal holes.