SOLAR PHOTOSPHERE

SOLAR PHOTOSPHERE

The photosphere is the visible surface of the Sun. All the light received from the Sun, in fact, comes from the photosphere.

Why do not we receive the radiation in the same form as generated in the interior of the Sun? 

At the centre of the Sun, the energy is generated in the form of high energy photons called γ- rays. As these photons travel outwards, they collide with particles of matter and lose energy continuously. By the time these photons reach the surface − the photosphere − they are reduced to photons of visible region of electromagnetic spectrum. So, visible radiation is emitted from the photosphere.

The density of photosphere is 3400 times less than the density of the air we breathe. The thickness of the photosphere is about 500 km and the temperature at its base is ~ 6500 K. The temperature decreases upward and reaches a minimum value of ~ 4400 K at the top. This assumption is corroborated by the Sun’s absorption spectrum which indicates that the light we receive must be passing through a cool gas in which photons get absorbed. 

The photosphere is not a quiet region. It shows a granular structure. 

Photograph of the photosphere showing Granules structure

 Diagram showing granules and their boundaries

If you look at carefully  you can see that the photosphere consists of bright and irregularly shaped granules; each granule surrounded by dark edges. It has been found that these granules are very hot and their typical size is ~ 1500 km. The hot gas in the granules rises up with a speed of the order of 500 ms−1 and bursts apart by releasing energy. The cool material subsequently sinks downward along the dark edges or lanes between granules. The rising hot granules are seen only for a very short time (~ 10 minutes) before they dissolve. 

What causes granulation of the photosphere? 

It is caused due to convection (a mode of energy transport by matter). The granulation can be visualised as the top layer of a region where, due to convection, hot gas from below the photosphere moves upward. Thus, the centre of the granule is hotter and it emits more radiation and looks brighter in comparison to the edges which are relatively cooler and emit less radiation. Convection based explanation seems valid because the spectra of granules indicate that their centres are much hotter than the edges. Further, the solar granulation provides observable evidence supporting the idea that there exists a convection zone below the photosphere. 

What is the chemical composition of the photosphere?

It consists of 79 percent hydrogen and the remaining 21 percent consists of nearly 60 other chemical elements. Interestingly, all the elements of the photosphere are known elements and their proportion in the earth is more or less the same as that in the photosphere. 

This similarity in the chemical compositions of the photosphere and the earth is of utmost importance for understanding the formation of the solar system. 

Though the photographs of the Sun give the impression that it has a clear edge, such clear and distinct edge does not exist. Outside the apparent edge are the Sun’s outer layers, collectively known as the Sun’s atmosphere. These layers can be seen and probed and valuable information about their physical characterist.