The Sun’s Corona
The White-Light Corona - The Corona is the Sun's outer atmosphere. It is visible during total eclipses of the Sun as a pearly white crown surrounding the Sun. The corona displays a variety of features including streamers, plumes, and loops. These features change from eclipse to eclipse and the overall shape of the corona changes with the sunspot cycle. However, during the few fleeting minutes of totality few, if any, changes are seen in these coronal features.
The Emission Line Corona - Early observations of the visible spectrum of the corona revealed bright emission lines at wavelengths that did not correspond to any known materials. This led astronomers to propose the existence of 'coronium' as the principal gas in the corona. The true nature of the corona remained a mystery until it was determined that the coronal gases are super-heated to temperatures greater than 1,000,000C (1,800,000F). At these high temperatures both hydrogen and helium (the two dominant elements) are completely stripped of their electrons. Even minor elements like carbon, nitrogen, and oxygen are stripped down to bare nuclei. Only the heavier trace elements like iron and calcium are able to retain a few of their electrons in this intense heat. It is emission from these highly ionized elements that produces the spectral emission lines that were so mysterious to early astronomers. We can now produce artificial eclipses in coronagraphs that cover the disk of the Sun and filter out everything except the emission due to these coronal ions.
The X-Ray Corona - The corona shines brightly in x-rays because of its high temperature. On the other hand, the 'cool' solar photosphere emits very few x-rays. This allows us to view the corona across the disk of the Sun when we observe the Sun in X-rays. To do this we must first design optics that can image x-rays and then we must get above the Earth's atmosphere. In the early 70's Skylab carried an x-ray telescope that revealed coronal holes and coronal bright points for the first time. During the last decade Yohkoh, provided a wealth of information and images on the sun's corona. Today we have the SOHO and TRACE satellites obtaining new and exciting observations of the Sun's corona, its features, and its dynamic character.