Editing Sol (section)

===Atmosphere===
During a total solar eclipse, the solar corona can be seen with the naked eye.
The parts of the Sun above the photosphere are referred to collectively as the ''solar atmosphere''. 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 solar transition region, the corona, and the heliosphere. 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 waveshock front boundary with the interstellar medium. The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason why has not been conclusively proven; evidence suggests that Alfvén waves may have enough energy to heat the corona. 

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,000 Kelvin. 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.

Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. 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 solar eclipsetotal eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.

Above the chromosphere is a solar transition region in which the temperature rises rapidly from around 100,000 kelvin to coronal temperatures closer to one million K. The increase is because of a phase transition as helium within the region becomes fully ionized by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of Halo (optical phenomenon)nimbus around chromospheric features such as Spicule (solar physics)spicules and Solar filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from outer space by instruments sensitive to the far ultraviolet portion of the electromagnetic spectrum.

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the Solar System and heliosphere. The low corona, which is very near the surface of the Sun, has a very low partical density compared to the particle density of Earth's atmosphere near sea level. The temperature of the corona is several million kelvin. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.

The heliosphere extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the Solar System. Its inner boundary is defined as the layer in which the flow of the solar wind becomes ''superalfvénic'';that is, where the flow becomes faster than the speed of Alfvén waves. Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a Parker spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.