THE PEARLY CORONA
Above the Sun's photosphere lies a thin, cool, pinkish layer called the chromosphere. It is visible only during solar eclipses or through an H-alpha filter. The top of the chromosphere merges into the corona, the Sun's vast outer atmosphere, which reaches outward at least 10 times the solar radius.
It, too, can usually be seen from Earth only during solar eclipses, when its pearly light is a beautiful sight to behold. The coronal gas is very thinly spread but extremely hot— about 2 million degrees Fahrenheit (1,000,000° C). It is heated by processes that remain unclear.
Beyond the corona, the Sun sends out a stream of charged particles—mostly negatively charged electrons and positively charged protons—into space in what is called the solar wind. Near the Sun, this wind blows at 2,500 miles per second (4,000 km/s), but at Earth's distance, the velocity is a tenth of that figure.
The solar wind sometimes interacts with Earth's atmosphere and creates auroras. It also controls the shape and direction of a comet's ionized gas tail.