This image of the solar "moss" was taken by NASA's Transition Region and Coronal Explorer (TRACE) spacecraft October 8, 1999. The image is false color, looking almost directly down over coronal loops, immense magnetic arches of hot gas that are anchored in the Sun's visible surface and could span dozens of Earths laid end to end. The bases of the coronal loops appear as white, feathery objects in the bottom left and top right of this image. The moss is the blue, black and white spongy structure between the bases of the coronal loops.
Solar moss consists of hot gas at about two million degrees Fahrenheit which emits extreme ultraviolet light observed by the TRACE instrument. It occurs in large patches, about 6,000 - 12,000 miles in extent, and appears between 1,000 - 1,500 miles above the Sun's visible surface, sometimes reaching more than 3,000 miles high. It looks "spongy" because the patches are composed of small bright elements interlaced with dark voids in the TRACE images. These voids are caused by jets of cooler gas from the Sun's lower atmosphere, the chromosphere, which is at about 10,000 degrees Fahrenheit.
The solar moss appears only below high pressure coronal loops in active regions, typically persisting for tens of hours, but has been seen to form rapidly and spread in association with loops that arise after a solar explosion, called a flare.