Atmosphere

Atmosphere
See also: Corona and Coronal loop
During a total solar eclipse, the solar corona can be seen with the naked eye, during the brief period of totality.

The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere.[48] They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere.[48] The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The chromosphere, transition region, and corona are much hotter than the surface of the Sun.[48] The reason has not been conclusively proven; evidence suggests that Alfvén waves may have enough energy to heat the corona.[52]

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,100 K.[48] This part of the Sun is cool enough to support simple molecules such as carbon monoxide and water, which can be detected by their absorption spectra.[53]

Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines.[48] It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun.[45] The temperature in the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top.[48] In the upper part of chromosphere helium becomes partially ionized.[54]


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