Main Index Section IndexExtract from the Sacramento Peak Observatory page on the World Wide Web
The solar disk we see (in white light) is called the photosphere. It is a thin layer, that produces the radiation transmitted into space and we see. At this distance from the sun, photons are more likely to escape than be absorbed.
(i) height about 100 kilometers
(ii) radiation temperature is about 5780 Kelvin (peak output is the center of visible light)
(iii) photospheric temperature ranges from 6600 Kelvin at the bottom to 4300 Kelvin (temperature minimum) at the top
(iv) density is about one hundred millionth of water
(v) pressure is one hundredth of an atmosphere.
The photosphere is the transition from the convection zone to a convectively stable atmosphere. For photons to freely leave the sun, the atmosphere does not have as many absorbing atoms per unit volume. Decreasing density contributes to the transparency. Also, the photosphere starts just above the area where ionized hydrogen has recombined to hydrogen. In the photosphere, the neutral hydrogen readily combines with electrons to form negatively ionized hydrogen which is an effective scatterer. The electron density is a strong function of density and the negatively ionized hydrogen drops off rapidly above the photosphere.
The sun's limb is darker than the center. This is called limb darkening. Photospheric light travels through an absorptive medium. This means that one can see only see so far into the photosphere. At the limb, the line of sight sees the top of the photosphere, or about 4300 Kelvin. At the center, the line of sight sees deeper into the sun, and a higher temperature. So the limb looks darker because it is higher in the atmosphere which is cooler than at disc center.
Another effect which can be observed in the photosphere are the electron transitions which form absorption lines. Absorption lines form at different heights in the atmosphere, so they can be used to map different layers in the photosphere. Also, velocity along the line of sight can be deduced by the line broadening.
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