Corona, << kuh ROH nuh, >> is the outermost layer of the sun’s atmosphere. It is visible to the unaided eye only during a total solar eclipse, when the remainder of the sun is hidden by the moon. At such times, the corona appears as an irregularly shaped halo of light. Many stars other than the sun also have coronas.
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The sun’s corona consists chiefly of electrons, hydrogen ions (electrically charged atoms), and ions of heavier atoms that have lost many electrons. These ionized particles and electrons form a gaslike substance known as a plasma. Atoms become ionized when their electrons absorb enough energy to leave the atoms. The corona is greatly ionized because it has an extremely high temperature, measured at 4,000,000 °F (2,200,000 °C).
Activity in the corona increases and decreases in an 11-year cycle called the sunspot cycle. Streaks of coronal plasma known as polar plumes spread outward from the sun’s poles. Long, gaseous rays called coronal streamers radiate from areas closer to the equator. See Sunspot.
The corona continually expands into space, forming a flow of ionized particles called the solar wind. Astronomers believe that the solar wind flows mainly from coronal holes, regions of relatively low temperature and density. Coronal holes occur primarily around the sun’s poles, but they sometimes appear at lower latitudes. The solar wind extends to the earth’s orbit and beyond.
Astronomers observe the corona with instruments on the ground and in space. The best time to observe the corona from the ground is during a solar eclipse, an event that occurs about every 18 months. One of the main instruments used for viewing when there is no eclipse is a coronagraph, a telescope with a disk in the middle of its tube. The disk blocks out the light from lower layers of the sun’s atmosphere. In 1995, the National Aeronautics and Space Administration (NASA) of the United States and the European Space Agency launched the Solar and Heliospheric Observatory (SOHO). This spacecraft observes the corona with coronagraphs and other instruments. SOHO observations show that, about once every day, the sun throws off a huge ball of plasma known as a coronal mass ejection.
NASA’s Transition Region and Coronal Explorer (TRACE) satellite, launched in 1998, took detailed pictures of the corona until 2010. Japan’s Yohkoh spacecraft mapped X rays from the corona from 1991 to 2001. The Japanese-led Hinode mission began studying the corona in 2006. The spacecraft found evidence that magnetic waves and rapid changes in the sun’s magnetic field play a significant role in heating the corona.
