The outer layers of the Sun are the Photosphere, the Chromosphere, the Transition Region and the Corona.
Photosphere – The deepest layer of the Sun that we can observe directly.
It reaches from the surface visible at the centre of the solar disk to about 250 miles (400 km) above that.
The temperature in the photosphere varies between about 6500 K at the bottom and 4000 kelvin at the top (11,000 and 6700°F, 6200 and 3700°C).
Most of the photosphere is covered by granulation.
Chromosphere – A layer in the Sun between about 250 miles (400 km) and 1300 miles (2100 km) above the solar surface (the photosphere).
The temperature in the chromosphere varies between about 4000 kelvin at the bottom (the so-called temperature minimum) and 8000 kelvin at the top (6700 and 14,000°F, 3700 and 7700°C).
So in this layer (and higher layers) it actually gets hotter if you go further away from the Sun, unlike in the lower layers, where it gets hotter if you go closer to the centre of the Sun.
Transition Region – A very narrow (60 miles / 100 km) layer between the chromosphere and the corona where the temperature rises abruptly from about 8000 to about 500,000 kelvin (14,000 to 900,000°F, 7700 to 500,000°C).
Corona – The corona is the outermost layer of the Sun, starting at about 1300 miles (2100 km) above the solar surface (the photosphere).
The temperature in the corona is 500,000 kelvin (900,000°F, 500,000°C) or more, up to a few million K.
The corona cannot be seen with the naked eye except during a total solar eclipse, or with the use of a coronagraph.
The corona does not have an upper limit.
Source: NASA
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