Corona
Corona, the outer portion of the sun's atmosphere. Normally, the corona is visible only during a total solar eclipse, when it appears as an irregular whitish halo around the darkened sphere of the sun. The corona's shape varies, to some extent, with the 11-year-long sunspot cycle. During periods of the greatest sunspot activity, the corona appears as a nearly circular halo. When sunspots are least numerous, long streamers radiate from the sun's equatorial region, and somewhat shorter plumes project from the poles. Because total solar eclipses are infrequent and last for only a few minutes at any one place on earth, astronomers use a device called a coronagraph to observe the corona at other times. The coronagraph blocks out the light from the sun's disk, allowing the corona to be seen and photographed.
The corona is made up of widely scattered ions (electrically charged atoms) and atomic particles that form a gas of very low density In sharp contrast to the relatively cool surface and lower atmosphere of the sun, the corona has a temperature of 2,000,000 F. (1,100,000 C.) or more. Some astronomers believe this extremely high temperature may be caused by shock waves formed as rapidly moving jets of gas surge upward from the sun's surface. Ions constantly flow outward from the corona, spreading across the solar system in what is known as the solar wind.