Properties of a coronal hole derived from extreme-ultraviolet observations

A coronal <`hole" is characterized by significant deficiency in the intensity of coronal emission lines, yet there is little change in lines and continua formed below 800,000 K except for the He I and He ii emission features. Comparison of the hole with the normal quiet Sun indicates that the electron pressure is reduced hy a factor of 3, the coronal temperature is lower hy 600,000 K, and the temperature gradient in the chromospheric-coronal transition layer is less steep by an order of magnitude. The apparent insensitivity of the lines formed in the transition layer to the physical conditions of the hole implies that the conductive flux from the corona is proportional to the square of the electron pressure.