corona

The outermost layers of the Sun, which become visible as a white halo during a total solar eclipse. It extends out to many times the Sun's radius, until it merges into the interplanetary medium. It consists of a number of components. The K corona (electron corona or continuum corona) is seen as white light scattered from the photosphere by high-energy electrons, which are at a temperature of a million degrees. The K corona is not uniform, but shows variable structures such as streamers, condensations, plumes and rays. Because the electrons are moving at high speeds, the Fraunhofer lines are blurred out in the spectrum of the reflected light. The F corona (Fraunhofer corona or dust corona) is light from the photosphere scattered by slower-moving dust particles around the Sun. Fraunhofer lines are seen in its spectrum. The extension of the F corona into interplanetary space is seen as the zodiacal light. The E corona (emission line corona) consists of the light from discrete emission lines produced by highly ionized atoms, particularly iron and calcium. It is detected out to two solar radii only. The corona also emits radiation in the extreme ultraviolet and soft X-ray regions of the spectrum. The extent and shape of the corona changes during the course of the solar cycle, mainly due to the streamers produced in active regions.